Variable hybridization outcomes in trout are predicted by historical fish stocking and environmental context

Abstract

Hybridization can profoundly affect the genomic composition and phenotypes of closely related species, and provides an opportunity to identify mechanisms that maintain reproductive isolation between species. Recent evidence suggests that hybridization outcomes within a species pair can vary across locations. However, we still do not know how variable outcomes of hybridization are across geographic replicates, and what mechanisms drive that variation. In this study, we described hybridization outcomes across 27 locations in the North Fork Shoshone River basin (Wyoming, USA) where native Yellowstone cutthroat trout and introduced rainbow trout co‐occur. We used genomic data and hierarchical Bayesian models to precisely identify ancestry of hybrid individuals. Hybridization outcomes varied across locations. In some locations, only rainbow trout and advanced backcrossed hybrids towards rainbow trout were present, while trout in other locations had a broader range of ancestry, including both parental species and first‐generation hybrids. Later‐generation intermediate hybrids were rare relative to backcrossed hybrids and rainbow trout individuals. Using an individual‐based simulation, we found that outcomes of hybridization in the North Fork Shoshone River basin deviate substantially from what we would expect under null expectations of random mating and no selection against hybrids. Since this deviation implies that some mechanisms of reproductive isolation function to maintain parental taxa and a diversity of hybrid types, we then modelled hybridization outcomes as a function of environmental variables and stocking history that are likely to affect prezygotic barriers to hybridization. Variables associated with history of fish stocking were the strongest predictors of hybridization outcomes, followed by environmental variables that might affect overlap in spawning time and location.

Newly discovered cichlid fish biodiversity threatened by hybridization with non‐native species

“…A specific issue is hybridization between introduced species and native fish species. This has been reported in closely related species from multiple fish families, including the salmonids (Mandeville et al., 2019; Muhlfeld et al., 2014), cichlids (Firmat et al., 2013; Shechonge et al., 2018) and cyprinids (Almodóvar et al., 2012; Hata et al., 2019), and is likely to become increasingly common due to the spread of freshwater species for aquaculture and inland fisheries enhancement (Deines et al., 2014). However, the full evolutionary and ecological consequences of hybridization between invasive and native species are typically unclear, and further studies of the impact of hybridization events on native biodiversity are required.…”

Genomic differentiation across the speciation continuum in three hummingbird species pairs

“…Calculating d xy across the genome and locating F ST peaks that are associated with elevated d xy can help narrow down the potential causes of some islands of divergence (e.g. [18,[31][32][33][34]).…”

Abiotic conditions are unlikely to mediate hybridization between invasive rainbow trout and native Yellowstone cutthroat trout in a high-elevation metapopulation

“…For example, over 20 million rainbow trout were stocked in the Flathead River watershed, Montana (mostly at lower elevations), and this has played a major role in the distribution of present-day hybridization, which has been spreading rapidly upstream over the past 20 years from naturalized populations (Boyer et al 2008;Muhlfeld et al 2014). Throughout western North America, this pattern of low-elevation-biased propagule pressure is common leading to naturalized rainbow trout sources in lower portions of watersheds (Bennett et al 2010;Mandeville et al 2019). An important question is raised: Is hybridization absent or limited at cold, highelevation sites because of ecological differences between taxa and ecological resistance, or is it because these sites have had less exposure to rainbow trout?…”