Quantitative genetic analysis of late spring mortality in triploid Crassostrea virginica

Triploid oysters, bred by crossing tetraploid and diploid oysters, are common worldwide in commercial oyster aquaculture and make up much of the hatchery-produced Crassostrea virginica farmed in the mid-Atlantic and southeast of the United States. Breeding diploid and tetraploid animals for genetic improvement of triploid progeny is unique to oysters and can proceed via several possible breeding strategies. Triploid oysters, along with their diploid or tetraploid relatives, have yet been subject to quantitative genetic analyses that could inform a breeding strategy of triploid improvement. The importance of quantitative genetic analyses involving triploid C. virginica has been emphasized by the occurrence of mortality events of near-market sized triploids in late spring.

Genetic parameters for survival and weight of triploid and tetraploid C. virginica were estimated from twenty paternal half-sib triploid families and thirty-nine full-sib tetraploid families reared at three sites in the Chesapeake Bay (USA). Traits were analyzed using linear mixed models in ASReml-R. Genetic relationship matrices appropriate for pedigrees with triploid and tetraploid animals were produced using the polyAinv package in R.

A mortality event in triploids occurred at one site located on the bayside of the Eastern Shore of Virginia. Between early May and early July, three triploid families had survival of less than 0.70, while most had survival greater than 0.90. The heritability for survival during this period in triploids at this affected site was 0.57 ± 0.23. Triploid survival at the affected site was adversely related to triploid survival at the low salinity site (- 0.50 ± 0.23) and unrelated to tetraploid survival at the site with similar salinity (0.05 ± 0.39).

Survival during a late spring mortality event in triploids had a substantial additive genetic basis, suggesting selective breeding of tetraploids can reduce triploid mortalities. Genetic correlations revealed evidence of genotype by environment interactions for triploid survival and weak genetic correlations between survival of tetraploids and triploids. A selective breeding strategy with phenotyping of tetraploid and triploid half-sibs is recommended for genetic improvement of triploid oysters.