Causes and consequences of introgressive hybridization in an endemic minnow

Background/Question/Methods

Despite accounting for less than 1% of the world’s surface area, freshwaters host over half of the world’s known fish species. Many of these fishes are increasingly imperiled but the causes and consequences of their decline remain poorly understood. The Owens Tui Chub is a minnow endemic to the Owens River Valley in California’s eastern Sierra Nevada Mountains, a region which has experienced severe anthropogenic disturbance to its freshwater habitats over the last 100 years. Most prominent of these disturbances are the reduction in habitat extent following the construction of the Los Angeles Aqueducts, which siphon much of the region’s water for urban use, and also the widespread introduction of non-native game fishes. At the same time, the Owens Tui Chub has been driven to the verge of extinction by introgressive hybridization with a closely related subspecies, which was presumably introduced by recreational fishermen using them as bait. This widespread introgression following a limited introduction is indicative of a selective sweep, but the fitness advantage provided to hybrids is unknown.

Results/Conclusions

I propose two hypotheses to explain this rapid introgession: (1) Introgression facilitated adaptation to flowing versus still water following the canalization of the Owens River and the loss of marshy off-channel habitat; and (2) Introgression facilitated the evolution of anti-predator defense traits following the widespread introduction of predatory fishes. I used a comparative survey of body shape in hybrid and parental populations from lake and stream habitats and found that introgression eroded morphological divergence between lake and stream habitats. In a reciprocal transplant experiment, I found no evidence that introgressed populations were locally adapted by habitat type, together indicating that adaptation to flowing versus still water is likely not the driver of rapid introgression. I used behavioral assays to test for adaptation to non-native predators. I found that nearly all of the behavioral variation was contained within populations, and that the proportion of bold versus shy individuals did not vary by predation risk, providing no support for our second hypothesis. I will discuss our recent progress using whole genome sequencing to characterize patterns of introgression in relation to environmental conditions and fish phenotype, as well as the implications of our work for conservation in a rapidly changing world.