Recent local adaptation of sockeye salmon to glacial spawning habitats

Abstract

Salmonids spawn in highly diverse habitats, exhibit strong genetic population structuring, and can quickly colonize newly created habitats with few founders. Spawning traits often differ among populations, but it is largely unknown if these differences are adaptive or due to genetic drift. To test if sockeye salmon (Oncorhynchus nerka) populations are adapted to glacial, beach, and tributary spawning habitats, we examined variation in heritable phenotypic traits associated with spawning in 13 populations of wild sockeye salmon in Lake Clark, Alaska. These populations were commonly founded between 100 and 400 hundred sockeye salmon generations ago and exhibit low genetic divergence at 11 microsatellite loci (F ST \ 0.024) that is uncorrelated with spawning habitat type. We found that mean P ST (phenotypic divergence among populations) exceeded neutral F ST for most phenotypic traits measured, indicating that phenotypic differences among populations could not be explained by genetic drift alone. Phenotypic divergence among populations was associated with spawning habitat differences, but not with neutral genetic divergence. For example, female body color was lighter and egg color was darker in glacial than non-glacial habitats. This may be due to reduced sexual selection for red spawning color in glacial habitats and an apparent trade-off in carotenoid allocation to body and egg color in females. Phenotypic plasticity is an unlikely source of phenotypic differences because Lake Clark sockeye salmon spend nearly all their lives in a common environment. Our data suggest that Lake Clark sockeye salmon populations are adapted to spawning in glacial, beach and tributary habitats and provide the first evidence of a glacial spawning ecotype in salmonids.Glacial spawning habitats are often young (i.e.,\200 years old) and ephemeral. Thus, local adaptation of sockeye salmon to glacial habitats appears to have occurred recently.

Reconstructing Turbidity in a Glacially Influenced Lake Using the Landsat TM and ETM+ Surface Reflectance Climate Data Record Archive, Lake Clark, Alaska

“…Ramstad et al [36] found that increased turbidity correlated with longer snouts in breeding adults and females having pink body coloration as opposed to red. These attributes are attributed to reduced visibility.…”

“…These attributes are attributed to reduced visibility. Sexual signaling via red body color is less effective in turbid waters and therefore an unnecessary use of valuable carotenoids [36]. Pink coloration could make spawning fish less visible to predators [36].…”

“…Sexual signaling via red body color is less effective in turbid waters and therefore an unnecessary use of valuable carotenoids [36]. Pink coloration could make spawning fish less visible to predators [36]. In turbid waters, the chance of physical encounters increases and the need for longer snouts as weapons and appears to be an evolutionary response [36].…”

“…Pink coloration could make spawning fish less visible to predators [36]. In turbid waters, the chance of physical encounters increases and the need for longer snouts as weapons and appears to be an evolutionary response [36]. Long-term and significant changes (positive or negative) in water clarity within Lake Clark would therefore provide significant selective pressure on sockeye salmon phenotypes.…”

Introduction to a Special Section: Genetic Adaptation of Natural Salmonid Populations

“…Studies of wild salmonids to date have used the variation in heritable phenotypic traits that are easily measured (e.g., reproductive timing, adult size, and flesh color) and compared with the variation at neutral genetic markers (e.g., O'Malley et al 2010;Ramstad et al 2010). While informative, these studies are constrained by the limited number of easily measured phenotypic traits known to be highly heritable.…”