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epigenetic control – control of development that
occurs through the products of genes other than the
gene(s) which actually are responsible for formation
of the structure itself.
heterochrony – change due to change in timing
heterotopy – change in the position at which a
character is expressed.(spatial) (exp. on thorax rather
than abdomen)
heterometry – a change in quantity or degree of gene
expression. (exp. two pairs of wings instead of one)
heterotypy- phenotypic change from one type to
another (exp. walking legs to swimming legs)
How many gene changes are needed to account for the diversity of forms
seen in the animal and plant kingdoms?
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New discoveries are providing clues to the answer
Research shows that there are often very few and
small genetic differences between species that
exhibit very different adult forms. (example
humans, apes and chimps)
similar genetic and cellular mechanisms underlie
the development of embryos in species whose
adult forms are very different
3.
4.
It is now possible to identify small genetic
changes which are responsible for large
phenotypic variations
A history of these changes is also being pursued
through phylogenetic analysis
A class of genes that code for proteins that bind to
DNA and regulate the expression of a wide range of
other genes.
 The actual binding capability resides in a particular
location of the regulatory protein called the
homeodomain.
 The homeobox is a nucleotide sequence that codes
for this homeodomain
 The homeobox is very similar in many eukaryotic
organisms and is about 180 base pairs.
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Homeotic gene products provide positional
information in a multicellular embryo.
 They are involved in evolutionary change when....
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New features of multicellular organisms arise
due to manipulation of pre-existing cell
types. For example when…
 the same cells arrive at new locations
(heterotopy) OR
 the same cells are expressed at different times of
development (heterochrony)
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Multicellular organisms need a system for
arranging cells in 3 dimensional space to
assure …
 proper organization of symmetry,
segmentation, and body cavities
 that cleavage and gastrulation occur correctly
 correct differentiation of tissues (such as
nervous, muscle, gut etc.)
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Cells need to be identified based on their location in
relation to …
 other cells
 time of expression
Genes that carry information for this control are called
homeotic loci
 In animals these loci are called
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 HOM loci in invertebrates
 Hox loci in the vertebrates
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Genes collectively
referred to as Hox genes
Study of these genes is a new and hot area of
research
Found in all major animal phyla
organized in gene complexes i.e. they are found
in close proximity to each other on the
chromosome
 Appear to be the result of duplication events
 each taxa surveyed shows unique patterns of
duplication or loss in these loci
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Hox genes have temporal and spatial
colinearity which is unique to HOM and Hox
genes
 Have perfect correlation between the order of genes
along the chromosome and the anterior-posterior
location of their gene products in the embryo.
▪ Genes at the 3’ end are expressed in the head region
and genes at the 5’ end in the posterior part of the
embryo
 Also 3’ genes are expressed earlier in development than
those located toward the 5’ end
 Finally the 3’ end produces greater quantity of product.
 Called spatial, temporal and quantitative colinearity. This
is unique to Hox sequences.
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Hox Genes regulate the location of appendages on
body segments.
Work in embryo to specify the location where appendages
should be located
 Also determine when in embryonic development these
structures will be formed
 Hox genes do not control the actual formation of
appendages
 Other genes control the formation of the actual structure
whether a wing, leg, antenna, etc
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 However, these other genes are thought to be regulated by the
regulatory protein products of Hox loci
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These genes are found in not only segmented
animals but also in…
 plants
 fungi
 roundworms
 other non-segmented animals
 sponges!
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Origin may have accompanied the
development of multicellularity
Predates the development of a differentiated
body axis
Therefore these genes also influence
processes other than the specification of
anterior to posterior cell fates
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Use of phylogenetic mapping has helped
determine which genes in the Hox complex
have been gained or lost at key branching
points in the tree
Each major clade in the bilateral animals is
characterized by a particular suite of Hox
genes
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Examples
 addition of the locus called Abdominal-B is
associated with the evolution of bilateral
animals
 duplication of the entire Hox complex several
times leads to the mouse and other
vertebrates
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In the lower animals (sponges and cnidarians)
there is a correlation between the number of
HOM loci and the complexity of the animal’s
body plan
 The most primitive sponges and Cnidarians have
just 5 loci
 Sea urchins have 10
 mice have 39
This would support the idea that increase and
elaboration of the number of Hox genes helped make
the Cambrian “explosion” possible
 However, in the Bilateria, this trend does not hold up
and there is no real correlation between
morphological complexity and number of Hox loci
 Most of the post-Cambrian diversification is due to
changes in the timing or spatial location of Hox gene
expression rather than in the number of the loci
present
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Initial diversification of the Arthropods occurred
in the Cambrian, with even more diversification
occurring much later.
 Diversification of Arthropods relies heavily on
differentiation of body segments from posterior
to anterior
 This differentiation is due to Hox gene
expression
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Still, all of the arthropod taxa have the same
complement of 9 Hox genes
 Morphological diversification is due to changes in
localized expression of genes rather than the
addition of new HOM loci
 The genes which control morphological
diversification are activated or suppressed by the
action of a large number of HOM gene products
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In turn the Hox genes influence the
expression of a large number of other
genes and developmental processes
Much of the evolutionary diversification of
the arthropods probably occurred as a
result of changes in where Hox genes are
expressed
Fig 19.4 the arthropod groups
Fig 19.5
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Tetrapod limbs are a variation on a theme
Found on birds, mammals, reptiles,
amphibians
First developed to allow mobility on land
and then have undergone enormous
variation
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sister group of tetrapods is the
Panderichthyidae
 lobe-finned large predators in shallow fresh water
habitats
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There are structural homologies between the
groups
Fig 17.7 Member of the Panderichthyidae is
the recently discovered Tiktaalik roseae
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All tetrapods show the same basic features of limb
development
 A bud forms from mesodermal cells Fig 19.8a
 At the tip of the bud is the AER (apical
ectodermal ridge)
▪ AER secretes molecules that keep cells in an
undifferentiated state, this area is called the progress
zone
▪ The progress zone grows outward and defines the long
axis of limb development
▪ Fig 19.8 b and c
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At the base of the bud is a group of cells, ZPA
(zone of polarizing activity).
 molecules secreted from the ZPA diffuse into the
surrounding tissue and establish a gradient that
supplies positional information to cells in the
structure
 the concentration of the molecule is critical and
controls the timing and the 3-dimensional spacing
of cell types
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Spatial dimensions are defined in relation to
the main body orientation
 From anterior to posterior
 dorsal to ventral
(thumb to little finger)
(back of hand to palm)
 proximal to distal (arm to fingertips)
The concentrations of the molecules coming from the
AER and ZPA coordinate the system and tell the cells
where they are in 4 dimensional space ( time plus 3
dimensional space)
 The molecules that control the 3 spatial coordinates are
known (shh, Wnt7a, and Fgf-2 ) the ones that control the
timing (temporal element) are not yet known
 Hox genes are responsible for telling the cells where
they are along the limb and control development of
limb parts
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Homologous genes and developmental
pathways underlie the structural homology of
tetrapod limbs
Adaptive changes could be due to changes in
the timing of or level of expression of
pattern-forming genes (shh, Fgf-2 or Wnt7a)
or the Hox complex
In fact it has been demonstrated that
tetrapods have gained hands and feet as a
result of a change in timing and location of
homeotic gene expression
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Lecture 3 chapter 28 36 min  50 min
Lecture 4 chapter 10 12 min  24 min
Made the transition to land in the Silurian
there have been four major radiations
Rhyniophyta –first land plants
Ferns –first vascular tissue for conducting water
Early seed plants without flowers – seeds, pollen and
spores freed them from their need for water during
reproduction
 angiosperms – floral diversity and association with
insects allows wider distribution.
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Figure 19.15 parts of the flower
Flower morphology is dependent on
homeotic genes
 specify which organs appear in which locations
 have DNA-binding regions called MADS
analogous to homeobox of the Hox loci
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Have identified three types of mutants based on
various homeotic mutations
 Class A Fig 19.16 a
 Class B Fig 19.16 b
 Class C Fig 19.16 c
Combinations of mutations lead to the replacement
of floral organs with leaf-like structures Fig 19.17
 Act very much like homeotic mutants in animals
where one limb type can be replaced by another or
limb differentiation can be prevented altogether
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AP1 ( APETALA1) – expressed early in floral
development controls formation of outer two
whorls, sepals and petals. (Class A)
Failure of this gene
leads to no sepals or
petals and a class A
mutant
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AP3 (APETALA3) – expressed later and
involved in middle whorls of the flower bud.
(Class B)
Failure of this
gene leads to
lack of stamens
and petals and
a Class B
mutant
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AG (AGAMOUS) – expressed late and
involves the center of the flower bud. (Class
C)
Failure of the AG
gene leads to loss of
stamens and carpels
and a class C mutant
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LFY (LEAFY) is a master control gene on
which the other 3 are dependent
The LFY protein activates AP1, AP3 and AG
expression in the appropriate cells of the
four whorls Fig 19.19b
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AP1 without AG or AP3 induces sepals
AP1 with AP3 induces petals
AP3 with AG induces stamens
AG without AP1 or AP3 induces carpels
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Control of Flower formation could evolve
with a small number of changes in timing or
location of gene expression which could
result in large morphological changes in the
flower
Coevolution with insects may lead to the
rapid selection of some of these changes
LFY and its target genes ( AP1, AP3, AG) are not the genes
which form the flower but seem instead to indicate the
location and timing of floral parts
 In fact LFY, AG and MADS-boxes have been identified in
non-flowering plants such as pines and ferns
 In these other plants the genes involve the formation of
reproductive structures but not flowers
 Like the HOM/Hox genes in animals, the MADS-box genes
of plants may have evolved for some other function and
then been co-opted later for the control of flower
formation
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Though Arthropod limbs and tetrapod limbs
are not normally considered to be
homologous, at a much deeper level in the
developmental process controlled by genes,
they do share a common control mechanism,
the homeotic loci and the developmental
genes such as Distal-less.
Onychophorans
Uniramians
Chelicerates
Trilobites
Crustaceans
Figure 19.8 a pg 737
carpels, stamens, stamens, carpels
sepals, sepals, carpels, carpels
sepals, petals, petals, sepals
Normal
Triple mutant, lacks
expression of AP2,
AP1 and AG loci
Figure 19.3 pg 732