Abstract
The persistence of mixed mating systems in plants, in spite of theory suggesting that either complete outcrossing or selfing should evolve depending on the level of inbreeding depression, has become a classic puzzle in evolutionary biology. Despite the central role of inbreeding depression in the theory for the evolution of mating systems, the majority of published estimates of inbreeding depression are for annual species, are incomplete, or are measured in artificial environments, calling into question their general utility. I measured inbreeding depression in the field and greenhouse, for all life history stages including male and female reproductive fitness, of a perennial cleistogamous species. I found greater inbreeding depression in the greenhouse (33%) than in the field (11%), but in both cases, inbreeding depression was lower than the threshold required by models to promote outcrossing; however there was considerable variation in the magnitude of inbreeding depression expressed among maternal plants. Inbreeding depression alone is unlikely to be sufficient to explain the persistence of mixed mating in this species.
Cleistogamy, a system of dimorphic flowering in which individuals produce both open flowers, and closed obligately selfing flowers, is a mixed mating system which appears to be stable. I reviewed mathematical and verbal models of the maintenance of cleistogamy, as well as relevant published empirical data. Mathematical models require an inherent advantage of CH outcrossing, but do not address the mechanism for such an advantage. I found that CL flowers are generally cheaper to produce, have a higher probability of fertilization, and produce progeny that experience relatively low levels of inbreeding depression. Verbal bet-hedging models provide the most general explanation for the maintenance of CH flower production, given the many advantages of CL reproduction. Future theoretical and empirical work should address how temporal variation, in selection for genetically variable progeny, or in male outcrossing success may act to stabilize mixed CH/CL.
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