Mapping Hox gene sequences onto a phylogeny to see if
gains and losses can explain arthropod diversity
8 Hox genes in ancestor
of bilaterally symmetrical
animals
Human Hox genes
4 clusters
Chr 1: 9 Hox genes
Chr 7: 11
Chr 12: 9
Chr 17: 10
4 clusters
39 Hox
genes
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 relative
To Ubx
Evolutionary change in where
a Hox gene (e.g. pb) is expressed
Insect Ubx (has an alanine-rich region)
Supresses abdominal leg development.
Crustacean Ubx (no alanine-rich area) No suppresion legs
Ubx and abdA not expressed
in posterior segments
Tetrapod limb
A Devonian lobe-finned fish
With a prototypical tetrapod
limb.(Tiktaalik)
LF fishes: sister group to tetrapods
Lungfishes: closest extant group to
tetrapods
Remember that bat wings to whale flippers are based on this same architecture.
Homologous genes and developmental pathways  homologous structures.
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).
Diffusion gradients of signal molecules provides positional information to cells.
AER: Signal molecule: maintains mitotic activity of Progress Zone
Progress zone grows distally defining the long axis of the limb.
Fibroblast growth factors proteins 4 and 8: proximal-distal axis
ZPA (zone of polarizing activity)
Sonic hedghog gene product: anterior-posterior axis.
Dorsal surface of limb bud.
Wnt7a gene product: dorsal-ventral axis
Hox genes respond to signals molecules as distal growth takes place.
Homeotic genes and Flower Evolution
C. 300,000 sps. of Angiosperms
Four concentric whorls of modified leaves
Normal order: sepals, petals, stamens, carpels
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
Flower development model
Interactions of protein products of A, B, and C-class genes
produce the four flower organs.
Flower homeotic genes sequenced. All produce transcription factors
with a MADS box (DNA binding region)
The homeotic genes are controlled by a master control gene: LEAFY