Sustaining genetic diversity in an imperiled pelagophilic fish despite genetic drift

Patterns of genetic variation within species may differ in space and time, particularly in the face of rapid anthropogenic changes. Genomic time-series population-level data can reveal patterns of allele frequency change and can help understanding on-going evolutionary processes. This study used a time-series spanning 6 years to evaluate genomic variability and trends in effective population size, and to assess the influence of genetic drift on Macrhybopsis tetranema genomic diversity. Temporal trends were evaluated from samples collected from 2015 to 2020. Results from genomic variation and variance effective population size across time suggests M. tetranema upstream (New Mexico) are affected by genetic drift, but genetic diversity did not change significantly across the time-series. Absolute allele frequency differences across time-series revealed that drift is a result of many loci with moderate allele frequency changes, particularly after 2018, rather than few loci with high frequency changes. Also, linkage disequilibrium effective population size showed fluctuations and a general increasing trend. Overall, the results presented here demonstrated that genetic drift is an overarching evolutionary force driving trends in genetic variation in M. tetranema . Together, genetic data reported here and ecological data from recent studies, suggest that M. tetranema in the upstream portion of the remnant distribution experiences genetic drift likely due of downstream-biased gene flow, while upstream movement of adults can explain low relatedness and maintenance of genetic diversity upstream in the face of genetic drift.