A growing body of research suggests that many clonal populations maintain genetic diversity even without occasional sexual reproduction. The purpose of our study was to document variation in SNP diversity, DNA content, and pathogen susceptibility in clonal lineages of the New Zealand freshwater snail, Potamopyrgus antipodarum. We studied snails that were collected from multiple field sites around a single lake (Lake Alexandrina), as well as isofemale clonal lineages that had been isolated and maintained in the laboratory. We used the KASP method to genotype our samples at 46 nuclear SNP sites, and we used flow cytometry to estimate DNA content. We found high levels of SNP diversity, both in our field samples and in our clonal laboratory lines. We also found evidence of high variation in DNA content among clones, even among clones with identical genotypes across all SNP sites. Controlled pathogen exposures of the laboratory populations revealed variation in susceptibility among distinct clonal genotypes, which was independent of DNA content. Taken together, these results show high levels of diversity among asexual snails, especially for DNA content, and they suggest rapid genome evolution in asexuals.

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Coevolution between a host and its symbiont can lead mutualist partners to specialize on each other, which may lead to the evolution of barriers to associating with novel partners. Barriers to partner switching may be pre-association or post-association, analogous to pre-and post-zygotic barriers in the formation of hybrids between incipient species (Coyne & Orr, 1997). This analogy is especially applicable for horizontally transmitted endosymbionts in which preassociation barriers might result from impediments to successful

The maintenance of sexual reproduction remains an important question in evolutionary biology (Neiman et al., 2018). The question arises in part because sexual females produce male offspring, which do not bear young, whereas asexual females produce only daughters. This gives asexuals an intrinsic twofold advantage over their sexual counterparts, which is called the 'cost of males ' (Lively & Lloyd, 1990;Maynard Smith, 1978). The cost of males has been demonstrated in several systems, including Amazon Mollies (Schlupp et al., 2010) and Daphnia pulex (Wolinska & Lively, 2008). In addition to this intrinsic cost, there may be additional ecological costs of sex, such as the lower overwinter survival observed in sexual Boechera wildflowers compared to asexuals (Rushworth et al., 2020). The long-term persistence of sexual reproduction in the face of these costs implies that sexual reproduction must confer significant short-term fitness advantages (Maynard Smith, 1978). The ecological hypotheses for the source of such advantages have tended to focus on the potential value of producing genetically variable offspring in the face of temporal changes in the abiotic environment [Lottery Model (e.g. Williams)], competition for variable resources [Tangled Bank Model (e.g. Bell, 1982)] and coevolving parasites [The Red Queen Hypothesis (e.g. Hamilton, 1980)].The New Zealand freshwater snail, Potamopyrgus antipodarum, is a suitable system for testing the alternative ecological hypotheses for the maintenance of sex. These snails are found in mixed populations of sexual and asexual females in nature (Dybdahl & Lively, 1995), making direct comparisons possible, both within and between populations. In addition, both experiments and field data have shown that P. antipodarum exhibit a twofold cost of males (Gibson et al., 2017;Jokela et al., 1997), as originally predicted by Maynard Smith (1971, 1978). Thus far, the snail system has been used to test the ecological hypotheses for the maintenance of sex, of which the Red Queen hypothesis is the best supported. The Red Queen hypothesis suggests that the cost of males in sexuals is mitigated by the benefits of

Following a host shift, repeated co‐passaging of a mutualistic pair is expected to increase fitness over time in one or both species. Without adaptation, a novel association may be evolutionarily short‐lived as it is likely to be outcompeted by native pairings. Here, we test whether experimental evolution can rescue a low‐fitness novel pairing between two sympatric species of Steinernema nematodes and their symbiotic Xenorhabdus bacteria. Despite low mean fitness in the novel association, considerable variation in nematode reproduction was observed across replicate populations. We selected the most productive infections, co‐passaging this novel mutualism nine times to determine whether selection could improve the fitness of either or both partners. We found that neither partner showed increased fitness over time. Our results suggest that the variation in association success was not heritable and that mutational input was insufficient to allow evolution to facilitate this host shift. Thus, post‐association costs of host switching may represent a formidable barrier to novel partnerships among sympatric mutualists.