Significant deviations from the Hardy-Weinberg equilibrium in natural populations of bivalves may be due to a number of factors such as the Wahlund effect, null alleles and selection, and it is often difficult to decide which of these factors is operating. The use of laboratory controlled single crosses and mass matings, where the genotypes of the parents are known, can provide strong evidence for selection when deviations from expected genotype frequencies are observed in the offspring. Several cultures of offspring from a single cross and from a mass mating of the mussel, Mytilus edulis, were reared through the larval stage at either 32 or 25 parts per thousand salinity. At the early post-larval stage salinities were altered for some cultures while others remained unchanged. Samples were taken for electrophoresis as post-larvae and again as juveniles and were scored at the Lap, Pgm, Odh, Hex, Pgk and Gpi loci. Significant deviations from expected frequencies were observed at many loci in the offspring but these deviations did not appear to be related to salinity except at the Lap locus. The allele Lap94 was selected against at low salinity during the post-larval to juvenile stage, but no such selection occurred at low salinity during the larval stage. The data fit Koehn's (1985) model of post-larval selection at the Lap locus in mussels in Long Island Sound, USA, but suggest that there may be ontogenetic variation in fitness at this locus. Results at the Pgm and Gpi loci indicate that these loci are probably acting as markers for other loci on the same chromosome which have a strong effect on fitness. Density dependent factors may have caused some of the non-random mortality amongst offspring which was apparently independent of salinity.

Laboratory cultures of offspring from single families and a mass mating of Mytilus eduIis were reared to the spat stage at different temperatures. Some cultures were reared further to the juvenile stage at the same, or at an altered temperature. 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. Significant deviations from expected genotype frequencies were observed at most loci in at least some cultures, but in no cases were these deviations related to the temperature at which the cultures had been reared. We speculate that these differences may have been due to variations in the density of mussel spat in the cultures. Very extensive and consistent Pgm genotype-specific mortalities were evident in one family, and, in another family, consistent deviations were seen at the Hex locus. Weaker, but nevertheless significant, deviations from expected genotype frequencies occurred at most other loci and it is concluded that, in most cases, the loci were acting as markers for a locus, or loci, on the same chromosome which had a strong effect on fitness. It is thought unlikely that evidence for similar levels of selection would be found in natural populations because, in wild populations, particular marker-locus genotypes would be associated with a range of genetic backgrounds rather than a single chromosome. the relative importance of natural selection compared with stochastic effects such as inbreeding, null alleles or migration. The development of bivalve larval-rearing techniques (Loosanoff and Davis 1963, Bayne 1965)has enabled offspring from single families and restricted matings to be reared under controlled conditions for the study of the genetics of M. edulis. In such studies, expected genotypes at a locus, and their proportions amongst offspring, can be found by genotyping the progenitors, and selection, whether direct or indirect, must be the major factor causing any significant deviation from expected proportions.Earlier studies involving laboratory-reared bivalves have produced evidence for direct or indirect selection at a number of loci , Adamkewicz et al. 1984, Gaffney and Scott 1984, Hvilsom and Theisen 1984, Mallet et al. 1985. 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. However, our data on fitness of specific genotypes at the Lap locus suggest that selection differentials observed in laboratory-reared individuals are very different from those which may apply in the wild. In this paper, we present data on genotype-specific mortalities at various loci in spat and juvenile mussels which were reared at different temperatures.

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.