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

The evolution and maintenance of sexual systems in plants is often driven by resource allocation and pollinator preferences, and very little is known about their role in determining floral sex expression in plants. In annual, entomophilous plants three major constraints can be identified towards optimal reproduction: 1) nutrient resources available from the environment, 2) nutrient resources allocated towards reproduction, i.e., fruits vs. flowers, and 3) pollinator visitations. Andromonoecy is a sexual system where plants bear both staminate and hermaphrodite flowers on the same inflorescence. The optimal resource allocation hypothesis suggests that under nutrient constraints, plants will produce more male flowers since they are energetically cheaper to produce over the more expensive hermaphrodite flowers. We test this hypothesis in the andromonoecious Murdannia simplex (Commelinaceae) by quantifying male and hermaphrodite flowers in a natural population and contrasting the distribution of the two sexes in plants from two resource conditions (stream population vs. plateau population). We next carried out choice experiments to test pollinator preference towards a specific sex. We found that in M. simplex , production of hermaphrodite flowers is resource-dependent and under resource constraints fewer numbers of flowers were produced and most of them were males. We failed to observe pollinator preference towards either sex but Amegilla spp . and Apis cerana showed higher visitation towards the most abundant sex within a trial, suggesting frequency-dependent visitation. Thus, we conclude that environmentally driven resource constraints play a bigger role in driving floral sex expression in Murdannia over direct pollinator-driven constraints.