EvoDevo: Evolution and Development

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Evo-Devo Evolution and Development
Dll expression
En/Inv expression
Eyespots on a butterfly wing
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Homeotic Genes and Animal Body Plans

• Multicellular animals develop in four dimensions.
• Each cell has to know
• 1. spatial where it is relative to other cells
• 2. temporal what is occurring in the
  developmental sequence.
• Homeotic genes (Hox genes) (1)transcription
  factors determine which (2) structural genes are
  activated to produce (3) particular structures.

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Hox genes in Drosophila (body segmentation) Occur
in clusters (gene duplication)
Where genes are expressed

Colinearity 1. Expressed first 2. Anterior to
posterior 3. Greater quantity of transcription
factors
Gene location in hox cluster
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Hox Genes

• 1. Basis of colinear Hox gene expression
• 1. The transcription factor from the 3 end
  required to express all of the upstream genes.
• 2. The transcription factors effectiveness in
  initiating upstream gene expression declines with
  distance.
• 2. Each gene in complex contains a highly
  conserved 180 bp sequence the homeobox.
• Codes for a DNA binding segment (aa sequence) in
  the transcription factor.
• 3. Hox genes provide information on location.
  Mutations in Hox genes result in inappropriate
  structures for that location.

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Hox gene products activates genes responsible for
making a particular structure.
Mutations in Hox genes bx, pbx, and abx
Mutation of Hox gene antp
Wings normally appear on T2 Hox mutations change
identity of T3 to T2 Ancestors of dipteran
flies had 4 wings.
Identity of a head segment changed to that of a
thoracic segment
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Hox gene diversification ? diversification of
animals

• Hox homologues in everything from sponges to
  humans AND fungi and plants.
• Therefore, Hox genes predate the origin of
  animals.

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Mapping Hox gene sequences onto a phylogeny
8 Hox genes in ancestor of bilaterally
symmetrical animals
4 clusters 39 Hox genes
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• Representative arthropods diversity of form and
  function
• 1 million sp. described 50 x still to be named
• Exoskeleton segmented body (H T A)
• Pairs of appendages on each body segment open
  circulatory system

Crustaceans
Hexapods
Myriapods
An onychophoran (velvet worm) Closest living
relative of arthropods 1 pr. unjointed legs on
each of the similar body segments
Chilicerates
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Evolutionary diversification of arthropods partly
based on sites of Hox gene expression
Hox cluster of 9 loci for all arthropods
abdA always expressed on ventral side of segment
Ubx and abdA not expressed in posterior segments
Evolutionary change in where a Hox gene is
expressed
Mutation legless abdominal segments