Snakes and ladders: the ups and downs of animal segmentation

摘要

Segmentation, the division of the body into repetitive modular subunits or metameres, is ubiquitous throughout the animal kingdom. This morphological motif appeared several times in widely divergent phyla, some without common segmented ancestors (Bateson 1894; Willmer 1990). Segmentation presents challenges to standard evolutionary narratives (Minelli and Fusco 2004), in part because segments are discrete structures, like rungs in a ladder, that are added or subtracted in an all-or-none fashion, and also because large changes in segment number can occur in evolutionary lineages with little sign of intermediate forms. Two recent papers, one on snakes (Gomez et al. 2008) and one on centipedes (Vedel et al. 2008), shed some light on these important questions.