Different reproductive modes are characterized by costs and benefits which depend on ecological contexts. For example, sex can provide benefits under complex biotic interactions, while its costs increase under mate limitation. Furthermore, ecological contexts often vary along abiotic gradients. Here, we study how these factors simultaneously influence the frequency of sex in the facultatively parthenogenetic mayfly Alainites muticus . We first verified that parthenogenesis translates into female-biased population sex ratios. We then measured the density of individuals (a proxy for mate limitation) and community diversity (biotic interaction complexity) for 159 A. muticus populations covering a broad altitudinal gradient and used structural equation modeling to investigate their direct and indirect influences on sex ratios. We found no effect of community diversity or altitude on sex ratios. Furthermore, even when females can reproduce parthenogenetically, they generally reproduce sexually, indicating that the benefits of sex exceed its costs in most situations. Sex ratios only become female-biased under low population densities, as expected if mate limitation was the main factor selecting for parthenogenesis. Mate limitation might be widespread in mayflies because of their short adult lifespan and limited dispersal, which can generate strong selection for reproductive assurance and may provide a stepping-stone towards obligate parthenogenesis.

We used samples of Alainites muticus from 159 sites (i.e., populations). These samples stem from a biodiversity monitoring survey performed at about 500 sites across Switzerland, following the IBCH method. In addition, 20 of these populations have been resampled to first verified that parthenogenesis translates into female-biased sex ratios in natural populations.

Programs/Software required to open the data files : Adobe Acrobat (.pdf file), TextEdit (.txt and .rtf files) and R software (.R files).