| Sea cucumbers (Class: Holothuroidea) are one of six extant classes of the exclusively marine phylum Echinodermata. Unfortunately, these organisms are poorly represented in the fossil record and consequently, the taxonomy and systematics of this class remains problematic. As a preliminary investigation, a molecular phylogeny was constructed focusing on members of the family Cucumariidae from the eastern Pacific. Fourteen hundred nucleotides spanning portions of two mitochondrial genes, encoding the large ribosomal RNA subunit and cytochrome oxidase subunit one, were sequenced from 15 species, representing six families from two orders. A number of important phylogenetic issues were resolved, particularly the relationships among brooding species.;Following this phylogenetic assessment, an analysis of population structure in two species of sea cucumber as a function of their mode of development was undertaken, based on four hundred nucleotides of mitochondrial sequence with presumed homology to the replication control region. Cucumaria pseudocurata exhibits a brooding form of development while Cucumaria miniata passes through a pelagic nonfeeding larval stage lasting up to 14 days. These two species occupy extensive, overlapping ranges along the west coast of North America and populations were sampled between Pescadero Point, California and Juneau, Alaska. Significant geographic structuring of populations was evident in C. pseudocurata indicating limited dispersal. Despite the general trend of strong geographic structuring of populations, there is evidence of significant gene flow between adjacent sites separated by as little as four kilometres of ocean or as much as one hundred kilometres along the coast. The most significant genetic disjunction observed separated Alaskan and Queen Charlotte Island (Haida Gwaii) samples from Vancouver Island and all sample sites further south. The northern population samples were found to contain equal or greater genetic variability compared to many southern sites. The most plausible explanation of this genetic disjunction is therefore survival in northern glacial refugia in addition to the persistence of populations south of the furthest extent of the Cordilleran ice sheet. Presumably, these surviving populations expanded to their present apparently continuous distribution of these externally indistinguishable types following the retreat of the ice sheet. The degree of sequence divergence suggests that this separation existed well prior to the last glaciation and may have been maintained for several million years.;Conversely C. miniata population samples exhibited a panmictic structure over their entire range indicating high levels of gene flow despite a relatively short dispersal phase. These results suggest that mode of development and life history traits have had a profound influence on gene flow.;Finally, mtDNA sequence from C. miniata and C. pseudocurata has revealed a novel tRNA gene arrangement compared to other echinoderms. Analysis of mtDNA indicates this novel arrangement has arisen by a nontandem duplication of an ancestral tRNA gene cluster including putative control elements. Gene sequence analysis indicated potential utility of the tRNAs as a phylogenetic marker at deeper levels than other mitochondrial genes which, in conjunction with tRNA gene order rearrangements, make this a region of promising utility in studies of molecular evolution. |
