Genetic studies of laboratory reared Mytilus edulis.: II. Selection at the leucine amino peptidase (Lap) locus

Abstract

Offspring from several families and a mass mating of Mytilus edulis were cultured in the laboratory up to the juvenile stage under different temperature regimes. Mussels were genotyped at the leucine aminopeptidase (Lap) locus at the spat stage (post-metamorphosis) using cellulose acetate electrophoresis and at the juvenile stage using starch gel electrophoresis. Significant deviations from expected genotype frequencies were found in many cultures at both stages and certain genotypes were generally found to be favoured irrespective of temperature regime during culture. Estimates of relative fitnesses of the six common Lap genotypes suggest overdominance for the alleles Lap94 and Lap and semi-dominance for Lap98, but calculations based on this array of estimated fitnesses indicate that Lap98 would eventually become fixed in a panmictic population. Since a balanced Lap polymorphism exists in the wild we conclude that selection differentials at the Lap locus in laboratory cultures may be very different from those in nature, and suggest that nutritional constraints may account for some of these differences in selection. Also, in contrast to wild populations, the laboratory cultures showed a significant excess of Lap heterozygotes.

Genetic studies of laboratory-rearedMytilus edulis

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Laboratory cultures of offspring from single families and a mass mating of Mytilus eduIis were reared to the spat stage at different temperatures. Previous papers in this series (Beaumont et al , 1989 have demonstrated indirect selection, unrelated to salinity, at the Pgm and Gpi loci, and direct salinity-related selection at the Lap locus. Using electrophoresis, frequencies of genotypes at the Odh, Pgm, Gpi, Hex, EsD, Pgk and Dia loci were recorded at the spat stage and, again, where possible, in juveniles.

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“…Secondly, although laboratory conditions would be expected to be "better" for mussels, than those in the wild, in the sense that no predators are present, as pointed out elsewhere (Beaumont et al 1989), one source of stress in laboratory culture may be nutrition. Microalgae were provided for larval cultures at higher densities than would normally be found in the wild, but only two species were used.…”

Linkage studies in Mytilus edulis, the mussel

“…Details of electrophoresis and allele nomenclature are given in Beaumont et al (1983Beaumont et al ( , 1988Beaumont et al ( , 1989Beaumont et al ( , 1990 accidental exchange of larvae between cultures and the ingress of wild larvae into juvenile rearing systems. Zouros et al (1992) also noted these constraints for genetic studies in bivalves.…”

“…( , 1988. In some families, 3-day-old veliger larvae were partitioned into several separate cultures which were reared from then on under different conditions of salinity or temperature (Beaumont et at., 1988(Beaumont et at., , 1989(Beaumont et at., , 1990; Table 1). …”

“…Allozyme data are available from a number of laboratory-reared single-family crosses (Beaumont et at., 1983(Beaumont et at., , 1988(Beaumont et at., , 1989(Beaumont et at., , 1990 and are here tested for linkage.…”