Over the Falls? Rapid Evolution of Ecotypic Differentiation in Steelhead/Rainbow Trout (Oncorhynchus mykiss)

Abstract

Adaptation to novel habitats and phenotypic plasticity can be counteracting forces in evolution, but both are key characteristics of the life history of steelhead/rainbow trout (Oncorhynchus mykiss). Anadromous steelhead reproduce in freshwater river systems and small coastal streams but grow and mature in the ocean. Resident rainbow trout, either sympatric with steelhead or isolated above barrier dams or waterfalls, represent an alternative life-history form that lives entirely in freshwater. We analyzed population genetic data from 1486 anadromous and resident O. mykiss from a small stream in coastal California with multiple barrier waterfalls. Based on data from 18 highly variable microsatellite loci (He = 0.68), we conclude that the resident population above one barrier, Big Creek Falls, is the result of a recent anthropogenic introduction from the anadromous population of O. mykiss below the falls. Furthermore, fish from this above-barrier population occasionally descend over the falls and have established a genetically differentiated below-barrier subpopulation at the base of the falls, which appears to remain reproductively isolated from their now-sympatric anadromous ancestors. These results support a hypothesis of rapid evolution of a purely resident life history in the above-barrier population in response to strong selection against downstream movement.

Rapid parallel evolution of standing variation in a single, complex, genomic region is associated with life history in steelhead/rainbow trout

“…Patterns of genetic divergence based on the 32 non-Omy5 loci, as measured by F ST and phylogenetic network analysis (electronic supplementary material, figure S1), were consistent with previous studies showing close relationships between populations above and below barriers within a watershed and increasing divergence with distance along the California coast [39,40,[57][58][59][60]. By contrast, analysis of loci located on Omy5 showed high divergence between above-and belowbarrier populations within the same basin, and nearly complete reciprocal monophyly between above-and below-barrier populations (electronic supplementary material, figure S1).…”

Genetic relationships and gene flow between resident and migratory brook trout in the Salmon Trout River

“…D'Amelio and also found that brook trout above and below barrier water falls were more highly differentiated than were brook trout from below a barrier. Similar phenomena have been described for other salmonid species (e.g., Pearse et al, 2009) where reproductive isolation imposed by elevational gradients (water falls) has been partially overcome by bi-directional gene flow of natural (downstream) and anthropogenic (upstream) origin.…”

Microsatellite DNA variation among samples of bronze gudgeon, Coreius heterodon, in the mainstem of the Yangtze River, China

“…Genetic subdivision of fish populations may be a common phenomenon in the upper Yangtze River (Cheng et al 2012) and should be carefully considered in management and conservation of fishery resources. Damming has been shown to fragment habitats and thereby to affect the genetic structure of fish populations in river systems (Deiner et al 2007;Masuda et al 2009;Pearse et al 2009). Construction of the Gezhouba and Three Gorges dams fragmented and isolated the bronze gudgeon populations above and below the dams (Leng et al 1984;Liu et al 1990;Zhuang and Cao 1999).…”