Founder effect, genetic variability, and weight in the cultivated Portuguese oyster Crassostrea angulata

Existence of genetic variability is a prerequisite for successful implement ation of breeding programs, and clarification of the relationships in such programs to quantitative traits is of great economic interest. We have stud ied the relationship between multilocus heterozygosity and/or allozyme geno types and weight in the Portuguese oyster Crassostrea angulata (Lamark). Tw o cohorts were obtained in a commercial hatchery by mass-spawning from wild oysters. Loss of genetic variability was shown in cultured oysters as comp ared with the wild population because of a founder effect caused by a low e ffective population size. Significant effects on growth rate were detected for the Me-2, Xdh, Lap, Pgm, and Est loci. However, these effects were not retained in the two cohorts, nor in the two ages of the same cohort, nor we re differentiated effects detected in weight classes of the same age. At th e same time, differences between genotypes were not associated with differe nces between heterozygous and homozygous genotypes. Positive correlations b etween multilocus heterozygosities and growth rate, as well as significant differences between mean body weights for different degrees of heterozygosi ty, were found only in the largest weight class. Moreover, significant resu lts were obtained when the mean weight of different heterozygosity classes for total weight, body weight,and shell weight were compared only in the oy sters selected for their larger size. This result points to the isozymes as markers for quantitative traits and confirms the existence of heterosis in C. angulata, indicating the possibility of establishing breeding programs based on the maintenance of inbred lines and crossing them to obtain hybrid vigor.