Can asexuality confer a short term advantage? Investigating apparent biogeographic success in the apomictic triploid fern Myriopteris gracilis

Although asexual taxa are generally seen as evolutionary dead ends, asexuality appears to provide a short-term benefit in some taxa. The most frequently cited examples involve geographical parthenogenesis, in which asexual species display a wider distribution than their sexual relatives. However, such broad distributions may be an illusion created by multiple, morphologically cryptic, asexual lineages, each occupying a relatively small area. In this study we investigate the role of multiple lineages in the biogeography of Myriopteris gracilis Fée (Pteridaceae), a North American apomictic triploid fern species with a particularly large range.Methods: Rangewide asexuality was assessed by counting spores/sporangium in 573 M. gracilis specimens from across the species range, and lineage structure was assessed with both plastid DNA sequence and Genotyping By Sequencing (GBS) SNP datasets.Key Results: Spore counting identified no sexual populations, establishing that M. gracilis is exclusively asexual. The plastid data estimated the crown age of M. gracilis at ca. 2.5 mya and identified two lineages, each largely confined to the eastern or western portions of the range. These groups were further subdivided by the GBS data, revealing at least seven asexual lineages of varying geographic distributions, each occupying a relatively small portion of the total range of M. gracilis.Conclusions: Although maintained exclusively through asexual reproduction, the broad distribution of M. gracilis overstates the success of any one independently formed asexual lineage, and by extension, the short-term benefit of asexuality in this taxon.