同域分布的辽东栎和蒙古栎的强化选择与合子后繁殖隔离机制研究

A major goal of evolutionary biology is to understand the origin of species. Numerous empirical examples provide evidence for the allopatric speciation since the speciation is an inevitable consequence of populations evolving in allopatry given enough time. However, theoretical studies suggest that populations evolving in sympatry can also become genetically isolated and result in sympatric speciation. Unfortunately, so far, there is only less than five case studies supporting the scenario of sympatric speciation. If two closely related species experience secondary contact and grow in sympatry, hybridization is inevitable with the production of maladaptive hybrids. Under such conditions, reinforcement, a process by which selection against hybrids or mating between related taxa leads to the evolution of greater prezygotic reproductive isolation between emerging species, will be responsible for the sympatric speciation. Besides, postzygotic reproductive isolation may occur to reduce the viability or fertility of hybrid offspring. In this project, we aim to evaluate the role of reinforcing selection for prezygotic reproductive isolation and postzygotic isolation in increasing reproductive isolation between two closely related Chinese oaks, Quercus liaotungensis and Q. mongolica. There are two different kinds of contact zones for these two oaks. One is a historical contact zone located in Northeast China, the other one is a new contact zone located in North China. One of our former studies suggested that selection against hybrids has had sufficient time to reinforce the reproductive barriers between Q. liaotungensis and Q. mongolica in Northeast China but not in North China, based on the data of gene flow. This pattern means that the Q. liaotungensis and Q. mongolica provide a good system to study reinforcement and postzygotic reproductive isolation. This project plans to carry on a comparative study on reinforcing selection and postzygotic isolation in their historical and new contact zones. We will 1) perform hand pollination experiment to determine the difference in seed set between inter- and intra-fertilization, 2) perform SSR analyses and determine the multilocus genotypes of natural pollinated seeds to evaluate the hybridization rate and the direction of gene introgression, 3) genotype the seedlings and trees with different size using SSR markers to estimate the unfits of hybrids and to evaluate the postzygotic isolation, 4) compare the diverging of flowering time, pollen-stigma mechanisms, and other ecological and reproductive traits to evaluate the reinforcing selection for prezygotic isolation, and 5) compare the reinforcement and postzygotic isolation between historical and new contact zones. We try to know the role of both the reinforcement and postzygotic reproductive isolation in the increase of reproductive isolation between the two oaks.

同域分布的近缘种的杂交通常伴随着较大的杂交代价，因此近缘种可能强化合子前机制或者形成合子后机制进一步增强繁殖隔离。先前的研究大都针对异域种群开展，迄今仅有几项研究证实了同域的植物近缘种强化了合子前繁殖隔离机制，而合子后淘汰杂交个体还鲜有实证。我们之前的研究表明辽东栎和蒙古栎存在广泛的杂交，在我国东北地区，这两个物种的合子前繁殖隔离机制可能得到了一定程度的强化，共存着相当比例的纯种辽东栎和蒙古栎个体；而在华北地区，主要是两种栎树的杂交个体。这为强化选择和合子后隔离机制的研究提供了理想的比较研究实验系统。本项目拟在这两个近缘种的东北古老接触带和华北新近接触带开展人工授粉和分子标记实验，衡量杂交代价，评估合子后隔离机制的作用；比较开花时间、花粉-柱头识别、自交率等性状，评估强化选择的作用。最后比较东北古老接触带和华北新近接触带繁殖隔离机制的差异，全面衡量同域分布的辽东栎和蒙古栎的繁殖隔离机制。

同域分布的近缘种的杂交通常伴随着较大的杂交代价，因此近缘种可能强化合子前机制或者形成合子后机制进一步增强繁殖隔离。先前的研究大都针对异域种群开展，迄今仅有几项研究证实了同域的植物近缘种强化了合子前繁殖隔离机制，而合子后淘汰杂交个体还鲜有实证。我们以近缘种辽东栎和蒙古栎为对象，在其东北古老杂交带和北京新近杂交带开展了繁殖隔离机制的比较研究。结果表明东北和北京地区辽东栎和蒙古栎没有发生明显的花期分化；种间授粉和种内授粉的花粉管生长速率差别不显著；人工授粉种内和种间授粉结实率没有显著差异。这些结果表明在两个杂交带，辽东栎和蒙古栎之间都不存在明显的合子前隔离机制。利用SSR标记对自然授粉状态下的杂交率分析表明，北京地区，2013年，辽东栎自然状态的杂交率为0.551 ± 0.054，蒙古栎自然状态的杂交率为0.740 ± 0.060；2014年结果与2013年类似，即辽东栎自然状态的杂交率为0.588 ± 0.104，蒙古栎自然状态的杂交率为0.721 ± 0.045。东北地区的数据分析表明，辽东栎自然状态的杂交率为0.202 ± 0.032，蒙古栎自然状态的杂交率为0.143 ± 0.034；以辽东栎和蒙古栎分别为母本时的东北地区杂交率都显著低于北京地区；这一结果说明在古老杂交带，从合子到成熟种子的这一发育过程中，辽东栎和蒙古栎之间存在更强烈的合子后隔离机制。这一结果完全支持了我们提出的科学假说，即，相比于北京新近杂交带，在东北古老杂交带，由于二者共存时间更长，两个物种之间选择出了更强烈的种间繁殖隔离机制，从而维持了两个同域物种的种间界限。