Dispersal and the rainbow trout takeover

I’m going to keep rolling on the dispersal theme from last week and share a new paper by Ryan Kovach and colleagues that demonstrates the balance between dispersal and selection. Specifically, the authors show that this balance dictates the hybridization between a native and invasive trout species.
The authors utilized data from two populations of cutthroat trout that spans 24 years in order to detect changes in rainbow trout ancestry and quantify associated phenotypic variation. In this case, the danger for cutthroat trout populations is very real: too much hybridization with rainbow trout can lead to a hybrid soup of genomes in which native genomes dissappear (Allendorf and Leary 1988).

Figure 1 from Kovach et al. (2015)

Figure 1 from Kovach et al. (2015) showing the relationship between rainbow trout (RBT) admixture and length (a) or early migration (b)

The identification of genetic introgression from rainbow trout increased dramatically from 1984 to 2003 (from 0% to 87% in one adult population!). And if you are a hybrid salmon, the more rainbow trout genes you can get, the better. As the proportion of rainbow trout alleles goes up, body size goes up and time until migration goes down: two factors strongly associated with fitness.
However, the proportion of rainbow trout alleles entering the cutthrout populations was much greater than the proportion of alleles leaving, indicating selection against hybrids. And the selection coeffiicients against these hybrids were strong to boot, up to 0.88!
This left Kovach et al. with a simple explanation: dispersal by rainbow/cutthroat hybrids plays a huge role in the increase of hybrids over the past 24 years.

Thus, our study shows that combining data on fitness and dispersal is necessary to fully understand the mechanisms driving invasive hybridization and other eco-evolutionary dynamics [59]; the paucity of such data in wild animal populations makes this a novel step forward in our empirical understanding of how invasive introgression can spread in natural populations.

Allendorf, F. W., & Leary, R. F. (1988). Conservation and distribution of genetic variation in a polytypic species, the cutthroat trout. Conservation Biology, 170-184.
Kovach, R. P., Muhlfeld, C. C., Boyer, M. C., Lowe, W. H., Allendorf, F. W., & Luikart, G. (2015). Dispersal and selection mediate hybridization between a native and invasive species. Proceedings of the Royal Society of London B: Biological Sciences, 282(1799), 20142454.
[59] above Lowe, W. H., & McPeek, M. A. (2014). Is dispersal neutral?. Trends in ecology & evolution, 29(8), 444-450.

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