Meeting Abstract
A central question in evolutionary developmental biology is how the remarkable diversity found among distantly related species could be generated by highly conserved developmental systems. In part this paradox reflects our ignorance about the potential for developmental systems to generate novel variation. Limbs are an attractive system for addressing this question for a number of reasons. First, diversity in limb sizes, proportions and associated functions is in contrast to the deep homology in genetic patterning mechanisms shared from fish to birds to mice. Second, limbs are both easily quantified and frequent in the literature, enabling robust comparisons of potential to observed diversity. Third, limbs are serially homologous, enabling comparisons both between limbs (i.e., fore vs. hind) and species. Here I present results from a rich dataset representing >1,400 mammal, avian, and reptile taxa, focusing on patterns of both within and between limb variation. I demonstrate that: (1) within limb proportions are heavily biased such that proximal and distal segments function as tradeoffs while the middle segment is more conservative, a signal that is evident from early in development, and (2) functional dissociation of fore and hind limb is linked to increased diversity in between limb proportions. These under-appreciated patterns of diversity provide crucial clues about the developmental timing and mechanism by which species-specific limb variation is plausibly generated. One possibility is that early patterning contributes to amniote limb diversity via activation-inhibition dynamics, in which case a simple “design rule” found in a range of structures could explain both the generation and limitation of observed phenotypic outcomes. However, whether such a universal morphogenetic rule exists in limbs requires further validation in experimental systems.
