The evolutionary genetics of segmentation

Although a segmented body plan is a common feature shared between members of Chordata, Arthropoda and Annelida, the evolutionary  origins of segmentation are far from easy to decipher. Shared traits  among taxa can either represent an ancestral state, or be the result of  convergent evolution. In the case of segmentation, the story is puzzled by the fact that Chordata, Arthropoda and Annelida are each more closely related to non-segmented animals than they are to each other.

Where segmentation is assumed to be an ancestral state, related non-segmented taxa must have lost this trait at some stage during their evolutionary history. On the other hand, if a segmented body plan is the derived state, this trait must have   evolved separately in taxa sharing the trait. In the former scenario, it would be expected that the genetics and  developmental mechanisms behind segmentation would be conserved between Chordata, Arthropoda and Annelida.   However, in the latter scenario, this would not necessarily be the case.

New research, published in EvoDevo this week may shed further light on this area of ongoing scientific debate. In order to investigate further the evolutionary origins of segmentation in arthropods and annelids, Seaver et al. compared the  molecular mechanisms controlling segment generation across taxa, with the assumption that shared molecular mechanisms reflect a common evolutionary history. They investigated orthologs of Drosophila melanogaster pair-rule genes, runt, paired (Pax3/7) and eve from the polychaete Capitella teleta.

Pair-rule genes are involved in the development of segmentation in arthropod embryos, and by studying the expression of  orthologs to these genes in an annelid counterpart, Seaver et al. were able to infer whether they control a similar mechanism in annelid. Their investigation foundno evidence of pair-ruling patterning as part of the annelid segmentation program. The findings therefore suggest that there may be fundamental differences in how segments form in annelids and arthropods, lending weight to the possibility of a separate evolutionary origin of segmentation between the two groups.

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