Human Hox genes 4 clusters Chr 1: 9 Hox genes Chr 7: 11 Chr 12: 9 Chr 17: 10 8 Hox genes in ancestor of bilaterally symmetrical animals Mapping Hox gene sequences onto a phylogeny to see if gains and losses can explain arthropod diversity 4 clusters 39 Hox genes
Hox cluster of 9 loci for all arthropods Evolutionary diversification of arthropods partly based on sites of Hox gene expression abdA always expressed on ventral side of segment relative To Ubx Ubx and abdA not expressed in posterior segments 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
Tetrapod limb Remember that bat wings to whale flippers are based on this same architecture. Homologous genes and developmental pathways homologous structures. A Devonian lobe-finned fish With a prototypical tetrapod limb.(Tiktaalik) LF fishes: sister group to tetrapods Lungfishes: closest extant group to tetrapods
Tetrapod limbs have a common ground plan Derived from a shared developmental program 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 Length of limb: Length of time of expression
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