Departamento de Biología Vegetal y Ecología, Universidad de Sevilla, C/ Profesor García González, 2, E41012, Seville, Spain.
Ann Bot. 2010 Dec;106(6):999-1008. doi: 10.1093/aob/mcq200. Epub 2010 Oct 5.
Variation in inbreeding depression (δ) 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 δ 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 δ at the individual plant level. This study assesses the extent of among-individual variation in PL and δ, the consistency of δ across life stages, and the relationships between individual PL and δ 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 δ varied considerably across life stages and individual plants. Cumulative δ was high across individuals (mean δ = 0·65), although there were no positive correlations between δ values at different life stages. Interestingly, maternal plants showing stronger δ for seed production were more pollen limited, but they were also less affected by δ for seedling growth because of a seed size/number trade-off.
Results show a general inconsistency in δ across life stages and individuals, suggesting that different deleterious loci are acting at different stages. The association between δ and PL at the individual level corroborates the idea that pollen limitation may be 'genotype-dependent' regardless of other factors.
个体植物间的近交衰退（δ）的变异被认为在交配系统进化和种群遗传学中起着核心作用。此外，这种变异可能与个体对花粉限制（PL）的易感性有关，因为那些因 δ 而强烈影响种子产量的个体，为了产生一定数量的果实或种子，将需要更多的异交花粉。然而，目前还没有研究明确检验个体植物水平上 PL 和 δ 之间的关联。本研究评估了 PL 和 δ 的个体间变异性程度、δ 在不同生命阶段的一致性，以及在混交灌木桃金娘中个体 PL 和 δ 之间的关系。
在自然的桃金娘种群中进行了人工控制的授粉实验。标记花朵，直到果实产生，并在温室中对产生的种子进行实验。
与自交相比，异交授粉增加了每果种子数、发芽率和幼苗生长，但不会增加结实率。只有每果种子数受到花粉限制，因此，累积花粉限制更多地取决于花粉质量（异交花粉），而不是花粉数量。δ 的效应在不同的生命阶段和个体植物中差异很大。个体间的累积 δ 很高（平均 δ = 0.65），尽管不同生命阶段的 δ 值之间没有正相关关系。有趣的是，在种子生产中表现出更强 δ 的母株，受到的花粉限制更大，但由于种子大小/数量的权衡，它们受到的 δ 对幼苗生长的影响也较小。
结果表明，δ 在不同的生命阶段和个体间存在普遍的不一致性，这表明不同的有害基因座在不同的阶段起作用。个体水平上 δ 和 PL 的关联证实了花粉限制可能是“基因型依赖”的观点，而不受其他因素的影响。
