Department of Biological Sciences, California State University, Chico, CA 95929-0515, USA.
Ann Bot. 2012 Feb;109(3):583-98. doi: 10.1093/aob/mcr160. Epub 2011 Aug 10.
Self-fertilizing taxa are often found at the range margins of their progenitors, where sub-optimal habitats may select for alternative physiological strategies. The extent to which self-fertilization is favoured directly vs. arising indirectly through correlations with other adaptive life history traits is unclear. Trait responses to selection depend on genetic variation and covariation, as well as phenotypic and genetic responses to altered environmental conditions. We tested predictions of the hypothesis that self-fertilization in Mimulus arises through direct selection on physiological and developmental traits that allow seasonal drought escape.
Phenotypic selection on mating system and drought escape traits was estimated in field populations of M. guttatus. In addition, trait phenotype and phenotypic selection were compared between experimental wet and dry soil in two greenhouse populations each of M. guttatus and M. nasutus. Finally, genetic variation and covariation for traits were compared between wet and dry soil treatments in a greenhouse population of M. guttatus.
Consistent with predictions, selection for early flowering was generally stronger than for mating system traits, and selection for early flowering was stronger in dry soil. Inconsistent with predictions, selection for water-use efficiency was largely absent; selection for large flowers was stronger than for drought escape in the field; and most drought escape and mating system traits were not genetically correlated. A positive genetic correlation between flowering time and flower size, which opposed the adaptive contour, emerged only in wet soil, suggesting that variation in water availability may maintain variation in these traits. Plastic responses to soil moisture treatments supported the idea that taxonomic divergence could have been facilitated by plasticity in flowering time and selfing.
The hypothesis that plant mating systems may evolve indirectly via selection on correlated life history characteristics is plausible and warrants increased attention.
自交种通常存在于其祖先的分布边缘，那里的次优生境可能选择替代的生理策略。自交是否直接受到青睐，还是通过与其他适应性生活史特征的间接关联而受到青睐，目前还不清楚。性状对选择的反应取决于遗传变异和协变，以及对环境变化的表型和遗传反应。我们测试了一个假设的预测，即在 Mimulus 中自交是通过对允许季节性干旱逃避的生理和发育特征的直接选择而产生的。
在 M. guttatus 的野外种群中，估计了交配系统和干旱逃避性状的表型选择。此外，在两个温室种群中，比较了 M. guttatus 和 M. nasutus 每个种群的实验湿土和干土条件下的性状表型和表型选择。最后，在 M. guttatus 的一个温室种群中，比较了干湿土处理下性状的遗传变异和协变。
与预测一致，早期开花的选择通常强于交配系统特征的选择，而在干旱土壤中，早期开花的选择更强。与预测不一致的是，对水利用效率的选择基本上不存在；在野外，大花的选择比干旱逃避的选择更强；大多数干旱逃避和交配系统特征没有遗传相关性。在湿土中，仅出现了与适应轮廓相反的开花时间和花大小之间的正遗传相关性，表明水分供应的变化可能维持了这些性状的变异。对土壤水分处理的可塑性反应支持了这样一种观点，即通过开花时间和自交的可塑性，可能促进了分类学的分化。
植物交配系统可能通过对相关生活史特征的间接选择而进化的假设是合理的，值得进一步关注。
