Does the remnant distribution of Macrhybopsis tetranema constitute a panmictic population?

Authors: Guilherme Caeiro-Dias, Thomas Turner, Megan Osborne

Year: 2024

Abstract

The pelagic-broadcast spawning guild of fishes is negatively impacted by fragmentation, altered flow regimes, habitat degradation of rivers of the North American Great Plains. Peppered Chub (Macrhybopsis tetranema) was the most recent member of this guild to be listed as endangered. This species has been extirpated from more than 94% of its historic range and is now only found in a 218-kilometer reach of the South Canadian River between Ute Lake (New Mexico) and Lake Meredith (Texas). Distribution of genetic diversity across the riverscape also depends on intrinsic traits such as fecundity, reproductive strategy, and migratory behavior. In addition, recent anthropogenic habitat changes, climate change, and stochastic events can disrupt populations connectivity, change population dynamics, and cause substantial fluctuations in population size, which in turn can affect levels of genetic diversity and its distribution across the riverscape. In dendritic systems, numerous studies have found an increase in genetic diversity downstream, due to downstream-biased dispersal, increased habitat availability/complexity and larger population size downstream. Using 1,908 loci we estimated genomic diversity from upstream localities (New Mexico) from 2015 to 2020 (n = 34, 31, 21, 9, 41, and 43, respectively) and from a single downstream site (Texas) in 2017 (n = 8); and assessed the impact of genetic drift upstream. In upstream localities, we found signs of strong genetic drift but no significant changes in genomic diversity across time. We also found an increased excess of heterozygotes in the downstream site not found upstream. Moreover, downstream population in 2017 exhibits a larger genomic variability when compared to upstream collections, particularly compared to samples collected in the same year. Given these results, we hypothesize that asymmetric movement due to egg drift coupled with limited juvenile/adult upstream movement might be a source of increased genetic drift associated with reduced population size upstream and increased genetic variability downstream. Although we cannot rule out that small sample size might be a source of bias, observed heterozigozity estimates for upstream reach across time were not significantly different and seemed relatively robust to sample size bias (e.g., results from nine samples collected in New Mexico in 2018 did not show significant heterozygosity estimates from other collections). Future sampling that includes both upstream and downstream populations for the same temporal collections is needed to test our hypothesis.