Genomic evidence of long-term asexuality at the northern range limit of wetland plant, Decodon verticillatus (Lythraceae)

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Thompson, Hana
Asexual reproduction , Range limits , Transcriptome
Many plant species shift from sexual to asexual reproduction at geographic range edges, but the impact of asexual reproduction on range expansion is largely unknown. If asexuality allows genotypes well-adapted to range edge conditions to persist long-term without recombination, this may allow plant species to expand beyond their sexual niche (“AEH: asexual expansion hypothesis”). Alternatively, asexual populations could represent short-lived genetic sinks that restrict further range expansion (“ASH: asexual sink hypothesis”). I investigated these hypotheses in the emergent wetland plant, Decodon vertcillatus (Lythraceae), which shifts from sexual to asexual reproduction at its northern range limit. I did a de novo transcriptome assembly and compared SNPs in sexual and asexual populations at and approaching the northern range limit to determine if range-edge, asexual populations show genomic signatures of long-term asexuality consistent with the AEH. Asexual genotypes exhibited higher heterozygosity, fewer unique homozygous SNPs (doubletons), and a breakdown of isolation by distance, supporting the AEH. Asexual genotypes also showed some evidence for an accumulation of deleterious non-synonymous and radical amino-acid altering mutations. In contrast to the AEH, the quantity of unique heterozygous SNPs (singletons) did not differ between sexual and asexual genotypes, and average genetic differentiation was unexpectedly higher among sexual than asexual genotypes. Yet, on the whole, my results support the asexual expansion hypothesis, suggesting that asexual reproduction allowed Decodon verticillatus to expand its range further northward than would have been possible under sexual reproduction alone. Understanding the factors that influence range limits and range expansion is becoming increasingly important to better anticipate the capacity of species to adapt, and potentially shift their ranges in response to anthropogenic environmental changes and prioritize range-edge populations for conservation.
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