Outbreeding effect on embryo development in hybrids of allopatric pink salmon (Oncorhynchus gorbuscha) populations, a potential consequence of stock translocation

Abstract

Previously, we demonstrated that F2 pink salmon (Oncorhynchus gorbuscha) hybrids between spatially separated even-broodline populations had significantly lower survivals than their controls and that both F1 and F2 hybrids between spatially separated odd-broodline pink salmon populations had reduced survivals. Those observations confirmed that outbreeding depression occurs, but did not provide insight into the mechanism. Development time, which often has a genetic basis in pink salmon, is probably locally adaptive. It is a trait that may contribute to outbreeding depression in intercrosses of genetically divergent fish populations. We analyzed development times (accumulation of temperature days; ATUs) at hatching of hybrids from the same even- and odd-year broodline crosses in which reduced survival had been observed. First-generation hybrids between Auke Creek (Juneau, eastern Gulf of Alaska) females and Pillar Creek (Kodiak, western Gulf of Alaska) males and Auke Creek controls were made in 1996 and 1997, incubated at Auke Creek, and released to the ocean. Second-generation hybrids, controls, and both backcrosses were made in 1998 and 1999. In 2001, F1 hybrids were made between Pillar Creek females and both Auke and Pillar creek males and incubated at Pillar Creek. All crosses of Pillar Creek fish (hybrids at Auke Creek and controls at Pillar Creek) developed more slowly (hatched after accumulating more ATUs) in both the first and second generations. Trajectories of development times of back-crossed salmon lay between those of F2 controls and hybrids. Comparison of development times of F2 crosses and backcrosses suggested that neither an ecological (locus-by-locus) nor a genetic (epistatic) model of outbreeding alone adequately explained observed differences. Prudent conservation of biodiversity requires consideration of biogeographic history as well as the genetic structure and local environment of the populations involved when actions are contemplated in which there is potential for intercrosses.