Developmental modularity and the evolutionary diversification of arthropodlimbs

Segmentation is one of the most salient characteristics of arthropods, and differentiation of segments along the body axis is the basis of arthropod d iversification. This article evaluates whether the evolution of segmentatio n involves the differentiation of already independent units, i.e., do segme nts evolve as modules? Because arthropod segmental differentiation is commo nly equated with differential character of appendages, we analyze appendage s by comparing similarities and differences in their development. The compa rison of arthropod limbs, even between species, is a comparison of serially repeated structures. Arthropod limbs are not only reiterated along the bod y axis, but limbs themselves can be viewed as being composed of reiterated parts. The interpretation of such reiterated structures from an evolutionar y viewpoint is far from obvious. One common view is that serial repetition is evidence of a modular organization, i.e., repeated structures with a com mon fundamental identity that develop semi-autonomously and are free to div ersify independently. In this article, we evaluate arthropod limbs from a d evelopmental perspective and ask: are all arthropod limbs patterned using a similar set of mechanisms which would reflect that they all share a generi c coordinate patterning system? Using Drosophila as a basis for comparison, we find that appendage primordia, positioned along the body using segmenta l patterning coordinates, do indeed have elements of common identity. Howev er, we do not find evidence of a single coordinate system shared either bet ween limbs or among limb branches. Data concerning the other diagnostic of developmental modularity-semi-autonomy of development-are not currently ava ilable for sufficient taxa. Nonetheless, some data comparing patterns of mo rphogenesis provide evidence that limbs cannot always be temporally or spat ially decoupled from the development of their neighbors, suggesting that se gment modularity is a derived character. (C) 2001 Wiley-Liss, Inc.