Recent theory indicates that plants may be able to increase their fitness by adjusting the relative investment of resources in growth and reproduction, through the male or female function, in response to alterations in resource availability throughout the growing season. Fruit production has been shown to be costly to plants in terms of carbon and mineral nutrients that, otherwise, could be used for further growth, defense, or pollen production and dispersal. Consequently, the failure of flowers to initiate and/or produce mature fruits (e.g., because lack of pollination or flower predation) should free resources for growth, defense, and additional flower production. This prediction, however, assumes that plants have the physiological mechanisms to appropriately reallocate unused resources from the female function. This study examines whether plants of Cucurbita pepo ssp. texana reallocate resources from fruit/seed production to growth and flower production in response to the removal of pistillate flowers 1 d after anthesis. This wild gourd is an annual, monoecious vine that produces a single flower per reproductive node. We found that when plants were not permitted to allocate resources to fruit and seed production, the vines grew faster, initiated more staminate and pistillate flower buds, and produced more pistillate flowers but had a smaller proportion of staminate buds that reached anthesis and fewer staminate flowers on their main shoot than plants with fruit/seed production. These findings indicate that the resources that would have gone into fruit maturation were reallocated to growth and flower bud production. However, despite an increased investment in the production of staminate flower buds, a relative surplus of resources did not result in an increase in the production of staminate flowers. Recent findings concerning the physiology of sex expression in Cucurbita indicate that the failure of staminate flower buds to develop to anthesis results from an increase in the hormonal signal (ethylene) that stimulates pistillate flower development on vines without developing fruits.

Defensive traits may evolve differently between sexes in dioecious plant species. Our current understanding of this process hinges on a partial view of the evolution of resistance traits that may result in male-biased herbivory in dioecious populations. Here, we present a critical summary of the current state of the knowledge of herbivory in dioecious species and propose alternative evolutionary scenarios that have been neglected. These scenarios consider the potential evolutionary and functional determinants of sexual dimorphism in patterns of resource allocation to reproduction, growth, and defence. We review the evidence upon which two previous reviews of sex-biased herbivory have concluded that male-biased herbivory is a rule for dioecious species, and we caution readers about a series of shortcomings of many of these studies. Lastly, we propose a minimal standard protocol that should be followed in any studies that intend to elucidate the (co)evolution of interactions between dioecious plants and their herbivores.

This study explores whether ecological factors, such as pollinators and pollen flow, or variation in pollen and ovule development account for the observed differences (approximately twofold) in the reproductive output of pin and thrum individuals of Erythroxylum havanense. The importance of ecological factors was assessed by means of comparison of the identity of pollinators and the rates of flower visitation, and by performing controlled hand pollinations and measurements of fruit set. In addition, we described the pollen and ovule development of thrum and pin individuals. Our results indicate that pollinators of E. havanense do not distinguish between floral morphs. The differences in fruit set between pin and thrum plants held even after hand pollination and, therefore, the observed differences in reproductive output between floral morphs of E. havanense cannot be explained in terms of asymmetrical pollen flow. There were no differences in the pattern of gynoecium development between the pin and thrum morphs, however androecium development showed marked differences between the morphs, and there was a resemblance between the developmental pathways leading to male sterility of the thrum morph of E. havanense with that of species with cytoplasmic male sterility (CMS).

Male sterility in hermaphroditic species may represent the first step in the evolution toward dioecy. However, gender specialization will not proceed unless the male-sterile individuals compensate for fitness lost through the male function with an increase in fitness through the female function. In the distylous shrub Erythroxylum havanense, thrum plants are partially male-sterile. Using data collected throughout eight years, we investigated whether thrum individuals have an increased performance as female parents, thereby compensating for their loss of male fitness. We found that thrum plants outperformed pins in the probabilities of seed maturation and germination and long-term growth of the seedlings. In turn, pollen from pin plants achieved greater pollen tube growth rates. Our results suggest that the superior performance of the progeny of thrum maternal plants is a consequence of better seed provisioning via effects of the maternal environment, cytotype or nuclear genes. Overall, our results suggest that E. havanense is evolving toward a dioecious state through a gynodioecious intermediate stage. This evolutionary pathway is characterized by an unusual pattern of gender dimorphism with thrums becoming females and pins becoming males. We propose that this pattern may be better explained by the interaction between male-sterility cytoplasmic genes and the heterostyly supergene.