A shift from sexual to asexual reproduction in a wetland plant: implications for range edge dynamics

Background/Question/Methods

All species have limits to their geographic distributions, thought to reflect a failure of adaptation to conditions beyond the range. The adaptive capacity of populations at range limits could be strongly influenced by the amount and distribution of genetic variation, which in turn is strongly affected by variability in reproductive system, particularly in plants which exhibit diverse reproductive modes. Plants can reproduce sexually, either through outcrossing (exchanging pollen with other individuals) or self-fertilization. Many plants also have the capacity for some form of asexual reproduction, by producing parthenogenic seeds or clonal reproduction via vegetative propagation. Sexual and asexual reproduction may have different ecological requirements, meaning that a species with both methods of reproduction can have separate sexual and asexual niches. In some species, the sexual niche is narrower than the asexual niche, thus shifts to asexuality might allow these species to thrive in environments where sexual reproduction and seed recruitment is inhibited. Decodon verticillatus is a wetland plant that reproduces sexually through most of its range and exhibits a shift to asexual clonal reproduction at the northern range margin. We investigated the population genomic consequences of this shift to asexuality, and the implications for range edge dynamics in Decodon verticillatus.

Results/Conclusions

We conducted a transcriptomic study to determine whether the asexual populations represent temporary sink populations, or long-lived, well-adapted populations that may have allowed for range expansion beyond the sexual niche of D. verticillatus. We did a de-novo transcriptome assembly and analyzed single nucleotide polymorphisms (SNPs) from sexual and asexual populations to determine whether the asexual populations show signatures of long-term asexuality, such as increased heterozygosity, an accumulation of deleterious mutations, and a breakdown of isolation by distance. We found that the asexual populations did show genetic signs of long-term asexuality, indicating that a shift to asexuality may have enabled range expansion in D. verticillatus beyond what would have been possible through 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.