植物交配系统的进化、资源分配对策与遗传多样性

影响植物自交率进化的选择力量主要体现在两个方面：当外来花粉量不足时，自交可以提高植物的结实率，即雌性适合度（繁殖保障）；而如果进行自交的花粉比异交花粉更易获得使胚珠受精的机会，那么自交也可以提高植物的雄性适合度（自动选择优势）。但是，鉴别什么时候是繁殖保障、什么时候是自动选择优势导致了自交的进化却是极其困难的。花粉贴现降低了自交植物通过异交花粉途径获得的适合度，即减弱了自动选择优势，而近交衰退既减少了自动选择优势也减少了繁殖保障给自交者带来的利益。具有不同交配系统的植物种群将具有不同的资源分配对策。理论研究已经说明，自交率增加将减少植物对雄性功能的资源分配比例，但将使繁殖分配加大，而且在一定条件下交配系统的改变甚至可以导致植物生活史发生剧烈变化，即从多年生变为一年生。文献中支持自交减少植物雄性投入的证据有很多，但是对繁殖分配与自交率的关系目前还没有系统的研究。资源分配理论可以解释植物繁育系统的多样性，尤其是能够说明为什么大多数植物都是雌雄同体的。自交对植物种群遗传结构的影响是减少种群内的遗传变异，增加种群间的遗传分化。长期以来人们一直猜测，自交者可能会丢掉一些长期进化的潜能，目前这个假说得到了一些支持。

MATING SYSTEM EVOLUTION, RESOURCE ALLOCATION, AND GENETIC DIVERSITY IN PLANTS

Two basic forces affect the evolution of plant mating systems.Selfing will be favored if it increases seed set when polle n is limiting (reproductive assurance) or increases siring success when pollen d evoted to selfing is more likely to accomplish fertilization than pollen devoted to outcrossing (automatic selection advantage). Experimental and observational studies, however, have yet to identify more precisely the set of ecological cond itions under which either of those advantages can be expressed. Pollen discounti ng reduces a selfers' success as an outcross pollen parent, i.e. lowering au tomatic selection advantage, and inbreeding depression is expected to reduce any advantage selfers might gain through reproductive assurance or automatic selection. Changes in a species' mating system are likely to induce evolutionary adjus tments in its resource allocation. Theory predicts that sex allocation, or the proportion of reproductive resources allocated to male function in hermaphroditic plants, decreases as the selfing rate increases, and that total reproductive al location increases with increased selfing if inbreeding depression is not so sev ere as to prevent the evolution of self-fertilization. In some extreme conditio ns, increased selfing may lead to severe change in life history, i.e. from peren niality to annuality. Sex allocation theory provides a general explanation for diversity in breeding systems, in particular, it explains why most flowering plan ts are hermaphroditic. While empirical evidence is abundant that supports the ex pected decrease in sex allocation as selfing rate increases, there has been no demonstration of the expected correlation between mating system and total reprodu ctive effort. Selfing tends both to reduce the level of genetic variability with in populations and to increase the amount of genetic differentiation among popul ations. A longtime conjecture that selfers may give up some long-term evolution ary flexibility has now been partially confirmed.