Maintenance of Heterozygosity during Selective Breeding of Oysters for Resistance to MSX Disease

Abstract

Allozyme variation was examined in five strains of oysters (Crassostrea virginica) that had been subjected to several generations of intense selection for resistance to the protozoan parasite Haplosporidium nelsoni (MSX). Pedigree records permitted us to estimate expected levels of heterozygosity for the selected strains and to compare the observed levels of heterozygosity with those in geographically related wild stocks. In all five strains, mean heterozygosity across six highly polymorphic gene loci was not lost at the rate predicted by the inbreeding model. However, significant losses of rare alleles occurred in each strain, allowing us to exclude contamination of the selected strains by wild oysters. Both selective and nonselective explanations for the maintenance of heterozygosity in these oysters are discussed.