The evolution of cell lineage in echinoderms
SYNOPSIS. Metazoan embryos in various phyla and classes often utilize quite different processes to specify cell fates during embryogenesis. These differences have been interpreted either as constraints, necessary for fabricating distinct adult body plans, or as adaptations for particular life history strategies. This paper analyzes the evolution of echinoderm cell lineage within a phylogenetic context as a means of testing these hypotheses. Several features of echinoderm cell lineage are probably over 550 million years old, and have persisted despite extensive transformations in adult morphology. Other features are much more variable evolutionarily, and have changed on many separate occasions. Importantly, even some of the most ancient and conservative features of echinoderm cell lineage can still evolve. These transformations are correlated with a particular life history transformation, the switch from feeding to nonfeeding larvae. The results suggest that adaptation has played a significant role in the evolution of cell lineage in echinoderms: some ancient features have been maintained for functional reasons rather than because of constraints, and some derived features have evolved in response to particular environmental challenges. ©1994 by the American Society of Zoologists.
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