Asymmetric density‐dependent competition does not contribute to the maintenance of sex in a mixed population of sexual and asexualPotamopyrgus antipodarum

Abstract

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

Invasive freshwater snails are less sensitive to population density than native conspecifics

“…By contrast, there was a trend toward native lineages producing the most embryos in low density. This result is similar to that of Dinges and Lively ( 2022 ), where mesocosm experiments showed that reproduction of native P . antipodarum lineages was negatively affected by high population density.…”

“…By contrast, there was a trend toward native lineages producing the most embryos in low density. This result is similar to that of Dinges and Lively (2022), where mesocosm experiments showed that reproduction of native P. antipodarum lineages was negatively affected by high population density. In our experiment, nativelineage snails produced relatively few embryos in intermediate and high density, which echoes trends reported for native-range P. antipodarum in Cope and Winterbourn (2004).…”

“…A similar result for two lineages of P. antipodarum, one native and one invasive, was reported in Neiman et al (2013) when food availability was maintained at a relatively high level that would not independently limit reproduction. We also see a trend towards increased reproduction in native lineages at low density, similar to the outcome of the native-lineage P. antipodarum mesocosm experiment of Dinges and Lively (2022). By contrast, Zachar and Neiman (2013) found that reproduction was maximized at intermediate density in two P. antipodarum lineages.…”

“…These outcomes for reproductive status mirror our embryo production results – native P . antipodarum perform better in low‐density treatments compared to intermediate and high‐density treatments (also see Cope & Winterbourn, 2004 ; Dinges & Lively, 2022 ).…”