Analysis of coelom development in the sea urchin Holopneustes purpurescens yielding a deuterostome body plan.

Valerie B Morris

doi:10.1242/bio.015925

Abstract

ABSTRACTAn analysis of early coelom development in the echinoid Holopneustes purpurescens yields a deuterostome body plan that explains the disparity between the pentameral plan of echinoderms and the bilateral plans of chordates and hemichordates, the three major phyla of the monophyletic deuterostomes. The analysis shows an early separation into a medial hydrocoele and lateral coelomic mesoderm with an enteric channel between them before the hydrocoele forms the pentameral plan of five primary podia. The deuterostome body plan thus has a single axial or medial coelom and a pair of lateral coeloms, all surrounding an enteric channel, the gut channel. Applied to the phyla, the medial coelom is the hydrocoele in echinoderms, the notochord in chordates and the proboscis coelom in hemichordates: the lateral coeloms are the coelomic mesoderm in echinoderms, the paraxial mesoderm in chordates and the lateral coeloms in hemichordates. The plan fits frog and chick development and the echinoderm fossil record, and predicts genes involved in coelomogenesis as the source of deuterostome macroevolution.

Highlights

Deuterostomes are a supraphyletic group comprising three major phyla, the echinoderms, the chordates and the hemichordates
The results cover a period starting at the early development of the coeloms from the archenteron, progressing through the later development of the hydrocoele to the formation of the five primary podia and the advancing coelomic mesoderm
Closure of the hydrocoele ring With respect to closure of the hydrocoele ring, the interpretation here is that the AB podial group and the CDE podial group form from different regions of the anterior archenteron, that each has its Conclusion A deuterostome body plan has been derived from an analysis of early coelom development in H. purpurescens that is supported by observations of early coelom development in H. erythrogramma

Summary

Introduction

Deuterostomes are a supraphyletic group comprising three major phyla, the echinoderms, the chordates and the hemichordates. The three major phyla are problematic since the pentameral body plan of echinoderms seems to be quite different from the bilateral body plans of chordates and hemichordates. The fundamental morphological homology between the pentameral and the bilateral body plans is identified in the derivation of a body plan for deuterostomes. The body plan of echinoderms is in general pentamerous, that is to say it is made up of five arms or rays and its structure and evolutionary origin are better understood when treated as a pentameral body plan rather than, as it is often described, a radial plan. The ambulacra are the radial canals and tube feet and the interambulacra are the plates and tissues between them (Hyman, 1955). Mooi et al (1994) referred back to Jackson who in 1912 described each arm or ray as a School of Biological Sciences A12, University of Sydney, New South Wales 2006, Australia

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