Allozymic variability in the Iceland scallop Chlamys islandica: geographic variation and lack of growth-heterozygosity correlations

Abstract

Populations of the Iceland scallop Chlamys islandca from the waters off Jan Mayen, Spitsbergen, Bear Island and northern Norway were inveshgated for allelic variation at 6 polymorphic gene loci. The 3 loci encoding for glucose phosphate isomerase (GPI), phosphoglucornutase (PGM) and superoxide dismutase (SOD) substantiated earlier findings of exceptionally high polyrnorphism at these loci in the Iceland scallop, higher than for many other pectinids. Loci encoding for malate dehydrogenase (MDH) and leucine-arninopeptidase (LAP) also exhibited higher polymorphism than is known for other pectinids. The high variability at an increasing number of enzyme coding loci for the subarctic/arctic species appears to contrast with predictions of negative correlations between environmental homogeneity and gene polymorphism. Allele frequencies at Mdh. Pgrn and Sod varied between populations over the geographical range surveyed, giving supportive evidence of at least partial geographic isolation between stocks. Data of the present study do not provide evidence that multiple-locus or single-locus heterozygosity affect longevity of C. islandica. Heterozygosity did not increase with age of the scallops, nor were positive growth-heterozygosity correlations demonstrated, either at singlelocus or multiple-locus heterozygosity. This contrasts with findings for several other bivalves, but is in accordance with observations for other pectinids.

The genetic structure of Pandalus borealis in the Northeast Atlantic determined by RAPD analysis

“…The genetic structure of P. borealis depicted by this work using RAPD markers is comparable with that described by Fevolden (1992) of Chlamys islandica, using allozyme variability in the same areas. C. islandica and P. borealis both have a planktonic larval stage that lasts for 2e3 months (Shumway et al, 1985) allowing large potential for dispersion while the larvae are carried by the currents.…”

Bivalve models of aging and the determination of molluscan lifespans

“…The authors speculate that heterozygous bivalves may be more apt to enter hypometabolic states, but this is not yet backed by experimental evidence. To the contrary, correlation of allozyme heterozygosity with either growth, reproduction, or survival (individual lifespan) is not found in pectinids (Volkaert and Zouros, 1989;Bricelj and Krause, 1992;Fevolden, 1992). This speaks against heterozygote superiority supporting predator avoidance in motile bivalves, and, indeed, from the marine ecologist's point of view, predator avoidance is a behavioural trait and presumably involves a multitude of genetic and physiological traits and cascades.…”

“…The trade-off for this specialization may be found in the relatively short reproductive lifespan of Antarctic bivalves, between late maturation on the one hand, as development of Antarctic bivalve trochophora larvae takes 3-15 times longer than in temperate species, and moderate longevity on the other (Peck et al, 2006). Data on heterozygosity in Arctic and Antarctic invertebrates are limited: the Iceland scallop, Chlamys islandica, is an Arctic and subarctic species with ''exceptional variation at several gene loci" including stress genes like superoxide dismustase and glycolytic genes (Fevolden, 1992). In contrast, three species of liparid fish in Spitsbergen showed extremely low allelic heterozygosity (Fevolden et al, 1989).…”

Microsatellite variation in Australian and Indonesian pearl oyster Pinctada maxima populations

“…This may be the result of either direct effects at specific loci (the over-dominance hypothesis) or indirect effects as a result of linkage between markers and fitness-related traits (the associative over-dominance hypothesis), as reviewed in Zouros & Pogson (1994). Subsequent work has shown considerable variation among species, with some studies showing no relationship between growth or survival and heterozygosity (Volckaert & Zouros 1989, Fevolden 1992) and others showing a strong effect (Krause & Bricelj 1995). Careful studies now available have suggested that positive relationships between heterozygosity and growth vary among annual cohorts and arise from inbreeding effects and associative over-dominance in Spisula ovalis (David et al 1995, 1997a, David & Jarne 1997 and in Crassotrea gigas (Thunberg, 1793) (McGoldrick & Hedgecock 1997).…”