Data from: Frequency-dependent selection acting on the widely fluctuating sex ratio of the aphid Prociphilus oriens

Frequency-dependent selection is a fundamental principle of adaptive
sex-ratio evolution in all sex ratio theories but has rarely been detected
in the wild. Through long-term censuses, we confirmed large fluctuations
in the population sex ratio of the aphid Prociphilus oriens and detected
frequency-dependent selection acting on these fluctuations. Fluctuations
in the population sex ratio were partly attributable to climatic factors
during the growing season. Climatic factors likely affected the growth
conditions of host plants, which in turn led to yearly fluctuations in
maternal conditions and sex ratios. In the process of frequency-dependent
selection, female proportion higher or lower than ca. 60% was associated
with a reduction or increase in female proportion, respectively, the next
year. The rearing of aphid clones in the laboratory indicated that mothers
of each clone produced an increasing number of females as maternal size
increased. However, the mean male number was not related to maternal size,
but varied largely among clones. Given genetic variance in the ability to
produce males among clones, selection should favor clones that can produce
more numerous males in years with a high female proportion.
Population-level sex allocation to females was on average 71%–73% for
three localities and more female-biased when maternal conditions were
better. This tendency was accounted for by the hypothesis of competition
among foundresses rather than the hypothesis of local mate competition. We
conclude that despite consistent operation of frequency-dependent
selection, the sex ratio continues to fluctuate because environmental
conditions always push it away from equilibrium.