Animals were radially symmetrical before bilaterality occurred:
The development of radial and biradial symmetry: The evolution of bilaterality
Martindale MQ, Henry JQ
AMERICAN ZOOLOGIST
38 (4): 672-684 SEP 1998
Abstract:
Understanding the evolutionary origin of novel metazoan body plans continues to be one of the most sought after answers in biology. Perhaps the most profound change that may have occurred in the Metazoa is the appearance of bilaterally symmetrical forms from a presumably radially symmetrical ancestor. The symmetry properties of bilaterally symmetrical larval and adult metazoans are generally set up during the cleavage period while most "radially" symmetrical cnidarians do not display a stereotyped cleavage program. Ctenophores display biradial symmetry and may represent one intermediate form in the transition to bilateral symmetry, The early development of cnidarians and ctenophores is compared with respect to the timing and mechanisms of axial determination. The origin of the dorsal-ventral axis, and indeed the relationships of the major longitudinal axes, in cnidarians, ctenophores, and bilaterian animals are far from certain. The realization that many of the molecular mechanisms of axial determination are conserved throughout the Bilateria allows one to formulate a set of predictions as to their possible role in the origins of bilaterian ancestors.
Some starfish are symmetrical, some are not!
Developmental regulatory genes and echinoderm evolution
Wray GA, Lowe CJ
SYSTEMATIC BIOLOGY
49 (1): 28-51 MAR 2000
Abstract
Modified interactions among developmental regulatory genes and changes in their expression domains are likely to be an important part of the developmental basis for evolutionary changes in morphology. Although developmental regulatory genes are now being studied in an increasing number of taxa, there has been little attempt to analyze the resulting data within an explicit phylogenetic context. Here we present comparative analyses of expression data from regulatory genes in the phylum Echinodermata, considering the implications for understanding both echinoderm evolution as well as the evolution of regulatory genes in general. Reconstructing the independent evolutionary histories of regulatory genes, their expression domains, their developmental roles, and the structures in which they are expressed reveals a number of distinct evolutionary patterns. A few of these patterns correspond to interpretations common in the literature, whereas others have received little prior mention. Together, the analyses indicate that the evolution of echinoderms involved: (1) the appearance of many apomorphic developmental roles and expression domains, some of which have plesiomorphic bilateral symmetry and others of which have apomorphic radial symmetry or left-right asymmetry; (2) the loss of some developmental roles and expression domains thought to be plesiomorphic for Bilateria; and (3) the retention of some developmental roles thought to be plesiomorphic for Bilateria, although with modification in expression domains. Some of the modifications within the Echinodermata concern adult structures; others, transient larval structures. Some changes apparently appeared early in echinoderm evolution (>450 Ma), whereas others probably happened more recently (
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