Shared features of leg development in vertebrates and flies. Proteins with similar domains of expression are listed as mouse/fly homologs in the gray box (top to bottom). Directions: D (dorsal), V (ventral), A (anterior), and P (posterior). Differing features ("quirks") are diagrammed in the margins. Black sectors of ovals (above) and black bars (below) denote zones of expression by the indicated proteins. a. Left side of a human embryo with arm and leg buds labeled and somites demarcated along the future spine. The left leg bud is cartooned below as an oval. The bud’s proximal-distal axis extends from the perimeter of the oval to its center (foot icon), with the other axes (A-P and D-V) indicated by arrows. Expression zones of signalling proteins (FGF8 and Shh) are inferred from data in mice. Abbreviations: AER (apical ectodermal ridge), FGF8 (Fibroblast Growth Factor 8), and ZPA (zone of polarizing activity). b. Left side of a fly larva with the second-leg disc enlarged and cartooned below as an oval. The disc's proximal-distal axis extends from the outer edge of the oval to its center (claws), with the other axes denoted by arrows. Abbreviations: Dpp (Decapentaplegic), PC (posterior compartment), and Wg (Wingless). Dpp and Wg domains are quite different in onychophorans. c. Proximal-distal axis of human leg, showing inferred zones of expression of "area code" transcription factors based on studies in mouse embryos. All six Dlx paralogs are expressed in the AER at the growing tip. d. Proximal-distal axis of the fly’s second leg, where expression zones of "area code" (or "gap gene") transcription factors are charted for the pupal period. This tripartite pattern is also seen in the unsegmented legs of velvet worms, but the smaller-scale zonation of the fly tarsus (not shown) is not.
Introduction: cover image
Body axes: figure 2 | figure 3 | figure 4 | figure 5 | figure 6
Nervous system: figure 7 | figure 8
Vision: figure 9 | figure 10 | figure 11 | figure 12 | figure 13
Touch and hearing: figure 14 | figure 15
Smell and taste: figure 16
Limbs: figure 17
Epilogue: figure 18
The Interactive Fly resides on the web server of the Society for Developmental Biology.