Genetic drift

A research consortium from British Columbia and Mexico establish a molecular phylogeny of 16 species of west-coast holothuroidea based on mitochondrial DNA sequences.  With a few exceptions, results support the existing taxonomy, based largely on the morphology of calcareous skin ossicles.  In addition to confirming that Cucumaria pseudocurata and C. curata are separate species, the researchers note that C. lubrica may actually represent 2 species, a subtidal one and an intertidal one (see differences in ossicle morphology in specimens from the Victoria, British Columbia area) and, finally, that brooding has arisen in at least 2 separate lineages within the Family Cucumariidae.  Arndt et al. 1996 Mol Phylogenetics Evol 6 (3): 425.

NOTE  of 5 families represented in the study, F. Cucumariidae is the largest with 9 species.  Of these, 7 (Cucumaria curata, C. lubrica [possibly represented by 2 species], C. pseudocurata, C. vegae, Pseudocnus astigmatus, and P. californicus) exhibit direct development

  black dotResearch study 2

map of western North America showing collection sites for sea cucumbers Cucumaria pseudocurata and C. miniata used in a study of genetic driftMolecular ecologists predict that marine invertebrates with widely disseminating planktonic larvae will have less genetic differentiation because of greater genetic mixing.  This is tested in 2 species of sea cucumbers Cucumaria miniata and C. pseudocurata that have overlapping geographical range from southern Alaska to northern California. The first species C. miniata has a pelagic, non-feeding larva that spend about 2wk drifting before settling and metamorphosing; the second species, C. pseudocurata, broods its embryos until they hatch into crawling juveniles.  The results show, as predicted, significantly greater genetic structure in the brooding species C. pseudocurata than in the species with pelagic larvae C. miniata.

A strong genetic disjunction is apparent for the brooding species C. pseudocurata between Haida Gwai (Queen Charlotte Islands) and Vancouver Island, British Columbia.  A high level of genetic diversity in the northern populations suggests survival in both northern and southern refugia during the Pleistocene Glaciation, some 15-18,000 yrs B.P.  In the illustration on the Right note the differences in ossicle structure between northern haplotypes (more rod- and dumbbell-shaped) and southern haplotypes (larger, perforated plates). 

Although the larval lifespan of C. miniata is relatively short, the authors note that the species is found in areas of moderate to high current flow, has high fecundity, and is distributed from the intertidal region to 200m depth – all potentially contributing factors to good larval dispersion and high gene flow.  The authors conclude that the limited dispersal potential of C. pseudocurata combined with possible survival in refugia during glaciation may have led to differences among populations approaching the level of speciation.  Arndt & Smith 1998 Molec Ecol 7: 1053.

NOTE nucleotide sequencing data from analysis of mitochondrial DNA (mtDNA)

NOTE  at least one such refugium has been identified to have existed through the last glaciation period on the east side of Graham Island in the Queen Charlotte Islands, British Columbia.  Warner et al. 1982 Science 218: 675. The potential expansion of west-coast marine-invertebrate species from post-glaciation refugia is considered elsewhere in the ODYSSEY: LEARN ABOUT LITTORINES: REPRODUCTION & DEVELOPMENT: GENETICS and LEARN ABOUT WHELKS: DISPERSAL, HETEROZYGOSITY, & GLACIAL REFUGIA

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