Heterogeneous genetic basis of age at maturity in salmonid fishes

Understanding the genetic basis of repeated evolution of the same
phenotype across taxa is a fundamental aim in evolutionary biology and has
applications to conservation and management. However, the extent to which
interspecific life-history trait polymorphisms share evolutionary pathways
remains under-explored. We address this gap by studying the genetic basis
of a key life-history trait, age at maturity, in four species of Pacific
salmon (genus Oncorhynchus) that exhibit intra- and interspecific
variation in this trait – Chinook Salmon, Coho Salmon, Sockeye Salmon, and
Steelhead Trout. We tested for associations in all four species between
age at maturity and two genome regions, six6 and vgll3, that are strongly
associated with the same trait in Atlantic Salmon (Salmo salar). We also
conducted a genome-wide association analysis in Steelhead to assess
whether additional regions were associated with this trait. We found the
genetic basis of age at maturity to be heterogeneous across salmonid
species. Significant associations between six6 and age at maturity were
observed in two of the four species, Sockeye and Steelhead, with the
association in Steelhead being particularly strong in both sexes (p =
4.46x10-9 after adjusting for genomic inflation). However, no significant
associations were detected between age at maturity and the vgll3 genome
region in any of the species, despite its strong association with the same
trait in Atlantic Salmon. We discuss possible explanations for the
heterogeneous nature of the genetic architecture of this key life-history
trait, as well as the implications of our findings for conservation and
management.