Williams J H, Boecklen W J, Howard D J
Department of Biology, MSC 3AF, PO Box 30001, New Mexico State University, Las Cruces, NM 88003, USA.
Heredity (Edinb). 2001 Dec;87(Pt 6):680-90. doi: 10.1046/j.1365-2540.2001.00968.x.
Patterns of reproductive isolation between two sympatric species of oaks, Quercus gambelii and Q. grisea, that exhibit strong ecological differentiation were examined. A full diallel cross using four trees of each species (i.e. all possible pollinations among eight trees) was performed. This design was repeated at two sites that represent different outcomes of sympatry: (1) a xeric mountain ridge where many hybrids are established (HZ); and (2) a mesic valley bottom where virtually no hybrids are established (MOCYN). By measuring fruit survival at several developmental stages, both the timing and strength of reproductive barriers within and between sites, species, cross types, and pollen dosage levels were examined. In three of four cases, heterospecific fruit set was significantly reduced compared to conspecific fruit set. This reduction occurred after the time of fertilization, but before the onset of embryo growth. Increasing the dose of pollen from an average of 9-194 grains/stigma did not affect this result. Thus, early postfertilization processes play a strong role in species fidelity in these oaks. Quercus gambelii experienced a five-fold decrease in conspecific fruit set at HZ relative to MOCYN. In contrast, heterospecific fruit set of Q. gambelii was the same at both sites. Poor Q. gambelii pollen performance is implicated as playing the major role in this result. One Q. gambelii individual at HZ was highly fecund, and had higher heterospecific than conspecific fruit set; slight introgression in this tree was detected uisng RAPD markers. The Environmental Emasculation Hypothesis that posits that environmental stress can increase the probability of hybrid formation by reducing the competitive ability of male gametes of one species is proposed.
对两种同域分布的栎树——甘贝尔栎（Quercus gambelii）和灰栎（Q. grisea）之间的生殖隔离模式进行了研究，这两种栎树表现出强烈的生态分化。使用每个物种的四棵树进行了完全双列杂交（即八棵树之间的所有可能授粉组合）。该设计在代表同域分布不同结果的两个地点重复进行：（1）一个干旱的山脊，那里有许多杂种得以形成（HZ）；（2）一个湿润的山谷底部，那里几乎没有杂种形成（MOCYN）。通过测量果实发育几个阶段的存活率，研究了地点内和地点间、物种间、杂交类型以及花粉剂量水平的生殖障碍的时间和强度。在四个案例中的三个案例中，与同种授粉相比，异种授粉的坐果率显著降低。这种降低发生在受精之后，但在胚胎生长开始之前。将花粉剂量从平均每柱头9粒增加到194粒并没有影响这一结果。因此，受精后的早期过程在这些栎树的物种保真度中起着重要作用。相对于MOCYN，甘贝尔栎在HZ的同种授粉坐果率下降了五倍。相比之下，甘贝尔栎在两个地点的异种授粉坐果率相同。甘贝尔栎花粉表现不佳被认为是导致这一结果的主要原因。HZ的一棵甘贝尔栎个体繁殖力很强，其异种授粉坐果率高于同种授粉坐果率；使用随机扩增多态性DNA（RAPD）标记检测到这棵树有轻微的基因渗入。提出了环境去雄假说，该假说认为环境压力可以通过降低一个物种雄配子的竞争能力来增加杂种形成的概率。
