Sex and flowers: testing the resource-dependent selection hypothesis for flower sex allocation

Context: Monoecious plants can adjust their proportional investment in male and female flowers to maximise reproductive fitness. The female reproductive function (seeds) often has greater resource costs than the male (pollen). Larger plants are generally thought to have greater resource availability and should have a female biased sex ratio, referred to as the size-dependent selection hypothesis. However, empirical tests of this hypothesis have found mixed support. This may be because size alone is not always a reliable proximate value for resource availability, which can be influenced by other abiotic factors. Aims: Breynia oblongifolia (Phyllanthaceae) is a perennial monoecious plant with unisexual moth-pollinated flowers from eastern Australia. Fruit production in Breynia is heavily influenced by rainfall, which is highly variable. We hypothesised that where soil moisture limits female function, Breynia would produce more male flowers (i.e. resource-dependent selection). Methods: We used a multi-year observational dataset to look for evidence of resource-dependent flower sex ratios in a wild population and conducted a manipulative glasshouse experiment to test alternative hypotheses for flower sex selection. Key results: In both our manipulative glasshouse experiment and observed wild population, decreasing soil water content resulted in higher proportions of male flowers, supporting the resource-dependent sex selection hypothesis. Conclusions: Soil moisture influences flower sex ratios but plant size does not. Implications: Future studies should not assume that height equates to resource wealth, as this is often overly simplistic and ignores the potential for key resources, like soil moisture or light, to fluctuate.