More evo-devo

Homeotic genes and flower formation c. 300,000 sps. of Angiosperms But Hox gene homologies even found in algae Four concentric whorls of modified leaves Normal order: sepals, petals, stamens, carpels

MADS-box mutants (Hox genes)

Arabidopsis thaliana screened for homeotic mutants. Class A mutants: sepals and petals replaced by carpels and stamens. Class B mutants: petals and stamens replaced by sepals and carpels. 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.

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

Tetrapod limbs have a common ground plan Derived from a shared developmental program 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: proximal-distal axis Sonic hedgehog gene product: anterior-posterior axis. 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.

Unique patterns of Hox gene expression