The bodyguard: Do hermaphrodite plants shield females from floral parasites?
Organisms that interact with plants, like pollinators and herbivores, affect plant fitness and may exert selection on plant sexual systems. Across plants with separate sexes (dioecy), females experience less herbivory than males and hermaphrodites. Most plant species are hermaphroditic, but dioecy has evolved independently up to 5000 times among the angiosperms. The first step in the evolution of dioecy from hermaphroditism is usually the invasion of females and the evolution of gynodioecy, where individuals are either female or hermaphrodite. Theory suggests that hermaphrodite-biased damage could promote female invasion and the evolution of gynodioecy, by reducing hermaphrodite seed production relative to females.
To test whether hermaphrodite-biased damage to flowers and seeds could lead to the evolution of gynodioecy, we used a novel interaction between Silene vulgaris plants and Hadena ectypa moths. The moth pollinates but also lays eggs in S. vulgaris flowers. Larvae feed on floral tissues and seeds. Hadena ectypa recently shifted from a hermaphrodite host, S. stellata, to the gynodioecious S. vulgaris. We conducted population surveys to assess whether H. ectypaprefers to oviposit into hermaphrodites vs. females and whether sexually-dimorphic floral traits could explain moth oviposition preference.
In a survey of five S. vulgaris populations, Hadena ectypa oviposited in hermaphrodite flowers significantly more often than expected in one population (X2 = 7.92, df = 1, p < 0.005). A survey of floral morphology in three populations revealed that hermaphrodite flowers had larger calyces (F1, 156 = 307.58, p < 0.0001) and smaller ovaries (F1, 156 = 120.16, p < 0.0001) than female flowers. Hadena ectypa’s preference for hermaphrodites is consistent with herbivore preference data from a variety of taxa. This bias likely reflects a preference for larger flowers in H. ectypa’s original host plant, S. stellata, which is a hermaphrodite. The moth’s preference for hermaphrodite S. vulgaris could be maladaptive. The larger ovaries of female plants should provide more resources to developing H. ectypa larvae than hermaphrodite ovaries, and larvae should require fewer large, rather than more small, ovaries to complete development.
Plans for future work include developing a mathematical model of S. vulgaris sex ratio evolution under different strengths of hermaphrodite-biased oviposition, and experiments to determine the spatial scale at which sex ratio affects oviposition and the effect of plant sex ratio on oviposition choices.