Authors

Discussion

Parasites are thought to be potent biotic selective agents in the adaptive evolution of animals, but little is known about the genetic basis of susceptibility to infection in wild organisms. Three-spined stickleback provide a good model for improving our understanding in this area, because of their well characterised variation in susceptibility and availability of diverse genetic tools. In the present study, the quantification of parasite abundance in natural and experimental environments and their association with selected candidate genes (NLRC5, PPARA, WNT7B and Eda) assist in elucidating the genetic regions underlying adaptation to infection. Candidate genes were selected because of previous work showing putatively adaptive divergence in allele frequencies between marine and freshwater. There was a strong relationship between some alleles and the abundance of ectoparasite and endoparasites of stickleback. Under natural conditions, Gyrodactylus sp. infection burden was associated with PPARA locus, plate morph and sex of fish while Diplostomum sp. had a significant relationship with plate morph and WNT7B locus. For experimental infections with the ectoparasite Gyrodactylus gasterostei, there was a significant association of parasite abundance with sex, plate morph and Eda locus, whereas growth rate of fish was associated with plate morph and PPARA locus. For the endoparasite Diplostomum pseudospathecum, there was a significant association of Eda locus with parasite burden of the fish under experimental condition. The divergent effects of parasites on different stickleback genes suggest parasite-mediated selection in the ecological adaptation of fish.