Hox gene diversification diversification of animals

1
Hox gene diversification ? diversification of
animals

• Hox homologues in everything from sponges to
  humans to fungi and plants (MADS-box genes).
• Therefore, Homeobox genes predate the origin of
  animals.

2
Mapping Hox gene sequences onto a phylogeny
8 Hox genes in ancestor of bilaterally
symmetrical animals
4 clusters 39 Hox genes
3

• Representative arthropods What is the basis of
  their diversity?
• 1 million sp. described maybe 50 million still
  to be named.
• Exoskeleton segmented body (H T A) and
  segmented legs
• Paired appendages on body segments 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
4
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
5
Tetrapod limb
A Devonian lobe-finned fish With a prototypical
tetrapod limb.
Remember that bat wings to whale flippers are
based on this same architecture. Homologous genes
and developmental pathways ? homologous
structures.
6
Tetrapod limbs have a common ground plan Derived
from a shared developmental program
Length of limb Length of time of expression
Mesoderm induces formation of the AER (apical
ectodermal ridge). AER cells signal molecule
cells maintain mitotic activity. ZPA (zone of
polarizing activity) cells secretes a diffusable
molecule. Diffusion gradient provides positional
information to cells. Progress zone grows
distally defining the long axis of the
limb. Fibroblast growth factors proteins 4 and 8
Establish proximal-distal axis Sonic hedghog
gene product anterior-posterior axis. Wnt7a gene
product in dorsal limb bud cells dorsal-ventral
axis
7
Hox genes respond to signals molecules as distal
growth takes place.
8
Homeotic genes and Flower Evolution C. 300,000
sps. of Angiosperms
Four concentric whorls of modified leaves Normal
order sepals, petals, stamens, carpels
9
Arabidopsis thaliana screened for homeotic
mutants.
Class A mutants sepals and petals replaced by
sex organs. Class B mutants middle two whorls
are altered. Class C mutants inner two whorls
are altered.
Combinations of A-C mutant genes Replacement of
sepals, petals, stamens, and carpels by leaflike
structures
10
Flower development model Interactions of protein
products of A, B, and C-class genes produce the
four flower organs.