Rieseberg Loren H, Baird Stuart J E, Desrochers Andrée M
Department of Biology, Indiana University, Bloomington, Indiana, 47405.
Department of Biological Sciences, Butler University, 4600 Sunrise Avenue, Indianapolis, Indiana, 46208.
Evolution. 1998 Jun;52(3):713-726. doi: 10.1111/j.1558-5646.1998.tb03696.x.
Theory predicts that homoploid hybrid speciation will be facilitated by selfing, yet most well-documented hybrid species are outcrossers. One possible explanation for this puzzle is that conditions in hybrid populations may favor selfing, even in otherwise outcrossing species. For example, in self-incompatible plants, mixtures of self and interspecific pollen often induce selfing. Here, we examine patterns of mating in three hybrid zones and four "pure" populations of Helianthus annuus and H. petiolaris, wild, self-incompatible sunflower species that are thought to have parented three homoploid hybrid species. Fourteen to 16 maternal families from each pure population and 44-46 maternal families from each hybrid zone were analyzed for seven polymorphic isozyme loci. Maximum-likelihood (ML) methods were used to estimate multilocus outcrossing rates (T ) and hybridization frequencies for each maternal family, each phenotypic group within each hybrid zone (annuus-like, hybrid, and petiolaris-like), and each population. As predicted for self-incompatible species, all four parental populations have outcrossing rate ML estimates of 1.0. Within the hybrid zones, outcrossing rates were lowest in the H. annuus-like fraction of the population (0.73, 0.72, and 0.74 in the three hybrid zones, respectively), largely intermediate in the H. petiolaris-like group (0.94, 0.90, and 0.94), and highest in the hybrid group (0.97, 0.93, and 0.97). Although outcrossing rates are lower in hybrid zones than in pure populations, it is unlikely that the observed decrease facilitates hybrid speciation because outcrossing rates in the critical hybrid fraction of the population do not differ significantly from 1.0. Dividing the outcrossed pollen pool into intraspecific and interspecific components revealed that maternal plants are largely fertilized by conspecific pollen, confirming an important role for pollen competition as a reproductive barrier. Highly sterile hybrid plants do not appear to discriminate between parental species pollen, but hybrids with higher fertility tend to be fertilized by pollen from the parental group they resemble genetically. Thus, gametic selection leads to substantial assortative mating in these hybrid zones.
理论预测，自交将促进同倍体杂交物种形成，但大多数有充分记录的杂交物种都是异交植物。对于这个谜题的一种可能解释是，杂交种群中的条件可能有利于自交，即使在其他方面为异交的物种中也是如此。例如，在自交不亲和的植物中，自花花粉和种间花粉的混合物常常会诱导自交。在此，我们研究了向日葵（Helianthus annuus）和叶柄向日葵（H. petiolaris）这两种野生、自交不亲和的向日葵物种的三个杂交区和四个“纯”种群的交配模式，这两种向日葵被认为是三个同倍体杂交物种的亲本。对每个纯种种群的14至16个母系家族以及每个杂交区的44至46个母系家族进行了7个多态同工酶位点的分析。使用最大似然（ML）方法估计每个母系家族、每个杂交区内每个表型组（类向日葵、杂交种和类叶柄向日葵）以及每个种群的多位点异交率（T）和杂交频率。正如对自交不亲和物种所预测的那样，所有四个亲本种群的异交率ML估计值均为1.0。在杂交区内，种群中类向日葵部分的异交率最低（三个杂交区分别为0.73、0.72和0.74），在类叶柄向日葵组中大多处于中间水平（0.94、0.90和0.94），而在杂交种组中最高（0.97、0.93和0.97）。尽管杂交区的异交率低于纯种种群，但观察到的异交率下降不太可能促进杂交物种形成，因为种群中关键杂交部分的异交率与1.0没有显著差异。将异交花粉库分为种内和种间成分表明，母本植物大多由同种花粉受精，这证实了花粉竞争作为一种生殖障碍的重要作用。高度不育的杂交植物似乎不会区分亲本物种的花粉，但具有较高育性的杂交种往往由与其遗传相似的亲本群体的花粉受精。因此，配子选择导致这些杂交区内大量的选型交配。
