Opposing health effects of hybridization for conservation

Abstract

The continuing decline of many natural plant and animal populations emphasizes the importance of conservation strategies. Hybridization as a management tool has proven successful in introducing gene flow to small, inbred populations, but can be also associated with health risks. For example, hybridization can change susceptibility to infection in either direction due to heterosis (hybrid vigor) and outbreeding depression, but such health effects have rarely been considered in the genetic management of populations. Here, we investigated the effects of experimental outcrossing between the critically endangered Saimaa landlocked salmon (Salmo salar m. sebago) and the genetically more diverse Atlantic salmon (S. salar) on infection susceptibility. We exposed the parent populations and their hybrids to two parasites that commonly infect these species, the bacterium Flavobacterium columnare and the macroparasitic fluke Diplostomum pseudospathaceum. We found that landlocked salmon had lower survival during the bacterial epidemic, but higher resistance against the fluke, compared with Atlantic salmon. Hybrids showed intermediate survival and resistance, suggesting that hybridization decreased susceptibility to one parasite, but concurrently increased it to another. Our results emphasize the importance of considering health effects of different types of infections when employing hybridization for conservation.