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Zoologischer Anzeiger 256 (2015) 2–13
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Review
Too many ways to make a muscle: Evolution of GRNs
governing myogenesis
Carmen Andrikou ∗ , Maria Ina Arnone
Cellular and Developmental Biology, Stazione Zoologica Anton Dohrn, Napoli 80121, Italy
a r t i c l e
i n f o
Article history:
Received 31 October 2014
Received in revised form 17 March 2015
Accepted 17 March 2015
Available online 18 March 2015
Keywords:
Myogenesis
Transcriptional regulators
Gene Regulatory Networks
Evolution
Development
Cooption
a b s t r a c t
Animal development is an elaborate process encoded in the genome. Regulatory genes encode transcription factors and signaling molecules, and their expression is under the control of cis-regulatory modules
that define spatially defined transcriptional regulatory states. The functional linkages among these genes
constitute the gene regulatory networks (GRNs) and changes in their architecture due to redeployment of
regulatory genes in new locations and/or at different times during embryogenesis results in evolutionary
changes. The focus of this review is a wide cross comparison of the GRNs orchestrating myogenesis in
several distant phyla in order to provide insights into the evolution of the myogenic regulatory landscape.
By comparing the core myogenic network architecture we reveal cases of deep homology, re-deployment
of plug-ins, change in hierarchy of action, cooption and novelty.
© 2015 Elsevier GmbH. All rights reserved.
Contents
1.
2.
3.
4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Every muscle has a different story . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.
Myogenesis in vertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2.
Myogenesis in invertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Conserved and divergent properties of the myogenic networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.
Molecular patterning of muscles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2.
Stable genetic toolkit and conserved evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3.
Modification of preexisting genetic repertoire and divergent evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Too many ways to make a muscle: teaching old genes new tricks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Muscle development involves complex series of cell morphogenetic rearrangements accompanied by the emergence of specific
gene regulatory circuits. In most triploblastic animals, different
Abbreviations: GRN, gene regulatory network; bHLH, basic helix-loop-helix;
MRF, smyogenic regulatory factors; TF, transcription factor; Shh, sonic Hedgehog;
MyoR, myogenic repressor.
∗ Corresponding author. Present address: Sars International Centre for Marine
Molecular Biology, University of Bergen, Bergen 5008, Norway. Tel.: +4755584292.
E-mail address: [email protected] (C. Andrikou).
http://dx.doi.org/10.1016/j.jcz.2015.03.005
0044-5231/© 2015 Elsevier GmbH. All rights reserved.
regions of the embryo generate progenitor populations of different muscles, which are categorized into two major cellular types
according to their structural and functional properties: striated
and non-striated muscles. In vertebrates and insects, striated muscles are further subdivided into multinucleated skeletal (somatic)
and cardiac muscle types while the non-striated are mainly the
smooth (visceral) muscle type. However, somatic muscles are not
always multinucleated or a product of cell fusion. For instance,
nematodes and tunicates possess single somatic cells. Also, the definition of ‘muscle’ varies within organisms; in the fruit fly Drosophila
melanogaster and the nematode Caenorhabditis elegans a single
myotube is defined as ‘a muscle’ while in vertebrates ‘a muscle’
consists of bundles of myotubes (Royuela et al., 2000).
C. Andrikou, M.I. Arnone / Zoologischer Anzeiger 256 (2015) 2–13
One important question in developmental biology is how single
progenitor cells are chosen to form certain tissues and myogenesis, has proved to be a powerful tool to provide that answer in
the case of muscle formation. The induction of a particular cell
fate can be in most development processes divided into two separate states where the cell is first specified and then determined to
form a given tissue. Specification is an early point and is mainly
regulated by extracellular signaling molecules that mediate the
activation of transcription factors specific for the cell type, which
eventually forms a given tissue. Myoblasts are the cells that are
specified to become muscles. Determination occurs when cells start
to form specific tissues and express specific proteins known as tissue molecular markers. If a cell is specified, its fate can be reversed
or transformed to another one, whereas in the state of determination, the cell’s fate cannot be changed anymore. The latest point is
differentiation and it often involves a change in appearance as well
as in function, such as, in the case of many muscle types, myocyte
fusion and fiber formation. The process of differentiation is typically
driven by activation or repression of a large set of genes (Taylor,
2002).
Since the information required for precisely building a tissue in each embryo involves the functional interaction between
extracellular signals, intracellular transcriptional regulators and
differentiation genes, in order to understand the molecular mechanisms of a developmental process one needs to dissect the
underlined genomic regulatory interactions. A systematic analysis of such type of interactions brings to the construction of a Gene
Regulatory Network (GRN), which is based on schematic representations of the functional linkages among specific genes in a given
time and tissue (regulatory state) and provides a causal explanation of the molecular interactions occurring during development
(Davidson et al., 2002). The understanding of the wiring properties
of a developmental GRN offers a comprehensive view of the relationship between the regulatory architecture and gene expression
dynamics and relates it to the dynamic processes of cell specification and differentiation (Ben-Tabou de-Leon and Davidson, 2006).
Moreover, since development is proceeded by the progressive
installation of different transcriptional regulatory states, the evolution of body plans must depend upon alterations in the architecture
of developmental GRNs, which makes the interspecies comparison
of GRNs an alternative mean of understanding evolution (Erwin and
Davidson, 2009).
This review focuses on the conservation and divergence of the
transcriptional networks that drive myogenesis among several
distant phyla using the recent determination of the GRNs governing myogenesis in early branching deuterostomes (the sea urchin
Strongylocentrotus purpuratus and the ascidian Ciona intestinalis),
as compared to protostomes (the fruit fly Drosophila melanogaster
and the nematode Caenorhabditis elegans) and vertebrates. We provide insights into the evolution of the properties of myogenic GRNs
and associate the degree of depth and density of developmental
networks with the level of organismic complexity.
3
over, the sister group of all metazoans, Ctenophora (Ryan et al.,
2013), appear to possess a fibre muscle cell type that significantly
differs from the ones found in triploblastic animals. These cells
indeed lack a nucleus, most organelles and the H bands region
typical of the sarcomere (Mackie et al., 1988). For these reasons,
independent evolution of striated muscle has also been suggested
(Burton, 2008; Oota and Saitou, 1999). A study using a detailed
genome analysis in a wide array of species has recently been published which strongly supports a dual origin of the striated muscle
type and provides an explanation for the existence of striated musculature found in Cnidaria and as well in the Ctenophora (Steinmetz
et al., 2012).
2.1. Myogenesis in vertebrates
In vertebrates, the different muscle types arise from different,
anatomically separated regions of mesoderm. The visceral (smooth
type) muscles develop from the inner, splanchnic layer of the lateral plate mesoderm, whilst cardiac and some craniofacial muscles
arise from bilaterally symmetrical regions of the lateral plate mesoderm. The skeletal (somatic) musculature originates from transient
structures of the paraxial mesoderm, called somites, located at
each side of the neural tube where different regions will form only
certain muscle types, such as dermomyotomal (skeletal muscles,
diaphragm etc.) and sclerotomal (cartilage and bone) progenitor
cells (Hollway and Currie, 2005). Other skeletal muscles that originate from different mesodermal populations are the craniofacial
type of muscles that are associated with head and neck structures.
These muscles derive from populations of both paraxial and lateral
mesoderm located anterior to the somites (Tzahor, 2015). Recent
studies have shown that the same progenitor populations (called
the cardiopharyngael mesoderm) contribute to a number of head
muscles and the heart (Lescroart et al., 2015).
The segmentation of the paraxial mesoderm into somites, as
well as the specification of the muscle progenitor cells (myoblasts),
are both induced by local oscillators in gene expression and morphogen gradients secreted from adjacent tissues, such as the neural
tube, the notochord, and the dorsal and lateral ectoderm. Myoblasts
start then to express a number of regulatory factors, resulting in the
transcriptional extinction of alternative mesodermal lineages and
the establishment of the myogenic regulatory state. Subsequently,
myoblasts fuse and form syncytial myocytes resulting in the formation of a scaffold of primary muscle fibers (primary myogenesis).
In this myogenic phase, distinct muscle populations start to differentiate and express certain muscle-specific structural genes. The
second step (secondary myogenesis) is the addition of extra muscle
fibers alongside the primary ones during which, a subset population of cells (satellite cells) is put aside as a reservoir for muscle
growth and repair (Hollway and Currie, 2003). The mode of muscle
development seen in vertebrates is schematically summarized on
top of Fig. 1.
2.2. Myogenesis in invertebrates
2. Every muscle has a different story
The origin and evolution of musculature is a debated subject.
Due to the strong ultrastructural similarities of striated muscles
and the conserved expression of regulatory and structural genes,
a common evolutionary origin has been often considered (Muller
et al., 2003; Seipel and Schmid, 2005; Spring et al., 2002). However,
the sister group of bilaterians, Cnidaria, possess only ectodermally
(tentacle longitudinal muscle) and endodermally derived epitheliomuscular and basiepithelial muscle cells (Jahnel et al., 2014),
which differentiate from regular epithelial cells; therefore are
epithelio-muscle-cells (EMC) and not true (fibre) muscles. More-
The regulatory landscape of myogenesis in invertebrates is
much less explored than in vertebrates. In non-bilateria nothing
is known about the molecular basis of muscle development whilst
in the remaining invertebrates poor molecular descriptions exist in
literature, with a few exceptions. The fruit fly D. melanogaster is the
invertebrate model for which muscle development has been so far
better described (see schematic representation in the bottom part
of Fig. 1). Flies also divide their mesoderm into distinct regions,
which give rise to separate muscle lineages with characteristic
properties; cardiac muscles develop from the most dorsal, external
mesoderm, visceral muscles derive from the internal, splanchnic
mesoderm and somatic muscles form from the external somatic
4
C. Andrikou, M.I. Arnone / Zoologischer Anzeiger 256 (2015) 2–13
Fig. 1. Comparative overview of muscle development in different organisms. (For interpretation of the references to color in this figure legend, the reader is referred to the
web version of this article.)
From top to down schematic representations of the development of the muscle lineage is reported for four embryos belonging to different phyla, as indicated in the
phylogenetic tree. (A) Vertebrates. Different regions of mesoderm will give rise to distinct muscle types. In mouse embryos, somites originate from the paraxial mesoderm
(red) and are further subdivided into the progenitors of different cell types. Skeletal (somatic) myoblasts are deposited from the myotome, which arise from the ventrolateral
lips of the dermomyotome. Myoblasts then fuse to form pioneer myocytes (bright red), which will later form a scaffold of small primary (red) and secondary muscle fibers
(yellow). Satellite cells are indicated as dark red (adapted from (Gilbert, 2006)). (B) Tunicates. The different muscle lineages, in an ascidian embryo, are descendants of specific
blastomeres (A4.1, B4.1 and b4.2). At the 64 cell-stage, the A-line and b-line secondary muscle precursors are formed in the most posterior lateral part of the neural plate
(orange and salmon pink, respectively) whilst the B-line primary muscle (red) including the heart progenitors (B7.5 descendant, in yellow) are located in the anterior lateral
part of the neural plate. The final position of muscle cells is indicated in the tailbud drawing: somatic muscles (red, pink and orange) and heart progenitors (yellow) (adapted
from (Hudson and Yasuo, 2008)). (C) Echinoderms. In the sea urchin embryo, circumesophagael muscle progenitors are arising from four, ventrolaterally located, secondary
mesenchyme (mesodermal) cells (red). At late gastrula stage, bilaterally symmetrical differentiated myocytes are formed at the oral vegetal domain of the archenteron.
Finally, at pluteus larva stage, processes of the myoblasts fuse in the midline of esophagus to form the muscle fibers (red). The muscles of three endodermally derived
myoepithelial sphincters (cardiac, pyloric and anal) are shown in yellow. (D) Insects. Developmental origins of distinct muscle types from different mesodermal regions
(orange) in a Drosophila embryo. Mesodermal hemisegment is subdivided into segmentally repeated domains, which will give rise to the progenitors of visceral (yellow) and
somatic (red) muscle progenitors. Within the somatic mesoderm, founder cells (red) and fusion-competent myoblasts (white) are produced and fuse to form multinucleated
muscle precursors. Finally, a fully differentiated array of distinct syncytial muscle fibers in each abdominal hemisegment is formed (adapted from (Taylor, 1996)).
mesoderm (Bate, 1990). The diversification of these segmented
units and the subsequent specification of myoblasts is influenced by
signals from adjacent regions and accompanied by the emergence
of specific transcription regulators. Within the somatic mesoderm
founder cells, fusion-competent myoblasts and adult muscle precursors are produced and have stereotyped position relatively to
each other. The adult muscle precursors differentiate into the
adult abdominal and thoracic musculature while each founder cell
fuses with fusion-competent myoblasts and form multinucleated
myotubes, which attach to specific sites on the epidermis (Baylies
et al., 1998).
The major two muscle cell types in the nematode C. elegans are
the striated body wall muscles and the non-striated muscles found
in the pharynx, intestine and reproductive organs and they all are
mesodermal derivatives. The subdivision of mesoderm (M lineage)
that leads to the formation of the different muscle types is a result
of an asymmetric cell division occurring due to inductive signals
coming from neighbor domains (Krause and Liu, 2012). Based on
their structural properties, the body wall muscles are thought to be
homologous to the vertebrate somatic type, the pharyngeal muscle
is considered to be the equivalent of the cardiac muscle type and
the intestinal muscles resemble smooth muscle type (Fukushige
and Krause, 2012). The majority of the body wall and all pharyngeal
and intestinal muscles have an embryonic origin whilst all gonadal
and sex specific muscles develop during larval stages (Sulston and
Horvitz, 1977).
In ascidian tailbuds, two different populations of striated muscle cells are found: the somatic muscles of the tail and the heart
muscles. Ascidian muscle cells derive from three out of the four
embryonic cell lineages, the B, A- and b- cells, due to a combination
of inductive signals and maternal factors that result in stereotypical asymmetric divisions and cell specification events. B-lineage
derived muscles form the bulk of the somatic larval muscle bands
(primary muscle cells), whereas those originating from A- and blineages (secondary muscle cells) form the posterior-most tail muscles (Nishida, 1987). Moreover, a pair of the B-lineage cells (B7.5)
C. Andrikou, M.I. Arnone / Zoologischer Anzeiger 256 (2015) 2–13
give birth to the anterior tail muscle cells and the heart muscle
progenitors (trunk ventral cells), which first differentiate on each
lateral side of the embryo and then fuse ventrally (Davidson, 2007).
Sea urchin embryos are characterized by two types of muscles,
the circumesophagael muscles and the myoepithelial type of cells
that compartmentalize the digestive tract (sphincters). The myoepithelial cells are endodermally derived and two of them (pyloric and
anal sphincter) share typical smooth type muscle properties whist
the third one (cardiac sphincter) is formed from a simple, striated
myoepithelium (Burke, 1981). The circumesophagael muscles arise
from a distinguished mesodermal population (four ventrolaterally
located secondary mesenchyme cells) (Andrikou et al., 2013) and
although they look morphologically similar to smooth (visceral)
type, their dense bodies appear to be periodically aligned across the
width of the filamentous region that resembles indistinct Z-lines,
typical of the skeletal muscle type (Burke, 1981). Circumesophagael
myoblast specification depends on inductive signals coming from
the ectoderm, which result in the activation of muscle specific transcriptional regulators (Andrikou et al., under review). These factors
will trigger the emergence of muscle differentiation gene expression and the subsequent fiber formation and fusion at the midline
of the esophagus.
The different modes of muscle development seen in vertebrates,
tunicates, echinoderms and insects are schematically summarized
in Fig. 1.
3. Conserved and divergent properties of the myogenic
networks
The molecular interplay that underlies muscle formation has
been a paradigm for transcriptional regulation since the discovery
of the basic helix-loop-helix (bHLH) domain – containing myogenic
regulatory factors (MRFs), which are able to convert undifferentiated non-mesodermal cells into muscle-like cells (Olson and Klein,
1994; Weintraub et al., 1989). The following sections review our
current understanding of the molecular regulation of myogenesis in vertebrates and a number of well-studied invertebrates and
provide a detailed interspecies evolutionary comparison of the
myogenic transcriptional programs.
3.1. Molecular patterning of muscles
Spatiotemporal somitogenesis in vertebrates followed by
deposit of muscle progenitors involves morphogen gradients of
Wnt Sonic Hedgehog (Shh), FGF, BMP4 and Retinoic acid, as well
as the Notch pathway, which directly or indirectly induce expression of myogenic genes (Borycki et al., 1999; Delfini et al., 2009;
Lewis et al., 2009; von Maltzahn et al., 2012). The specification and
differentiation of somatic myoblasts is depending mostly on the
sequential expression of the bHLH MRFs (summarized in Fig. 2).
Vertebrates have four MRFs: Myf5 (Braun et al., 1989) and Myod
(Sassoon et al., 1989), which play redundant roles in myoblast specification in the dorsal or ventral dermomyotome, respectively, and
MRF4 (Braun et al., 1990) and Myogenin (Edmondson and Olson,
1989), which instruct myoblasts for terminal differentiation. The
bHLH factor MyoR (myogenic repressor) or musculin is expressed at
high levels in proliferating skeletal myoblasts where it antagonizes
the action of Myod and is downregulated during muscle differentiation (Lu et al., 1999; Yu et al., 2003). MyoR is paralogous to the bHLH
protein capsulin, which is expressed in smooth muscle cell precursors (Lu et al., 1998). Twist, another bHLH factor, represses skeletal
myogenesis by sequestering E proteins – thus blocking the activity of Myod – and by inhibiting Mef2 and Myogenin transcription
factors and therefore is not expressed in the myotome (Hamamori
et al., 1997; Mastroyiannopoulos et al., 2013; Spicer et al., 1996).
5
Also, Twist is expressed in the lateral plate mesoderm of the embryo
(Tavares et al., 2001) from which the visceral muscles are derived.
Members of the paired-homeobox (Pax) family genes are also
involved in myogenesis. The myoblast precursors in the dermomyotome express the genes Pax7 and Pax3; the central region express
mostly Pax7 and the dorsal and ventral lips show highest level of
Pax3 (Kassar-Duchossoy et al., 2005). In the genetic cascade Pax
genes are acting upstream from Myod while Myf5 functions in parallel with them (Bryson-Richardson and Currie, 2008).
Other genes that are seen at the top of the genetic regulatory
hierarchy are the sine oculis related homeobox group 1 (Six1) and
4 (Six4), their cofactors eyes absent members 1 (Eya1) and 2 (Eya2)
as well as Dachshund (Dach2). This evolutionary conserved complex
(Six-Eya-Dach) (Christensen et al., 2008) activates Six target genes,
such as Pax3, Myod, MRF4 and Myogenin (Grifone et al., 2005).
The myocyte enhancer factor-2(Mef2) belongs to the MADS
family of transcription factors and is another key transcriptional
regulator of myogenesis. In vertebrates, Mef2 is directly regulated
by MRFs (Potthoff and Olson, 2007) and although it does not have
myogenic activity itself, it potentiates the function of MRFs through
transcriptional cooperation (Molkentin and Olson, 1996). A downstream target of Mef2 is a Serum Response Factor (SRF) Myocardin,
a gene necessary for muscle specification and differentiation in the
cardiac and smooth muscle cells (Creemers et al., 2006; Wang et al.,
2004).
Members of the Sex determining region Y (Sox) family are also
reported to have important roles during vertebrate myogenesis.
The vertebrate Sox8 and Sox9 (belonging to group E in invertebrates) (Bowles et al., 2000) act as specific negative regulators of
myogenic differentiation. Sox8 is strongly expressed in the proliferating myoblasts and interferes with the function of MRFs, which
results in a decreased activity of the Myogenin promoter. During
myogenic differentiation its expression drops and gets finally confined to satellite muscle cells (Schmidt et al., 2003). Moreover, Sox9
and Myocardin counteract each other in regulating vascular smooth
muscle cell differentiation (Xu et al., 2012).
Two groups of the Forkhead (Fox) family of transcription factors
are also involved in vertebrate myogenesis: FoxC and FoxF, which
are known to be terminal effectors of Shh signal (Hannenhalli and
Kaestner, 2009). The FoxC genes (FoxC1 and FoxC2) are expressed in
the dorsal, paraxial and intermediate mesoderm and play important roles in the development of somites and the cardiovascular
system (Kume, 2009; Wilm et al., 2004). FoxF1 and FoxF2 genes
are expressed in the developing lateral plate mesoderm and
the derived smooth muscle, where they regulate Myocardin and
Myocardin-related transcription factor activities (Hoggatt et al.,
2013; Mahlapuu et al., 2001).
Six of the seventeen T-box family members (Tbx1, Tbx18 and
Tbx20 of the Tbx1 subfamily, and Tbx2, Tbx3 and Tbx5 of the Tbx2
subfamily) are expressed and required in a combinatorial fashion
in cardiac muscle progenitors. Tbx1 is activated by FoxC1/C2 and,
together with Tbx20, activate two members of the Nkx homeobox
family, Nk2.5 and Nk2.3 (Buchberger et al., 1996) as well as Mef2
(Greulich et al., 2011).
An example of the hierarchy of action of the above-mentioned
regulators during somatic muscle development in vertebrates is
depicted in Fig. 2.
The main myogenic players in Drosophila are the bHLH transcription factor Twist and its Nkx target, Tinman (Baylies and
Bate, 1996; Bodmer, 1993). Twist is essential for mesoderm specification and its subsequent subdivision into different myogenic
domains (Baylies and Bate, 1996; Leptin, 1991) and is activated via
a Wg (Drosophila Wnt) signaling (Bate and Rushton, 1993) whilst
being negatively regulated by a Notch signaling pathway (TapanesCastillo and Baylies, 2004). The further subdivision of myogenic
domains and the specification of different types of muscles involve
6
C. Andrikou, M.I. Arnone / Zoologischer Anzeiger 256 (2015) 2–13
Fig. 2. Muscle fate acquisition steps in vertebrates and hierarchy of transcription factors through the skeletal myogenic lineage.
Somite formation and subdivision in different progenitor cells is regulated by FoxC factors. The early skeletal muscle progenitor cells are specified by the sequential expression
of Six1/4, Pax3/7, the bHLH factor MyoR and the two MRFs, Myf5 and Myod. During the step of commitment, the specified muscle progenitors are committed to become
myoblasts (determination) under the influence of the second peaks of expression of Myf5 and Myod, as well as the MADS box factor Mef2 and Sox8. The activated myoblasts
will eventually differentiate by the other two MRFs, Myogenin and MRF4. The graph reported in the lower part of the figure is a schematic representation of the hierarchical
temporal expression of regulatory genes derived from various sources (Bentzinger et al., 2012; Kume, 2009; Potthoff and Olson, 2007; Schmidt et al., 2003; Wilm et al., 2004;
Yu et al., 2003).
Source: Cell drawings are adapted from Bentzinger et al. (2012).
intrinsic FGF, EGF and Hh signaling events (Tixier et al., 2010).
More downstream of Twist and Tinman is Nautilus (nau), the
ortholog, based on aminoacid sequence, of the vertebrate MRFs
in flies (Michelson et al., 1990). Nautilus expression initiates at
the onset of myogenesis and has a restricted role in the specification of a subset of muscle precursors (Balagopalan et al., 2001).
The mesoderm-specific Pax gene, Pox-meso (Poxm), is downstream
of MRFs in Drosophila and loss of-function mutants display mild
muscle defects in a subset of ventral and lateral muscles (Duan
et al., 2007). Moreover, in Drosophila, Six4/5 and its cofactor Clift
(orthologous to Eya) are essential for ventral and lateral muscle
development and are under the control of Tinman (Liu et al., 2009).
Mef2 function is restricted to muscle development in Drosophila,
and is directly activated by Twist (Cripps et al., 1998). The induction
of visceral musculature includes the activation of the Nkx homeobox genes Tinman and Bagpipe (Bap) (Azpiazu and Frasch, 1993),
the FoxF ortholog Biniou (Zaffran et al., 2001; Zinzen et al., 2009)
and the Tbx1 ortholog, org-1 (Lee et al., 2003). Org-1 is involved in
the development of the circular visceral muscles and is regulated
by Dpp and Tinman and is a direct upstream regulator of Dpp and
Wg expression (Schaub and Frasch, 2013) while Biniou is essential for the differentiation of the splanchnic mesoderm into midgut
musculature and is downstream of Twist and Tinman (Zaffran et al.,
2001).
The bHLH protein bHLH54F (ortholog of MyoR) is expressed
specifically both in visceral and skeletal muscle cell precursors
(Georgias et al., 1997; Ismat et al., 2010) and in the visceral muscles is under the control of Biniou (Jakobsen et al., 2007). Finally, a
Drosophila specific factor, Him (holes in muscle), is known to inhibit
Mef2 activity (Soler and Taylor, 2009).
In C. elegans, the specification of the M-lineage is regulated by
the T-box factor, Tbx35 and its direct target ceh-51 (ortholog of
Nkx2) (Broitman-Maduro et al., 2009), whose combinatorial activity initiates the expression of the Six family related factors, ceh-34
and unc-39 (Krause and Liu, 2012). Ceh-34 and eya-1 act together in
the mesodermal lineage in the determination of either the anterior
body wall muscles or the non-muscle coelomocytes (Amin et al.,
2009). Body wall muscle fate is later specified by convergent Wnt
(mom-2) and TCF/LEF (pop-1) signals as well as the Caudal-related
factor PAL-1 that activate the MRF ortholog hlh-1 (Chen et al., 1992;
Lei et al., 2009). Hlh-1 is upstream of the MADS box containing SRF,
unc-120, and together they redundantly participate in the determination of the body wall musculature (Fukushige et al., 2006).
An additional factor involved in the BMW specification is the zinc
finger protein FOZI-1 that works redundantly with the hlh-1 gene
(Amin et al., 2007). The gonadal muscle specification and diversification occurs due to a Notch (LIN-12) pathway (Hale et al., 2014).
Moreover, the C.elegans genome contains a single SoxC gene (sem2), which is necessary for embryonic vulval myoblast proliferation
and specification (Tian et al., 2011). Finally, the Twist ortholog of
C. elegans, hlh-8, plays a critical role in the formation of vulval and
enteric muscles (Corsi et al., 2000).
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In ascidian tailbuds the primary body muscles develop
autonomously under the control of the asymmetrically localized
maternal determinant Macho-1 (Nishida and Sawada, 2001) and its
target T-box gene Tbx6 (Yagi et al., 2005) that suppresses Twist-like
expression and subsequent mesenchyme fate induction (Kumano
et al., 2014). The secondary muscles develop conditionally in
response to FGF, Notch and Wnt signals (in Ciona) (Hudson and
Yasuo, 2006; Imai et al., 2002; Tokuoka et al., 2007). Both primary
and secondary muscle differentiation needs Ci-MRF (or AMD1 in
Halocynthia), the only MRF factor that ascidians possess and that is
equally related by sequence to the four vertebrate MRFs (Meedel
et al., 2007; Satoh et al., 1996). Finally, the heart precursor cells are
induced by an FGF signal (Davidson et al., 2006), which activates
FoxF (Beh et al., 2007) followed by Tbx1/10 and Nk2.5 (Wang et al.,
2013).
Sea urchin non-skeletogenic mesoderm specification is regulated by a Notch signal (Materna et al., 2013). Further myogenesis is
triggered by an FGF signal and is governed by sequential activation
and interplay of four Fox family factors: FoxY (sea urchin specific
Fox member), FoxC, FoxF and FoxL1. FoxY is upstream in the hierarchy and together with FoxC they are activating, among FoxF and
FoxL1 (Andrikou et al., under review), a core set of myogenic regulators such as the SRF Myocardin (Andrikou et al., 2013), the Sox
family member SoxE, the Scratch family member, SchratchX, Six1/2
(Andrikou et al., under review) and the bHLH factor Twist (Andrikou
et al., 2013). Also, Hh signal, known to be involved in sea urchin
myogenesis (Walton et al., 2009), seems to be activated by FoxC
(Andrikou et al., under review). In the sea urchin genome, three
Myod orthologous genes are found, due a lineage duplication event,
Myod1 (SUM1), Myod2 and Myod3 (Andrikou et al., 2013). Myod2 is
the only one expressed in the myoblasts and triggers muscle differentiation together with the T-box gene Tbx6 (Andrikou et al., under
review).
3.2. Stable genetic toolkit and conserved evolution
Conservation is defined by the existence of similar or identical sequences that occur across species (orthologous sequences),
which indicates that a particular sequence is retained during evolutionary time despite speciation. Extremely conserved sequences
are likely to have a very important functional role, since generally,
they seem to evolve more slowly than the ‘less important’ ones and
possess higher domain architecture conservation (Forslund et al.,
2011). This principle of evolutionary stability of the ‘more important’ sequences can be supported by the long-term retention of
several family groups of transcription factors that are known to act
during development. These families show an exceptional conservation within their DNA binding domains rather than their overall
structure and besides of a common origin they often share a common function too.
A nice example of that principle is the bHLH MRF group since
the myogenic factors from an echinoderm and a nematode can efficiently activate myogenesis in mammalian cells (Chen et al., 1992;
Venuti et al., 1991). Vertebrates have four MRFs that were derived
by gene duplications from a single ancestral MRF (Atchley et al.,
1994). This is nicely documented in invertebrates, where only a single member of the MyoD/MRF gene family occurs and its myogenic
role is often conserved like in tunicates (Ci-MRF), Drosophila (nautilus) and C.elegans (hlh-1) raising the possibility of an ancestral
MRF-dependent myogenic regulatory network. MyoR is another
bHLH transcription factor often involved in myogenesis and is
known to repress muscle differentiation. In vertebrates, MyoR is
represented by two paralogs, musculin and capsulin, expressed in
proliferating skeletal and visceral myoblasts, respectively.
Musculin and capsulin share a striking sequence identity with
the Drosophila ortholog bHLH54F as well as similar functions since
7
bHLH54F is expressed in a subset of somatic and visceral muscle
cells resembling the combined expression patterns of capsulin
and musculin (Ismat et al., 2010). Finally, in vertebrates, the bHLH
regulator Twist represses skeletal myogenesis but seems to have
a positive regulation on the cranial mesoderm development by
maintaining the mesenchyme architecture and the progenitor
state of the mesoderm (Bildsoe et al., 2013). In sea urchins, Twist
is reported to have a positive regulation on muscle formation (Wu
et al., 2008) but it is also expressed in other mesodermal tissues
(Andrikou et al., 2013). In ascidians Twist is reported to inhibit
myogenesis and promote instead mesenchyme fate induction.
Finally, in both Drosophila and C. elegans, the only protostomes for
which muscle formation has been studied in great detail at the
molecular level, Twist is a myogenic activator. This contrasting
activating and repressing activities of Twist in myogenesis in different species is probably due to its interaction with different bHLH
binding partners depending on the cellular context (Castanon and
Baylies, 2002).
One more large gene family with members often involved in the
somatic and visceral mesoderm differentiation is the Fox family of
transcription factors. FoxF and FoxC orthologs are highly conserved
classes among diverse metazoans and they are clustered in both
protostomes and deuterostomes together with FoxQ and FoxL genes
(Mazet et al., 2006; Shimeld et al., 2010). This cluster was probably
ancestrally expressed in the developing endo-mesodermal derivatives; given that all these Fox genes show conserved expression in
developing endo-mesodermal tissues and play roles in mesoderm
differentiation and muscle development. Moreover, a sequentially
activation and an overlapping domain is reported in many cases,
as in vertebrates where FoxC genes mark the dorsal mesoderm
and derivatives, while the FoxF genes mark the lateral mesoderm
and derivatives, as well as in sea urchins, where during muscle
specification an inter-regulation among FoxC and FoxF is observed.
In a similar fashion, in Drosophila, the FoxC ortholog Crocodile is
required for the formation of dorsal pharyngeal muscles (Hacker
et al., 1995) whilst Biniou (FoxF ortholog) is patterning the visceral musculature. Also, the two FoxC (FoxC1 and FoxC2) genes
identified in jawless vertebrates seem to have a conserved expression pattern in paraxial and intermediate mesoderm (Wotton and
Shimeld, 2011). Another point of similarity is the regulatory feedback loop between BMP family signals and the FoxF orthologs.
For instance, in Drosophila, Dpp (BMP4 relative) regulates Biniou
(Staehling-Hampton et al., 1994) and in vertebrates FoxF1 seems
to be downstream of a BMP4 signaling (Tseng et al., 2004). Moreover, a specific enhancer of Dpp is a direct target of Biniou (Zaffran
et al., 2001) while BMP4 expression in the vertebrate lateral plate
requires FoxF1 (Ormestad et al., 2006). Finally, FoxF orthologs
are reported to be upstream of capsulin (in vertebrates), HLH547
(in Drosophila) (Jakobsen et al., 2007) and MyoR2 in sea urchin
(Andrikou et al., under review).
Sox proteins, another well-known ancient gene family of transcription factors, possess some well-characterized subfamilies with
important conserved roles in myogenesis. For instance, the vertebrate Sox8 and Sox9, members of the SoxE subgroup, are reported
to be strongly expressed during myogenesis and act as specific
negative regulators of myogenic differentiation. In the sea urchin,
we witness a similar role of SoxE, being transiently expressed in
the specified myoblasts before muscle differentiation. Finally, in
C.elegans the SoxC ortholog sem-2 is involved in vulval muscle precursor specification.
The homeodomain-containing Nkx family is another example
of functional conservation during myogenesis. In vertebrates, the
Nkx. 5 and Nkx2.3 transcription factors play pivotal roles in cardiogenesis and the same is seen in the tunicate Ciona, where Ci-Nkx
is one of the basic components of the cardiogenic regulatory network. Moreover, in Drosophila Tinman and Bagpipe are involved in
8
C. Andrikou, M.I. Arnone / Zoologischer Anzeiger 256 (2015) 2–13
visceral muscle specification and in C. elegans ceh-51 is establishing
the muscle fate of the mesodermal lineage.
A family of transcription factors that, together with other
genes, shows a surprising conservation in several developmental processes, including the myogenic one, is the one of
sine oculis (Six). A striking example is the conservation of the
Pax–Six–Eya–Dachshund network that was originally identified for
its role in eye development, but which is also instrumental in myogenesis, nephrogenesis, and in the development of other organs
(Kawakami et al., 2000). The Six/Eya family consists of Six1/2, Six3/6
and Six4/5 subfamilies. In deuterostomes, both Six1/2 and Six1/4
members and their Eya cofactors are involved in the formation of
the myogenic lineage. In Drosophila, unlike vertebrates, only Six4
and its cofactor Eya are involved in the formation of the myogenic lineage, while the ortholog of Six1 is a central regulator of eye
development (Pignoni et al., 1997), a probably ancestral function of
this gene since its vertebrate ortholog is also expressed in the eye
(Hanson, 2001). The Six-Eya transcription complex works in parallel with Pax3 and Pax7 factors in vertebrates. In the same fashion,
the Drosophila ortholog Poxm is also collaborating with the Six-Eya
complex for the specification of the ventral and lateral mesoderm.
Finally, the Pax3/7 ortholog in Parhyale hawaiensis is expressed in
muscle satellite cells and shares with vertebrates a conserved role
in muscle regeneration (Konstantinides and Averof, 2014).
The MADS box transcription factor group includes members
known to be evolutionary conserved myogenic factors like Mef2
and Myocardin. Mef2 genes are characterized from the existence
of various alternatively spliced isoforms, and each of them is usually differentially expressed in various tissues, such as neurons
and muscles (Potthoff and Olson, 2007). Both muscle-specific and
neuron-specific forms appear to be present already in the sea
anemone, suggesting that the involvement of alternative splice
variants of Mef2 in endomesoderm and neuron differentiation predates the split between diploblasts and triploblasts (Genikhovich
and Technau, 2011). While Mef2 function is restricted to muscle
development in Drosophila, the four vertebrate orthologs have multiple roles, including neural crest and bone development (Potthoff
and Olson, 2007). Likewise, in C. elegans Mef2 is only expressed
in neurons and, in sea urchins, it is expressed in the neurogenic
ectoderm and in non-myogenic mesoderm, but not in myogenic
domains (Andrikou et al., 2013). However, the related MADS box
factor, SRF (Myocardin) seems to be involved in the determination of both C. elegans body wall muscle and sea urchin esophageal
muscles. Myocardin is also the major regulator of smooth muscle
development in vertebrates indicating that when a (myogenic) regulator is identified in one model organism, this gene (or a related
family member) is very likely to be involved in some aspects of the
same process in another species.
Finally, the recruitment of the same signaling molecules (Wnt
FGF, Hh, Notch) in myogenesis is evident. Therefore, even if they
show diverse ways of action, their repetitive recruitment in the
myogenic regulatory cascade represents an additional strong element of conservation.
A summary of all the known transcription factors involved in
myogenesis and conserved in protostomes, invertebrate deuterostomes and vertebrates is displayed in Fig. 3.
3.3. Modification of preexisting genetic repertoire and divergent
evolution
The sharing of the genetic regulatory apparatus is important
for the generation of divergence. Ancient regulatory genes or circuits provide a substrate from which a new function can develop
with duplication events. Gene duplication modes differ in contribution to genetic novelty and redundancy: the genes resulting from
whole genome and tandem duplications diverge usually slowest in
both coding sequences and gene expression and contribute most
to genetic redundancy, while other duplication mechanisms such
as proximal and transposed (DNA based or retro- transposition)
duplication contribute more to evolutionary novelty (Magadum
et al., 2013; Wang et al., 2011). Evolutionary novelties emerge
from three main principles: sub- and neo-functionalization that
contribute to the retention of duplicated genes by providing them
with complementary or new functions, respectively, and nonfunctionalization in which one copy is lost and the situation reverts
to its pre-duplication state (Force et al., 1999). In the light of
neo-functionalization, evolutionary novelties can also arise by the
emergence of species-specific (orphan) genes (Tautz and DomazetLoso, 2011).
Examples of neo- and sub-functionalization are nicely documented in the case of Fox family genes. Fox proteins are
subdivided in groups and despite the similarity in their DNAbinding domains, all various Fox proteins have evolved distinct
roles due to the expansion of the family through duplication events
which allowed them to acquire specialized functions (Haldar et al.,
2008). For instance, the sea urchin-specific Fox gene, FoxY, previously described as FoxC-like (Ransick et al., 2002), is a clear
example of neo-functionalization: in this case, the duplication of
Fox family within the sea urchin genome resulted in an evolutionary novelty and in the establishment of a FoxC duplicated gene
(FoxY) in both the genetic cascade of myoblast specification and
the small micromere derivatives (Andrikou et al., 2013; Song and
Wessel, 2012). A similar situation is seen in C.elegans where the
FoxF ortholog let-381 has been coopted to function in the specification of the non-muscle coelomocytes (Amin et al., 2010). Finally,
a nice example of sub-functionalization is documented in vertebrates, where the FoxF2 gene has a broader role compared to FoxF1
with the additional domains of expression to include the central
nervous system, eye, ear, and limb buds (Aitola et al., 2000).
Another example of cooption to a different mesodermal lineage is the case of Six1/2 both in sea urchins, where two Six1/2
isoforms exist, one involved in muscle specification (Andrikou
et al., under review) and another which is required for pigment
cell commitment (Ransick and Davidson, 2012), and in C. elegans,
where the Six1/2 orthologous gene is necessary for both non-muscle
coelomocytes (Amin et al., 2009) and body wall muscle specification.
Another case of multiple role acquisition is seen in the Sox gene
family. Members of the SoxE subfamily have important conserved
roles in muscle specification. However, they also have conserved
additional roles, such as involvement in sexually dimorphic gonadal
development. For instance, in vertebrates, Sox9 and Sox10 genes
are reported to play a role in spermatogenesis (Morais da Silva
et al., 1996) and in Drosophila the ortholog Sox100B is required for
somatic testis differentiation (Nanda et al., 2009). A similar situation is found in sea urchins, where SoxE is expressed during the
gonadal development of the adult (Juliano et al., 2006).
The molecular evolution of a family by duplication and divergence is often reflected in the frequent linkages of the family
members. For example, a tandem duplication event that gives rise
to at least two identical sequences (paralogs), one following the
other in a chromosome segment, indicates recent duplication. After
the duplication, mutations can cause divergences that give the individual members of the family new functions. This is the case of the
MyoD family in the sea urchin where, as already mentioned, SUM1
and MyoD2 are found one next to the other in the genome. This specific duplication event (within echinoids) resulted in two sequences
with a high nucleotide similarity but a quite dissimilar aminoacidic
one. At the protein level, the conservation is seen only in the bHLH
domain and not in the domains outside it, which coincides with the
general principle of the ‘more important’ sequence stability. This
transformation of the protein structure is explaining the switch in
C. Andrikou, M.I. Arnone / Zoologischer Anzeiger 256 (2015) 2–13
the function of SUM1, which is not part of the myogenic but has
been coopted as a regulatory element of the skeletogenic network
(Andrikou et al., 2013).
The integration of species-specific or orphan genes within
developmental regulatory cascades is another mean of evolutionary divergence and is repeatedly seen in the animal kingdom.
For instance, in sea urchins, a species specific Fox gene, FoxY, is
upstream of the myogenic network. In ascidians a key myogenic
factor that plays an important role in the primary muscle cell lineage specification, Macho-1, is a specific maternal factor of that
phylum. Also, in C.elegans, a unique transcription factor, FOZI-1,
functions in the M lineage for the proper myoblast specification of
body wall muscles. Moreover, Him a Drosophilid specific gene is
necessary for proper muscle differentiation.
Finally, non-functionalization created by gene loss events is also
contributing in alterations of genetic networks, with either the
ancestral role of the lost genes to be taken over by other factors
during evolutionary time or the interconnectivity of the preexisting regulatory cascade to be modified. Such an example is the case
of Pax3/7 that is absent from the sea urchin genome and therefore
has lost its conserved role as regulator of myogenesis.
4. Too many ways to make a muscle: teaching old genes
new tricks
Although the many similarities and differences among the
genetic landscapes found of the different animals so far analyzed
highlight a common muscle patterning, much more is needed
to make a proper developmental comparison: that one involves
not only the function of the actual genes but also their cisregulatory elements and circuits that independently evolved in
ancestral organisms. Thus, the nature of the evolutionary alterations that arises from the regulatory changes depends on the
hierarchical position of the change within a GRN and the reorganizations seen in different developmental GRNs that drive similar
9
processes. Understanding the evolutionary changes through the
logics of GRN architecture alterations and the comparison of them
in different organisms, represents one new type of study in the EvoDevo field named ‘synthetic experimental evolution’ (Davidson and
Erwin, 2009). This approach enable us to understand the evolutionary changes in animal morphology and body plan design by
simply indicating that the mechanistic foundation of the major
morphological changes lies in alterations in the GRN architecture.
However, despite their importance for understanding molecular
evolution of developmental processes, there is little direct comparison of the GRN architecture between distinct animals available
in the literature. In this section, an attempt is made to apply this
type of comparison using the up to date available data on GRN
driving myoblast specification in a vertebrate (mouse), a tunicate (ascidian), an echinoderm (sea urchin), an insect (fruit fly)
and a nematode (C. elegans), focusing mostly on the modules that
form the network and in the hierarchical position of the key factors reported so far. The outcome of this comparative analysis is
reported in Fig. 4 and described in detail in the following paragraphs.
GRNs are hierarchical and modular and their modular subcircuits can evolve at different rates and in different ways. For
instance, although the MRFs have a conserved myogenic role in
all organisms reported, their hierarchical position in the GRN is
different among the various organisms. In protostomes, MyoD is
not the main MRF since other genes appear at the top of the myogenic GRN hierarchy. Also, in sea urchins, from the three paralogs
found in the genome, only one (Myod2) proved to be an essential
MRF. We conclude that an MRF-dependent myogenic regulatory
network probably existed in the common ancestor of protostomes
and deuterostomes and early in the protostome lineage other genes
took over (e.g., Twist in flies), resulting in a less important role of
MyoD, while in the vertebrates this network expanded. The multiplication of MRFs in the vertebrate lineage highlights the need of a
multilevel regulation where the increase of complexity is accom-
Fig. 3. Orthologs of transcription factors (TFs) with conserved myogenic functions.(For interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
Table summarizing orthologs of several TF families that are known to have conserved functional roles in myogenesis in protostomes, invertebrate deuterostomes and vertebrates. In red are the factors whose myogenic function is documented. Sp, Strongylocentrotus purpuratus; Dm, Drosophila melanogaster; Ci, Ciona intestinalis; Ce, Caenorhabditis
elegans; Ph, Parhyale hawaiensis.
10
C. Andrikou, M.I. Arnone / Zoologischer Anzeiger 256 (2015) 2–13
Fig. 4. Comparative diagrams of myogenic GRNs between different organisms. (For interpretation of the references to color in this figure legend, the reader is referred to the
web version of this article.)
The core myogenic regulatory circuits in mouse, ascidian, sea urchin, fly and nematode embryos. Genes are represented as horizontal thick lines and their names are reported
below them. Arrows represent positive regulation and bars represent repression. Full lines show direct/indirect transcriptional regulation; dashed lines represent occurrence
of signaling events. In dark blue are components of Notch, in dark ochre of Wnt in dark brown of Hh and in red of FGF signaling pathways. In salmon pink are the myogenic
regulatory factors (MRFs), in dark green Fox family members, in light green Six-Eya proteins, in olive green Pax proteins, in orange MADS/SRF transcription factors, in light
blue are the Twist factors, in purple Sox family proteins, in light brown Nkx members, in light purple MyoR factors, in aquamarine T-box factors and in black are regulators
from other protein families. All five GRNs have been reconstructed using the program Biotapestry (www.biotapestry.org).
panied by the expansion and redundancy of the relevant regulatory
systems. In contrast, in less complex systems (such as those of most
invertebrates) a shallower regulation system can be provided by a
single MRF.
Other members of the myogenic regulatory network that appear
to switch hierarchical positions and roles are the Six/Eya transcription factors. These genes can be regarded as differentiation drivers
since they are often cross-regulated with other transcription factors
of the same tier of the myogenic GRN (e.g., Pax genes) and they provide multiple inputs into the downstream differentiation batteries.
In vertebrates, they are seen in the apex of the myogenic regulatory cascade; however, this is not the case in sea urchin, where they
occupy an intermediate position of the network topology. Also, in
both Drosophila and in C. elegans, Six and Eya orthologs act in the
periphery of the GRN and downstream of the Myod ortholog, the
opposite of what is seen in vertebrates, suggesting that the position
of their module within the myogenic regulatory network is evolving
quickly. Finally, individual vertebrate mutants of the Six-Eya system
appear to have milder defects, but the combined loss of function
of both genes synergistically aggravates the observed phenotype
(Grifone et al., 2007), highlighting once more the redundancy that
characterizes the vertebrate myogenesis.
Pax3/7 genes also occupy variable positions and act in redundancy within the myogenic network topology. In vertebrates they
are seen at the apex of the regulatory cascade: Pax3 gene is
genetically upstream of Myod and runs in parallel with Myf5
since Pax3:Myf5 mutants are devoid of all body muscles and lack
expression of Myod and all other downstream myogenic factors
(Tajbakhsh and Cossu, 1997). Moreover, both genes are able to compensate partially for each other during embryonic myogenesis since
muscle formation is more defective in Pax3:Pax7 double-knockout
embryos (Relaix et al., 2005). Single mutants of the Pax ortholog
Poxm, in Drosophila also display relatively mild effects and act in
parallel with Six/Eya complex but they are downstream of MRFs,
by contrast to vertebrates.
Members of the MADS box genes (e.g., Mef2 and SRFs) are also
alternatively used in the myogenic regulatory network in different
positions indicating that multiple MADS proteins have an ancestral role in regulating myogenesis. In vertebrates and nematodes,
the myogenic MADS box genes are directly regulated by Myod and
function synergistically with MRFs whilst in flies the Mef2 ortholog
acts in parallel and downstream of Twist, indicating a conserved
cooperative myogenic role of MADS box family downstream of the
master muscle regulators. However, this module is different in sea
urchins where the SRF Myocardin appears upstream of MRFs and
placed in a higher tier of the myogenic regulatory network.
Factors that are repetitively reused in similar tiers of the myogenic network include Fox family members as well as the bHLH
factor MyoR. Fox genes are placed high in the hierarchy of the
myogenic regulatory networks and commit the visceral/cardiac
muscle fate in most cases, with the exception of C. elegans where
the orthologous FoxF gene is coopted to serve another mesodermal lineage. The bHLH family gene, MyoR is genetically upstream
of the bHLH MRFs since is known to antagonize the MRF activity by
binding to the same DNA sequences. MyoR orthologs are expressed
in proliferating undifferentiated myogenic populations and they
are frequently part of a conserved subcircuit, downstream of FoxF
genes.
Finally, we witness Delta-Notch, FGF, Hh, and Notch signaling
circuits frequent re-deployment in muscle specification. All four
signals are reported to trigger the emergence of muscle lineage progenitors and promote their commitment. They determine which
set of genes will be active/inactive during the establishment of the
myogenic gene pool recruitment. Signaling devices are an example of ‘plug-in’ sub-circuits that can be used at different levels of a
GRN hierarchy since signaling molecules are known to be repeatedly coopted for utilization in diverse developmental contexts. Due
to the different level of complexity given for the reported animals, they are acting as check-points/switches in different positions
within the GRN in a non-conserved way regarding their downstream targets, a known property of ‘plug-in’ sub-circuits.
5. Conclusions and perspectives
This broad interspecies comparison of different myogenic regulatory networks revealed some interesting themes. High level of
conservation of the gene family apparatuses involved in muscle
specification among species is observed. It seems that as transcription factor families expanded, the ancestral myogenic function
may have been preserved in more distant family members, rather
than the homologous genes. For example, in the bHLH family,
Twist maintains more MRF activity in Drosophila than the closer
MyoD homolog, nautilus. This contributes to the robustness of the
system, by providing it with several regulatory alternatives, and
highlights the evolutionary plasticity of developmental GRNs architecture. However, the fact that the same factors are used over and
over in such different animal systems indicates that the modular
components are somehow obliged to keep their myogenic activity
during evolutionary time, supporting the intercalary theory of evolution (Gehring and Ikeo, 1999). According to this theory, different
gene functions were built on the same simple platform (prototype)
existing in the same common ancestor (pluripotency). In a similar
fashion, it was recently shown that not only large genetic networks
tend to be more robust, but also that the sequences adjacent to
such networks can bind more transcription factors, demonstrating that robustness can facilitate evolvability (Payne and Wagner,
C. Andrikou, M.I. Arnone / Zoologischer Anzeiger 256 (2015) 2–13
2014). Therefore, the several developmental pathways evolved by
the recruitment of additional genes (intercalation) in the genetic
cascade due to the increasing complexity of the organisms, resulting in evolutionary novelties.
Conserved sub-circuits and plug-ins are also revealed (e.g., FoxFMyoR) but so far any putative kernel has been identified, probably
due to the low resolution and small number of the available myogenic GRNs. Therefore, more data are needed to shed light on this
important question.
Finally, low level of conservation in the hierarchy of the gene
regulatory network cascade and of the gene interactions per se is
often observed, with orthologous genes occupying different positions in the GRN within different animals (e.g., Six1/2, MyoD) and
novel animal specific genes being placed in the apex of the cascade
(e.g., FoxY in sea urchins, Macho-1 in ascidians, FOZI-1 in nematodes). The hierarchical depth of the examined networks reveals
both cases of deep and shallow GRN structures. In vertebrates for
instance, we are witnessing a deep GRN, composed by dense gene
wiring and characterized by genetic redundancy whilst in smaller,
simpler organisms (e.g., sea urchins) the sequence of regulatory
steps required to complete the myogenic process is shorter. Therefore, the organismic complexity is reflected in the wiring density
and the GRN architecture.
Acknowledgements
This manuscript has been presented on the III International
Congress on Invertebrate Morphology in Berlin 2014 at the Symposium of ‘Development and Morphology of Mesodermal Derivatives’
. We thank the main organizers of the meeting Gerhard Scholtz,
Thomas Stach and Carsten Lüter and the organiser of the Symposium Andreas Hejnol. CA was supported by a fellowship of the
Marie Curie ITN EVONET (project 215,781) and a fellowship of POR
Campania FSE 2007-2013 Project MODO, Model Organism.
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