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

Existence of genetic variability is a prerequisite for successful implementation of breeding programs, and clarification of the relationships in such programs to quantitative traits is of great economic interest. We have studied the relationship between multilocus heterozygosity and/or allozyme genotypes and weight in the Portuguese oyster Crassostrea angulata (Lamark). Two cohorts were obtained in a commercial hatchery by mass-spawning from wild oysters. Loss of genetic variability was shown in cultured oysters as compared with the wild population because of a founder effect caused by a low effective 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 were differentiated effects detected in weight classes of the same age. At the same time, differences between genotypes were not associated with differences between heterozygous and homozygous genotypes. Positive correlations between multilocus heterozygosities and growth rate, as well as significant differences between mean body weights for different degrees of heterozygosity, were found only in the largest weight class. Moreover, significant results were obtained when the mean weight of different heterozygosity classes for total weight, body weight,and shell weight were compared only in the oysters 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.