Download Evolution and Development

3/6/11
The Goal of Evo-Devo
 
To understand the mechanisms by which
development has evolved, in terms of:
processes (i.e., what new cell or tissue
interactions are responsible for novel morphologies in
certain taxa) ex. Neural crest
  evolutionary processes (e.g., what selection pressures
promoted the evolution of these novel morphologies)
Types of Evidence
 
Developmental genetic data:
  Gene
  developmental
Phylogenetic tree
 
 
 
A hypothesis of the evolutionary
relationships among taxa.
Details the history of descent of groups of
taxa such as species from their common
ancestors
Metazoan Phylogeny
expression patterns
  Qualitative
and Quantitative expression of gene transcripts
or proteins
  Cis-Regulatory Reporter Constructs
Comparative Embryological data
Paleontological data
  Together these data help infer the developmental
genetic origins and histories of morphological
characters.
 
 
Homology – A Phylogenetic Concept
 
Homologous features are those that have been inherited, with more or less modification, from
a common ancestor in which the feature first evolved.
 
 
If they are shared by all members of a group/clade, these homologous structures are called synapomorphies
(shared derived characters)
Homology may be suggested by a combination of similarity in :
 
position
 
structure
Consensus derived from the
amalgamation of different biological
fields (e.g., Paleontology, Molecular
Biology, and Developmental genetics)
1
3/6/11
Biological Homology concept
 
Macroevolution
A feature that is homologous among species at one
level of organization (e.g., phenotypic), may or may
not be homologous at another level (e.g., genetic or
developmental).
organisms are constructed from a “genetic
toolkit”.
  The toolkit consists of:
 
Refers to the evolution of significant phenotypic
changes
  Great
enough to place the changed lineage and its
descendants in a distinct genus or higher taxon.
The Toolkit & its Evolution
 
The “genetic toolkit”
 
Toolkit Evolution
  Transcription
  Duplication
  Multicellular
  Regulatory
  Toolkit
 
genes  Gene regulatory networks
 
“tinkering” provides the basis of
  Morphological
 
Modularity
 
PARALOGS
 
genes within a species are called paralogues.
Their sequence similarities are due to descent from a common
ancestor and are not the result of convergence for a particular
function.
  Homologous
evolution (Macroevolution)
Hox genes
genes between species are called orthologues.
Example: Hox gene cluster
Pitx1 expression in Three-spine Sticklebacks and
evolutionary pelvic reduction
Modular units allow certain parts of the body to change
without interfering with the functions of other parts.
They are discrete and interacting modules.
Types of modularity:
  Anatomical
(e.g., imaginal discs, morphogenetic fields,
parasegments, vertebrate organ rudiments)
  Cellular (e.g., signal transduction pathways)
  DNA (e.g., enhancers [cis-reg. modules])
 
Carroll et al. 2001
and divergence  Expands Toolkit!
  Homologous
These changes are made possible by evolving utility
of the genetic toolkit
factors & Signaling factors
Example: Pitx1 expression in the Three-spine
Stickleback (Gasterosteus aculeatus)
ORTHOLOGS
Shapiro et al. 2004
2
3/6/11
Cis-reg construct and function
•  A reporter construct consists of:
 
•  regulatory DNA of interest including promoter
•  reporter gene - encodes protein (e.g., GFP) whose expression can be
easily visualized under a microscope
Enhancer
Heterotopy – Evolutionary changes in the
location of development
Mechanisms of Macroevolution
Duck feet and Bat wings: Extra skin membrane evolution
Toolkit “tinkering”
 
Merino et al. 1999
  Mutations
Chicken
that affect regulatory regions
  Heterotopy
- changes in location
- changes in time
  Heterometry - change in amount
  Heterochrony
Promoter
Duck
  Mutations
that affect coding regions
  Heterotypy
Bmp4
Gremlin
Apoptosis
- change in kind
Weatherbee et al. 2006
Alberts et al. 2002.
Heterochrony – Evolutionary changes in the
timing of development
Heterochrony: the paedomorphic Axolotl
(Ambystoma mexicanum)
Tiger Salamander
 
The retention of juvenile characteristics
into adulthood (e.g., tail fin and gills)
Heterometry - Evolutionary changes in the
amount during development
(Ambystoma tigrinum)
Wildtype
Terrestrial Adult
Aquatic Adult
Example: Salamanders exhibit heterochronic changes by which the larval
stage is either retained or truncated are caused by genetic changes in the
ability to induce or respond to hormones initiating metamorphosis.
Lets consider the Axolotle!
Bmp4
overexpressed
Bmp4
inhibited
The TRH Cascade
Thyroxin-releasing hormone (TRH)  Thyroid-stimulating hormone (TSH)  Thyroxin
Thyroxin  METAMORPHOSIS!
BMP4 expression is correlated with the
breadth and depth of the beak!
3
3/6/11
Co-option and the evolution of novel
characters
 
 
Co-Option: Hox-a9-13 genes
Summary
Recruitment (co-option) – refer to the evolution of
novel functions for pre-existing genes and
developmental pathways.
Involves the addition or evolution of novel enhancer
elements that allow genes and signaling pathways
to have multiple developmental roles.
 
Evo-Devo
  Allows
ba
fl
hl
ba
fl
hl
ba
au
Hox-a9
Hox-a9
us to understand Homology by uniting
Morphological and Genetic Data
  Provides insight into the developmental underpinings of
the mechanisms of macroevolution
  Highlights the importance of gene regulation (i.e.,
utilizing enhancer modularity) in morphological
evolution.
Hox-a9
4