Inbreeding Depression and Selfing Rates in Hermaphroditic Species of Schiedea (Caryophyllaceae): A Comparison to Diclinous Species

不像大多数动物的个体有单独的雄性和雌性，植物在繁殖系统中表现出很大的范围。大多数植物是雌雄同体的，在同一朵花上具有雄性和雌性的功能，但许多其他物种，包括一些重要的作物植物，在同一株植物上有单独的雄花和雌花，或者由单独的雄花和雌花组成（雌雄异株）。人们提出了许多不同的假说来解释植物育种系统的多样性和雌雄异株的进化。一个主要的论点认为，雌雄异株的进化是为了避免近亲繁殖的衰退。近亲繁殖衰退发生在近亲交配的个体，其后代表现出活力和繁殖能力的下降。如果近交抑制是一个因素，那么人们应该期望在进化为雌雄异株的种群中发现高水平的近交自花授粉和高水平的近交抑制。虽然已经提出了许多理论，但关于这个问题的经验数据很少。我们正在研究近交抑制在夏威夷植物属雪兰雌雄异株进化中的作用。这个群体显然是在进化雌雄异株，并在非常密切相关的物种中表现出一系列的繁殖系统。由于栖息地的破坏、食草动物的放牧以及与外来植物的竞争，夏威夷脆弱的生态系统中许多重要的中间物种濒临灭绝，因此这项研究具有特殊的紧迫性。了解促进植物育种系统变化的因素对农业和林业的实际问题具有许多潜在的育种应用。众所周知，近交衰退会影响许多重要经济作物的产量，如玉米。了解导致雌雄异株的因素可能会提高控制植物性别比例的能力，在许多作物品种的产量取决于雌性的果实或种子产量的情况下，这种知识具有巨大的价值。了解单一性的基础对杂交菌株的生产也很重要，因为雌性个体很容易杂交，而不需要对雌雄同体花进行耗时昂贵的阉割。

Unlike most animals which have individuals with separate male and female sexes, plants exhibit a great range in breeding systems. Most plant species are hermaphroditic with male and female function in the same flower, but many other species, including a number of important crop plants, have separate male and female flowers on the same plant or consist of populations of separate male and female plants (dioecy). Many different hypotheses have been proposed to explain the diversity of breeding systems in plants and the evolution of dioecy. One major argument suggests that dioecy has evolved to avoid inbreeding depression. Inbreeding depression occurs when closely related individuals mate and the resulting offspring show reduced vigor and ability to reproduce. If inbreeding depression is a factor, then one should expect to find both high levels of inbreeding self-pollination occurring as well as high levels of inbreeding depression in populations that are evolving dioecy. Although there have been many theories proposed, very little empirical data on this question is available. We are examining the role of inbreeding depression in the evolution of dioecy in the Hawaiian plant genus Schiedea. This group is apparently evolving dioecy and exhibits a range of breeding systems in very closely related species. This research is of special urgency because many important intermediate species from the fragile Hawaiian ecosystems are near extinction because of habitat destruction, grazing introduced by herbivores and competition with introduced plants. Knowledge of factors promoting changes in plant breeding systems has many potential breeding applications to practical problems in agriculture and forestry. Inbreeding depression is well known to influence yield in many economically important crop plants, such as corn. Knowledge of factors causing dioecy may lead to greater ability to manipulate sex ratios in plants and is of tremendous value where yield depends upon the production of fruit or seed by the females as is the case of many crop species. Understanding the basis of unisexaulity will also be important in production of hybrid strains, since female individuals are easily hybridized without the necessity of time-consuming costly emasculation of hermaphroditic flowers.