Tasmanian Pacific oysters, Crassostrea gigas, in Washington state: A morphological, physiological, and molecular genetic analysis of an introduced population

Domestication by selective breeding requires specific knowledge of performance-related traits in commercially exploited aquaculture species. A small number of deep-cupped Pacific oysters, Crassostrea gigas, were transferred from a Tasmanian shellfish hatchery to Washington state. Thirty-four of these were spawned to establish an F1 generation. F1 Tasmanian oysters were grown in the field in comparative growout trials and their survival, growth rate, and shell morphology were monitored. During the second year of the experimental growout, Tasmanian F1 and control oysters were examined for patterns of gonadogenesis and glycogen storage. Allozyme frequencies were calculated for 21 loci to assess genetic variability and the magnitude of a possible founder effect. Nuclear and mitochondrial DNA sequences were amplified by the polymerase chain reaction (PCR) and examined for restriction fragment length polymorphisms (RFLPs) and sequence variation.;The Tasmanian F1 oysters experienced 40% fewer mortalities and attained significantly greater shell length and higher whole weight and volume than controls. Seasonal proliferation of gonadal tissue and declines of glycogen stores showed no significant differences. There was no overall heterozygote deficiency or decreased heterozygosity or polymorphism among Tasmanian F1 oysters. However, there was a 19% overall reduction in allelic variation due to the loss of ten previously described rare alleles in the F1 generation. Restriction of a 2,100 bp fragment of the nuclear rDNA array with 45 endonucleases produced no population-specific haplotype. Partial sequencing of the mitochondrial cytochrome b gene revealed a C→T transition with a frequency of 50% among Tasmanian F 1 oysters. The introduced oysters may increase productivity as a pure stock in areas of high mortality or through hybrid vigor in crosses with selected lines of local C. gigas. Adequate numbers of broodstock animals should be used in maintaining the Tasmanian oyster population to prevent further erosion of genetic variability.