Abstract
Inbreeding depression is assumed to be a central factor contributing to the stability of plant mating systems. Predicting the fitness consequence of inbreeding in natural populations is complicated, however, because it may be affected by the mating histories of populations generating variation in the amount of purging of deleterious alleles. Furthermore, the level of inbreeding depression may depend on environmental conditions and the intensity of pollen competition. In a greenhouse experiment comparing four populations of the neotropical vine Dalechampia scandens (Euphorbiaceae), we tested whether inbreeding depression for early-life fitness depended on the inferred mating history of each population, as indicated by genetically determined differences in herkogamy and autofertility rates. We also tested whether the intensity of pollen competition and the level of stress encountered by the seeds and seedlings affected the amount of inbreeding depression observed. Herkogamy was a good predictor of autofertility in each population. However, we found only limited evidence for inbreeding depression in any population, and inbreeding depression varied independently of the intensity of pollen competition and amount of stress encountered by the seeds and seedlings. Thus, the population's rate of autofertility did not predict the amount of inbreeding depression. Overall, we found no evidence supporting the expectations that more inbred populations experience less inbreeding depression, and that pollen competition reduces the cost of inbreeding. These results suggest that additional factors may be responsible for the maintenance of the mixed mating systems of D. scandens populations.
Highlights
Despite more than a century of research into the causes of variation and evolution of plant mating systems, many questions remain unanswered (Karron et al 2012)
(37.6 %) of the variance in ASD occurred among populations, 16.4 % among plants within populations and the remaining 46 % within plants
For dichogamy only 7.7 % of the observed variance occurred among populations, 20.9 % among plants within populations and the remaining 71.4 % among blossoms within plants
Summary
Despite more than a century of research into the causes of variation and evolution of plant mating systems, many questions remain unanswered (Karron et al 2012). Inbreeding depression, the reduced fitness of inbred compared with outcrossed offspring, reduces the fitness advantage of selfing and is expected to be the main factor favouring outcrossing (Lloyd 1979; Charlesworth and Charlesworth 1987, but see Johnston et al 2009). Under the partial dominance hypothesis, inbreeding depression is assumed to be caused mainly by the expression of deleterious recessive alleles in inbred offspring (Charlesworth and Charlesworth 1987). The level of inbreeding depression can co-evolve with the mating system, complicating the prediction of its selective effect (Lande and Schemske 1985; Charlesworth et al 1990). The fitness costs of inbreeding are, expected to vary with the history of inbreeding in a population, and this variation might in turn influence the stability of mixed mating systems
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