Data from: Male offspring production by asexual Potamopyrgus antipodarum, a New Zealand snail

As only females contribute directly to population growth, sexual females
investing equally in sons and daughters experience a two-fold cost
relative to asexuals producing only daughters. Typically, researchers have
focused on benefits of sex that can counter this ‘cost of males’ and thus
explain its predominance. Here, we instead ask whether asexuals might also
pay a cost of males by quantifying the rate of son production in 45
experimental populations (‘lineages’) founded by obligately asexual female
Potamopyrgus antipodarum. This New Zealand snail is a powerful model for
studying sex because phenotypically similar sexual and asexual forms often
coexist, allowing direct comparisons between sexuals and asexuals. After 2
years of culture, 23 of the 45 lineages had produced males, demonstrating
that asexual P. antipodarum can make sons. We used maximum-likelihood
analysis of a model of male production in which only some lineages can
produce males to estimate that ~50% of lineages have the ability to
produce males and that ~5% of the offspring of male-producing lineages are
male. Lineages producing males in the first year of the experiment were
more likely to make males in the second, suggesting that some asexual
lineages might pay a cost of males relative to other asexual lineages.
Finally, we used a simple deterministic model of population dynamics to
evaluate how male production affects the rate of invasion of an asexual
lineage into a sexual population, and found that the estimated rate of
male production by asexual P. antipodarum is too low to influence invasion
dynamics.