Among-individual variation in pollen limitation and inbreeding depression in a mixed-mating shrub

Variation in inbreeding depression (delta) among individual plants is considered to play a central role in mating system evolution and population genetics. Moreover, such variation could be linked to individual susceptibility to pollen limitation (PL) because those individuals strongly affected by delta for seed production will require more outcross pollen for setting a given number of fruits or seeds. However, no study has tested explicitly for associations between PL and delta at the individual plant level. This study assesses the extent of among-individual variation in PL and delta, the consistency of delta across life stages, and the relationships between individual PL and delta in the mixed-mating shrub Myrtus communis. Controlled hand-pollinations were performed in a natural M. communis population. Marked flowers were monitored until fruit production and a greenhouse experiment was conducted with the seeds produced. Compared with selfing, outcross-pollination enhanced seed number per fruit, germination rate and seedling growth, but did not enhance fruit-set. Only seed number per fruit was pollen limited and, thus, cumulative pollen limitation depended more on pollen quality (outcross pollen) than on quantity. The effects of delta varied considerably across life stages and individual plants. Cumulative delta was high across individuals (mean delta = 0 center dot 65), although there were no positive correlations between delta values at different life stages. Interestingly, maternal plants showing stronger delta for seed production were more pollen limited, but they were also less affected by delta for seedling growth because of a seed size/number trade-off. Results show a general inconsistency in delta across life stages and individuals, suggesting that different deleterious loci are acting at different stages. The association between delta and PL at the individual level corroborates the idea that pollen limitation may be 'genotype-dependent' regardless of other factors.