When to be a male? Role of resource-limitation and pollinators in determining gender in an andromonoecious spiderwort

Abstract The evolution and maintenance of sexual systems in plants is often driven by resource allocation and pollinator preferences. Gender or sex in plants, therefore, represents a physiological, functional, and behavioral response of a plant to its environment. Annual, entomophilous plants have three major constraints towards optimal reproduction: 1) nutrient resources obtained from the environment, 2) nutrient resources allocated towards reproduction, i.e., fruits vs. flowers, and 3) pollinator visitations. We know very little about the role of nutrient resources and pollinators in driving gender expression in flowers in a natural environment. Andromonoecy is a sexual system where the plant bears both staminate and hermaphrodite flowers on the same inflorescence and it occurs in ～2% of flowering plants. Although it has been shown that andromonoecy has evolved multiple times within the angiosperms, our understanding of the underlying mechanisms of what determines gender expression in nature is limited. The optimal resource allocation hypothesis, one of the most widely accepted hypotheses which describes evolution of andromonoecy, suggests that under nutrient constraints, plants will produce more male flowers since they require lesser energy investment than the more expensive hermaphrodite flowers. We test this hypothesis in an andromonoecious plant Murdannia simplex (Commelinaceae) by measuring gender distributions in a large natural population and contrasting the gender distributions between two different resource conditions (stream population vs. plateau population). We also carried out choice experiments to quantify if pollinators showed any preference towards a gender. We found that in M. simplex, production of hermaphrodite flowers is resource-dependent and under resource constraints not only fewer numbers of flowers were produced but also there were a higher number of male flowers. We failed to observe pollinator preference towards either gender but Amegilla spp. (Zonamegilla) and Apis cerana showed frequency-dependent visitation which suggests that selection on inflorescence size may be an indirect form of selection pressure exerted by pollinators resulting in gender-bias in the plants. Thus, we conclude that environmentally driven resource constraints play a bigger role in driving floral gender expression in Murdannia over direct pollinator-driven constraints.