Abstract
Variation in size and age at maturity is an important component of life history that is influenced by both environmental and genetic factors. In salmonids, large size confers a direct reproductive advantage through increased fecundity and egg quality in females, while larger males gain a reproductive advantage by monopolizing access to females. In addition, variation in size and age at maturity in males can be associated with different reproductive strategies; younger smaller males may gain reproductive success by sneaking among mating pairs. In both sexes, there is a trade‐off between older age and increased reproductive success and increased risk of mortality by delaying reproduction. We identified four Y‐chromosome haplogroups that showed regional‐ and population‐specific variation in frequency using RADseq data for 21 populations of Alaska Chinook salmon. We then characterized the range‐wide distribution of these haplogroups using GT‐seq assays. These haplogroups exhibited associations with size at maturity in multiple populations, suggesting that lack of recombination between X and Y‐chromosomes has allowed Y‐chromosome haplogroups to capture different alleles that influence size at maturity. Ultimately, conservation of life history diversity in Chinook salmon may require conservation of Y‐chromosome haplotype diversity.
Highlights
Variation in life history within populations is common across taxa and is often associated with alternative strategies for increasing fitness
We examined patterns of linkage disequilibrium on the sex chromosome of Chinook salmon to determine if male-specific haplotype blocks (Y-chromosome haplotypes) existed, and if so, are these haplotypes associated with variation size and age at maturity which commonly differ between sexes in Chinook salmon
We identified Y-chromosome haplogroups that are associated with size, and likely age, at maturity in Chinook salmon throughout Alaska
Summary
Variation in life history within populations is common across taxa and is often associated with alternative strategies for increasing fitness This includes partial migration, where some individuals of a population migrate while others remain resident (Chapman et al 2011), reproductive morphs that exhibit different mating strategies or sexually selected traits (Shuster 1989, Johnston et al 2013, Küpper et al 2015), age and size at maturity (Gibbons et al 1981), and even length of life-span such as annual versus perennial plants (Hall et al 2006). Older larger males gain reproductive success by monopolizing access to females while younger smaller males gain reproductive success by sneaking in among mating pairs (Healey and Heard 1984, Berejikian et al 2010) For both sexes, there is a trade-off to delayed maturation where increased reproductive success is tempered by an increased risk of mortality before reproduction. Considerable diversity in age at maturation is maintained within many populations, presumably as a bethedging strategy to spread risks over the life-cycle of these fish
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