Carr David E, Dudash Michele R
Departments of Plant Biology and Zoology, University of Maryland, College Park, Maryland, 20742.
Maryland Agricultural Experiment Station, University of Maryland, College Park, Maryland, 20742.
Evolution. 1997 Dec;51(6):1797-1807. doi: 10.1111/j.1558-5646.1997.tb05103.x.
In prior work we detected no significant inbreeding depression for pollen and ovule production in the highly selfing Mimulus micranthus, but both characters showed high inbreeding depression in the mixed-mating M. guttatus. The goal of this study was to determine if the genetic load for these traits in M. guttatus could be purged in a program of enforced selfing. These characters should have been under much stronger selection in our artificial breeding program than previously reported characters such as biomass and total flower production because, for example, plants unable to produce viable pollen could not contribute to future generations. Purging of genetic load was investigated at the level of both the population and the individual maternal line within two populations of M. guttatus. Mean ovule number, pollen number, and pollen viability declined significantly as plants became more inbred. The mean performance of outcross progeny generated from crosses between pairs of maternal inbred lines always exceeded that of self progeny and was fairly constant for each trait through all five generations. The consistent performance of outcross progeny and the universally negative relationships between performance and degree of inbreeding are interpreted as evidence for the weakness of selection relative to the quick fixation of deleterious alleles due to drift during the inbreeding process. The selective removal (purging) of deleterious alleles from our population would have been revealed by an increase in performance of outcross progeny or an attenuation of the effects of increasing homozygosity. The relationships between the mean of each of these traits and the expected inbreeding coefficient were linear, but one population displayed a significant negative curvilinear relationship between the log of male fertility (a function of pollen number and viability) and the inbreeding coefficient. The generally linear form of the responses to inbreeding were taken as evidence consistent with an additive model of gene action, but the negative curvilinear relationship between male fertility and the inbreeding coefficient suggested reinforcing epistasis. Within both populations there was significant genetic variation among maternal lineages for the response to inbreeding in all traits. Although all inbred lineages declined at least somewhat in performance, several maternal lines maintained levels of performance just below outcross means even after four or five generations of selfing. We suggest that selection among maternal lines will have a greater effect than selecting within lines in lowering the genetic load of populations.
在之前的研究中，我们发现在高度自交的小花沟酸浆中，花粉和胚珠产量没有显著的近交衰退，但在混合交配的斑点沟酸浆中，这两个性状都表现出高度的近交衰退。本研究的目的是确定在强制自交程序中，斑点沟酸浆这些性状的遗传负荷是否可以被清除。在我们的人工育种程序中，这些性状应该比之前报道的生物量和总花产量等性状受到更强的选择，因为例如，无法产生有活力花粉的植物无法对后代有贡献。在斑点沟酸浆的两个种群中，我们在种群和单个母系水平上研究了遗传负荷的清除情况。随着植物自交程度的增加，平均胚珠数、花粉数和花粉活力显著下降。由成对母系自交系之间杂交产生的异交后代的平均表现总是超过自交后代，并且在所有五代中每个性状都相当稳定。异交后代的一致表现以及表现与近交程度之间普遍存在的负相关关系被解释为，相对于在近交过程中由于漂变导致有害等位基因的快速固定，选择作用较弱的证据。从我们的种群中选择性地去除（清除）有害等位基因，本应通过异交后代表现的提高或纯合度增加效应的减弱来揭示。这些性状各自的平均值与预期近交系数之间的关系是线性的，但有一个种群在雄性育性（花粉数和活力的函数）的对数与近交系数之间表现出显著的负曲线关系。对近交的反应通常呈线性形式，这被视为与基因作用的加性模型一致的证据，但雄性育性与近交系数之间的负曲线关系表明存在加强上位性。在两个种群中，所有性状对近交的反应在母系谱系之间都存在显著的遗传变异。尽管所有自交系的表现至少都有所下降，但即使经过四五代自交，仍有几个母系的表现水平略低于异交平均值。我们认为，在降低种群遗传负荷方面，选择母系比在系内选择会有更大的效果。
