Ramsey Justin, Bradshaw H D, Schemske Douglas W
Biology Department, Box 355325, University of Washington, Seattle, Washington 98195, USA.
Evolution. 2003 Jul;57(7):1520-34. doi: 10.1111/j.0014-3820.2003.tb00360.x.
Evolutionists have long recognized the role of reproductive isolation in speciation, but the relative contributions of different reproductive barriers are poorly understood. We examined the nature of isolation between Mimulus lewisii and M. cardinalis, sister species of monkeyflowers. Studied reproductive barriers include: ecogeographic isolation; pollinator isolation (pollinator fidelity in a natural mixed population); pollen competition (seed set and hybrid production from experimental interspecific, intraspecific, and mixed pollinations in the greenhouse); and relative hybrid fitness (germination, survivorship, percent flowering, biomass, pollen viability, and seed mass in the greenhouse). Additionally, the rate of hybridization in nature was estimated from seed collections in a sympatric population. We found substantial reproductive barriers at multiple stages in the life history of M. lewisii and M. cardinalis. Using range maps constructed from herbarium collections, we estimated that the different ecogeographic distributions of the species result in 58.7% reproductive isolation. Mimulus lewisii and M. cardinalis are visited by different pollinators, and in a region of sympatry 97.6% of pollinator foraging bouts were specific to one species or the other. In the greenhouse, interspecific pollinations generated nearly 50% fewer seeds than intraspecific controls. Mixed pollinations of M. cardinalis flowers yielded >75% parentals even when only one-quarter of the pollen treatment consisted of M. cardinalis pollen. In contrast, both species had similar siring success on M. lewisii flowers. The observed 99.915% occurrence of parental M. lewisii and M. cardinalis in seeds collected from a sympatric population is nearly identical to that expected, based upon our field observations of pollinator behavior and our laboratory experiments of pollen competition. F1 hybrids exhibited reduced germination rates, high survivorship and reproduction, and low pollen and ovule fertility. In aggregate, the studied reproductive barriers prevent, on average, 99.87% of gene flow, with most reproductive isolation occurring prior to hybrid formation. Our results suggest that ecological factors resulting from adaptive divergence are the primary isolating barriers in this system. Additional studies of taxa at varying degrees of evolutionary divergence are needed to identify the relative importance of pre- and postzygotic isolating mechanisms in speciation.
长期以来，进化论者一直认识到生殖隔离在物种形成中的作用，但对于不同生殖屏障的相对贡献却了解甚少。我们研究了猴面花属的两个姊妹物种——粉花猴面花（Mimulus lewisii）和深红猴面花（M. cardinalis）之间隔离的本质。所研究的生殖屏障包括：生态地理隔离；传粉者隔离（自然混合种群中传粉者的忠实性）；花粉竞争（在温室中进行种间、种内和混合授粉的结实率及杂种产生情况）；以及相对杂种适合度（在温室中的发芽率、存活率、开花率、生物量、花粉活力和种子质量）。此外，根据同域种群的种子收集情况估算了自然杂交率。我们发现在粉花猴面花和深红猴面花的生活史多个阶段存在显著的生殖屏障。利用从植物标本馆收集构建的分布图，我们估计物种不同的生态地理分布导致了58.7%的生殖隔离。粉花猴面花和深红猴面花由不同的传粉者访花，在同域分布区域，97.6%的传粉者访花行为只针对其中一个物种。在温室中，种间授粉产生的种子比种内对照少近50%。即使只有四分之一的花粉处理是深红猴面花的花粉，深红猴面花花朵的混合授粉产生的亲本后代仍超过75%。相比之下，两个物种对粉花猴面花花朵的父本成功情况相似。从同域种群收集的种子中观察到的粉花猴面花和深红猴面花亲本出现率为99.915%，这与基于我们对传粉者行为的野外观察和花粉竞争的实验室实验所预期的情况几乎相同。F1杂种表现出发芽率降低、高存活率和繁殖率以及低花粉和胚珠育性。总体而言，所研究的生殖屏障平均阻止了99.87%的基因流动，大多数生殖隔离发生在杂种形成之前。我们的结果表明，适应性分化产生的生态因素是该系统中的主要隔离屏障。需要对不同进化分化程度的分类群进行更多研究，以确定合子前和合子后隔离机制在物种形成中的相对重要性。
