Introgression between wild and selectively-bred Sydney rock oysters (Saccostrea glomerata)

Oysters provide vital ecosystem services for coastal ecosystems worldwide. Sydney rock oysters (Saccostrea glomerata) are among the most important native Australian oysters. In 1990, a selective breeding program for faster growth rates was implemented for S. glomerata. This has since expanded to include breeding for resistance to winter mortality and QX disease. Selectively-bred oysters are currently farmed in estuaries containing wild oyster populations, providing the opportunity for interbreeding that could alter the genetic variability and structure of the wild populations. Here, we use next-generation genotype by-sequencing to investigate the genetic structures of wild populations and the selectively bred B2 line of S. glomerata. These data are used to test for genetic introgression between the populations at two sites in the Georges River, an estuary in Sydney where selectively bred oysters have been farmed since 1990. Strong genetic partitioning was identified between wild and selectively-bred populations and there was no evidence of sustained gene flow in the form of introgression. Contrary to our expectations, we found significantly higher levels of genetic diversity and heterozygosity in the selectively-bred population than the wild population. These results potentially reflect the impact of population bottlenecks and are relevant to the resilience of this species to environmental change.