UVM Theses and Dissertations
Format:
Print
Author:
Clarke, Gretel Liesl
Dept./Program:
Biology
Year:
2014
Degree:
Ph. D.
Abstract:
Most flowering plants are hermaphroditic, reproducing through seed set and seed siring. In gynodioecious species, some individuals are female and only reproduce through seed set. Because females do not gain fitness through pollen, they must have a seed fitness advantage over hermaphrodites to be maintained. Mutualists and antagonists interacting with plants may generate this advantage. We evaluated whether pollinators (Bombus sp.), pre-dispersal seed predator flies (Hylemya sp.), and vertebrate herbivores help explain the persistence of females in the gynodioecious Polemonium foliosissimum.
We found no evidence that pollinator service differed between the sex morphs; neither fruit set nor seeds per fruit differed between them. However, fruits on female plants were attacked less often by Hylemya, and females set more seed. Physical removal of seed predator eggs increased seed set more than did supplemental pollen addition, and hand pollination and reduction in seed predation had additive effects on seed set. In addition, destruction of fruits by seed predators reduced seed set of hermaphrodites more than females. Thus, we concluded that seed predators, not pollinators, most strongly affect the female seed set advantage in P. foliosissimum.
We tested the mechanisms underlying sex-biased predation and found that flies preferentially oviposit on hermaphrodites and may be partly explained by earlier flowering phenology and larger flowers of hermaphrodites; oviposition rates decline over the season and with flower size. Larval survival was also higher on hermaphrodites, and may be partly explained by the earlier phenology and larger flowers of hermaphrodites; larval survival also declined over the season and with flower size. Thus flies appear to be 'wise' in preferentially ovipositing on hermaphrodites. From the plant point of view, females experience a lower rate of predation both because they receive fewer eggs and because larval mortality is higher on females than hermaphrodites.
All of the studies we know of to evaluate whether herbivores generate fitness differences between the sex morphs used seed set in one or a few years. However, one should ideally use demographic modeling to compare lifetime seed production, incorporating differences in survivorship or growth, likely important to lifetime seed production. Females had a higher germination rate of seeds, and greater survival and growth of medium sized reproductive plants. Females also produced more seed in high sex ratio populations (high proportion of female plants), where seed predation was hermaphrodite-biased. However, even inthese populations, females did not have a lifetime advantage (female: hermaphrodite [lambda] was <1) in preliminary models where we assumed females and hermaphrodites produced 30% and 5% female progeny, respectively. Females only had a lifetime advantage if they produced more than ~50% female offspring. Thus, female persistence in P. foliosissimum may be context-dependent, perhaps occurring where cytoplasmic male sterility alleles are at a high frequency due to stochastic processes such as drift and founder effects.
We found no evidence that pollinator service differed between the sex morphs; neither fruit set nor seeds per fruit differed between them. However, fruits on female plants were attacked less often by Hylemya, and females set more seed. Physical removal of seed predator eggs increased seed set more than did supplemental pollen addition, and hand pollination and reduction in seed predation had additive effects on seed set. In addition, destruction of fruits by seed predators reduced seed set of hermaphrodites more than females. Thus, we concluded that seed predators, not pollinators, most strongly affect the female seed set advantage in P. foliosissimum.
We tested the mechanisms underlying sex-biased predation and found that flies preferentially oviposit on hermaphrodites and may be partly explained by earlier flowering phenology and larger flowers of hermaphrodites; oviposition rates decline over the season and with flower size. Larval survival was also higher on hermaphrodites, and may be partly explained by the earlier phenology and larger flowers of hermaphrodites; larval survival also declined over the season and with flower size. Thus flies appear to be 'wise' in preferentially ovipositing on hermaphrodites. From the plant point of view, females experience a lower rate of predation both because they receive fewer eggs and because larval mortality is higher on females than hermaphrodites.
All of the studies we know of to evaluate whether herbivores generate fitness differences between the sex morphs used seed set in one or a few years. However, one should ideally use demographic modeling to compare lifetime seed production, incorporating differences in survivorship or growth, likely important to lifetime seed production. Females had a higher germination rate of seeds, and greater survival and growth of medium sized reproductive plants. Females also produced more seed in high sex ratio populations (high proportion of female plants), where seed predation was hermaphrodite-biased. However, even inthese populations, females did not have a lifetime advantage (female: hermaphrodite [lambda] was <1) in preliminary models where we assumed females and hermaphrodites produced 30% and 5% female progeny, respectively. Females only had a lifetime advantage if they produced more than ~50% female offspring. Thus, female persistence in P. foliosissimum may be context-dependent, perhaps occurring where cytoplasmic male sterility alleles are at a high frequency due to stochastic processes such as drift and founder effects.