Small fish, large river: Surprisingly minimal genetic structure in a dispersal‐limited, habitat specialist fish

Abstract

Genetic connectivity is expected to be lower in species with limited dispersal ability and a high degree of habitat specialization (intrinsic factors). Also, gene flow is predicted to be limited by habitat conditions such as physical barriers and geographic distance (extrinsic factors). We investigated the effects of distance, intervening pools, and rapids on gene flow in a species, the Tuxedo Darter (Etheostoma lemniscatum), a habitat specialist that is presumed to be dispersal‐limited. We predicted that the interplay between these intrinsic and extrinsic factors would limit dispersal and lead to genetic structure even at the small spatial scale of the species range (a 38.6 km river reach). The simple linear distribution of E. lemniscatum allowed for an ideal test of how these factors acted on gene flow and allowed us to test expectations (e.g., isolation‐by‐distance) of linearly distributed species. Using 20 microsatellites from 163 individuals collected from 18 habitat patches, we observed low levels of genetic structure that were related to geographic distance and rapids, though these factors were not barriers to gene flow. Pools separating habitat patches did not contribute to any observed genetic structure. Overall, E. lemniscatum maintains gene flow across its range and is comprised of a single population. Due to the linear distribution of the species, a stepping‐stone model of dispersal best explains the maintenance of gene flow across its small range. In general, our observation of higher‐than‐expected connectivity likely stems from an adaptation to disperse due to temporally unstable and patchy habitat.

Genome‐wide diversity and habitat underlie fine‐scale phenotypic differentiation in the rainbow darter (Etheostoma caeruleum)

“…Contemporary population structure can impact the spread of adaptive genetic variation through evolutionary processes such as gene flow or genetic drift (Lande, 1976; Slatkin, 1987; Tigano & Friesen, 2016; Willi et al., 2006). Despite the fine‐scale scope of our study, we did find some evidence of population structure, which contrasts to similar darter studies that found no discernable genetic structure (Davis et al., 2015; Washburn et al., 2020). The two genetic clusters identified did not correspond to geographic stream distance.…”

Riverscape genomics of cichlid fishes in the lower Congo: Uncovering mechanisms of diversification in an extreme hydrological regime

“…Simple models of such systems predict asymmetric gene flow and higher genetic diversity in downwind, downstream, and down‐current locations compared to source populations. In rivers, we expect the accumulation of genetic diversity from the headwater to downstream locations in a one‐dimensional linear stepping‐stone pattern (Maruyama, 1969; Thomaz et al, 2016; Washburn et al, 2020). However, numerous factors could cause deviations from this expectation, including the presence of barriers to gene flow and corridors or other landscape complexities that promote multidirectional gene flow (Petkova et al, 2016).…”

Genomic and phenotypic divergence informs translocation strategies for an endangered freshwater fish

“…Darter ecology and reproductive behaviour may also play a key role in maintaining population structure, as these fishes can have strong preferences for specific microhabitats (Alexander & Phillips, 2012) and sexual traits (Williams & Mendelson, 2011). Not all darters exhibit a strong signal of population structure, suggesting that the potential for diversification is a result of particular combinations of life historical, anthropogenic, and biogeographic conditions (Camak & Piller, 2018; Washburn et al, 2020). Here, the picture of freshwater fish diversification becomes local, complex, and contingent rather than general, unimodal, and singular (Mayden, 1988; Near & Keck, 2005).…”