Can female buffalo gourd plants always fend themselves better than hermaphrodites against herbivores?
For dioecious plants to evolve from hermaphroditic ancestors, a male– (or female–) sterility mutation must first arise and then spread through a population, which necessitates the unisexual mutant’s fitness to exceed that of hermaphrodites. In the case of male-sterile mutants, this fitness gain is expected to occur through increased seed production or increased offspring viability when selfing is prevalent among hermaphrodites and their progeny suffer severe inbreeding depression. However, greater seed production and offspring viability –and ultimately better reproductive performance– may result from evolutionary shifts in resource allocation that allow unisexual mutants to defend themselves better from herbivores. This purported advantage of unisexual individuals may become fixed in female individuals of gynodioecious species. To test whether female plants are more resistant against herbivores than hermaphrodites, we measured herbivory levels in natural populations of the gynodioecious, perennial species Cucurbita foetidissima (buffalo gourd) in Central Mexico in two different years. These plants can be severely attacked by beetles of the genera Acalymma and Diabrotica (Chrysomelidae), and also by sucking insects like the squash bug Anasa tristis (Coreidae). In a greenhouse study, we tested whether the progeny of females was more tolerant or resistant to herbivore damage than that of hermaphrodites.
While overall foliar damage levels were low, female individuals tended to have lower levels of damage than hermaphrodites in 2002, but the trend reversed and became statistically significant in 2005. This difference between years could be due to fluctuations in insect abundance or induced resistance in the morph with more damage. We found no difference in germination rates or growth rates during the first year between the progeny of females and hermaphrodites. Preliminary analyses suggest that the progeny of hermaphrodites grow slower during the vegetative period of their second growth season, but they are more tolerant to damage than the progeny of females. Longer-term data on the natural levels of herbivory at different times during the growing season are needed for a better understanding of the fluctuations in damage levels within and among years. At this point, our results show a slight advantage of females in terms of resistance to herbivore attack. If this is a more permanent than transient trait of females, it could be contributing towards their fitness compensation and maintenance in the population.