Asymmetrical viability in backcrosses between highly divergent populations of Atlantic salmon (Salmo salar): implications for conservation

Introductions of non-native conspecifics are widespread and intraspecific hybridisation may cause reduced offspring fitness, which could potentially be asymmetric in geographically widely separated populations. Atlantic salmon (Salmo salar L.) from North America and Europe have diverged for the past similar to 6,00,000 years and natural hybridisation between them is unlikely. However, the global nature of the aquaculture industry and related fish movements has increased the chances for intercontinental interbreeding to occur. Understanding the fitness implications of such outbreeding is important for the species' conservation. Here, we examined offspring viability in F-1 and backcrosses (BC1) between Canadian and Scottish salmon. BC1 to Canadian fish were completely unviable; while BC1 to Scottish fish and F-1 hybrids showed high viability. The fitness depression found, typical of interspecific salmonid crosses, may reflect genomic disruptions known as Dobzhansky-Muller incompatibilities. These observations highlight that outbreeding depression may be asymmetric in post-F-1 hybrid generations, even within a designated species.