The concept of homology arose from classical studies of comprative morphology, and took on a new signficance with the advent of evolutionary theory. It is currentlyl undergoing antoher metamorphosis: many developmental geneticists now dfine homology as shared patterns of gene expression. However, this ne usage conflaes difinition with criteri, and fails to recognize the meaninful asignments of homology must speify a biologcal level. We argue the although developmental genetic data can help identify homologus structures. they are niether necessary nor sufficient, (...) and do not in any case jutify a new definition of homology. (shrink)

The evolution of development can be made accessible to study by exploiting closely related species that exhibit distinct ontogenies. The direct‐developing sea urchin Heliocidaris erythrogramma is closely related to indirect‐developing sea urchins that develop via a feeding larval stage. Superficial consideration would suggest that simple heterochronies resulting in loss of larval features and acceleration of adult features could explain the substitution of direct for indirect development. However, our experiments show that early development has in fact been extensively remodeled, with modified (...) localization of maternal determinants coupled with dissociation of cell cleavage from axis formation resulting in novel patterns of cell lineage differentiation and fate map. Gene expression has undergone concomitant changes. (shrink)

There is renewed interest in how the different body plans of extant phyla are related. This question has traditionally been addressed by comparisons between vertebrates and Drosophila. Fortunately, there is now increasing emphasis on animals representing other phyla. Pentamerally symmetric echinoderms are a bilaterian metazoan phylum whose members exhibit secondarily derived radial symmetry. Precisely how their radially symmetric body plan originated from a bilaterally symmetric ancestor is unkown, however, two recent papers address this subject. Peterson et al.(1) propose a hypothesis (...) on evolution of the anteroposterior axis in echinoderms, and Arenas-Mena et al.(2) examine expression of five posterior Hox genes during development of the adult sea urchin. BioEssays 23:211–214, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

Observations of a sea urchin larvae show that most species adopt one of two life history strategies. One strategy is to make numerous small eggs, which develop into a larva with a required feeding period in the water column before metamorphosis. In contrast, the second strategy is to make fewer large eggs with a larva that does not feed, which reduces the time to metamorphosis and thus the time spent in the water column. The larvae associated with each strategy have (...) distinct morphologies and developmental processes that reflect their feeding requirements, so that those that feed exhibit indirect development with a complex larva, and those that do not feed form a morphologically simplified larva and exhibit direct development. Phylogenetic studies show that, in sea urchins, a feeding larva, the pluteus, is the ancestral form and the morphologically simplified direct‐developing larva is derived. The current hypothesis for evolution of the direct‐developing larval form in sea urchins suggests that major developmental changes occur by neutral loss of larval features after the crucial transition to a nonfeeding life history strategy. We present evidence from Clypeaster rosaceus, a sea urchin with a life history intermediate to the two strategies, which indicates that major developmental changes for accelerated development have been selected for in a larva that can still feed and maintains an outward, pluteus morphology. We suggest that transformation of larval form has resulted from strong selection on early initiation and acceleration of adult development. BioEssays 29:566–571, 2007. © 2007 Wiley Periodicals, Inc. (shrink)