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Evolutionary biology

Insights from the echinoderms

Nature volume 389, pages 679–680 (1997)Cite this article

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 Wray1 (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.

The data of Lowe and Wray support two significant conclusions. First, morphogenesis of the definitive phyletic characteristics of the echinoderm body plan involves (and could depend on) the expression of distal-less, engrailed and orthodenticle in ways that are specific to the echinoderms. The main features of the modern adult echinoderm body plan are: fivefold radial symmetry (the sea cucumber is only secondarily bilateral); a calcite endoskeleton (test, spicules and/or spines); and a water vascular system, of which the tube feet (podia) are external locomotory projections. The authors found that all three of the genes are transcribed in fivefold radially symmetrical patterns during morphogenesis; that distal-less and orthodenticle are expressed during the early development of the podia; and, in brittle stars, that engrailed is specifically expressed in rows of ectodermal cells that overlie the gaps between the endoskeletal calcite plates. Thus, in echinoderms, these regulatory genes play some developmental roles that are distinct from those that they execute in either arthropods or chordates.

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Figure 1: Origin of the main elements of the adult echinoid body plan in the imaginal rudiment.

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  1. the Division of Biology, California Institute of Technology, 156-29, Pasadena, 91125, California, USA

    Eric H. Davidson

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Davidson, E. Insights from the echinoderms. Nature 389, 679–680 (1997). https://doi.org/10.1038/39484

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