Evolutionary biology: Insights from the echinoderms

Abstract

The evolutionary diversification of animal body plans has evidently devolved from alterations in the genetic regulatory programmes that control morphogenesis. This realization is now producing a tide of comparative descriptions of transcription-factor expression patterns, recorded at early stages of morphogenesis in various animals. Although these observations are visually compelling, the same is not always true of the accompanying interpretations. These often depend on the assumption that regulatory gene expression, even if called into play at the very beginning of a developmental process, is likely to be dedicated to given morphogenetic functions far downstream, and that such functional linkages have remained immutable throughout evolution. There is much disagreement on this issue, which bears on the meaning of evolutionary homology in a developmental process and, indeed, on the very nature of developmental regulatory evolution. The paper by Lowe and Wray (page718 of this issue) now contributes to this discussion. They describe the expression of three homeodomain transcription factors — distal-less, engrailed and an orthodenticle orthologue — in the larval and juvenile development of sea urchins, starfish, sea cucumbers and brittle stars. These are four different classes of echinoderm.