Milne R I, Terzioglu S, Abbott R J
Division of Environmental and Evolutionary Biology, School of Biology, University of St Andrews, St Andrews, Fife KY16 9TH, UK.
Mol Ecol. 2003 Oct;12(10):2719-29. doi: 10.1046/j.1365-294x.2003.01942.x.
Isolating barriers between interbreeding sympatric or parapatric interfertile species are maintained by processes that occur within their hybrid zones. Although the effects of intrinsic selection on hybrid fitness are well known, less is understood about extrinsic fitness variation. At Tiryal Dag, northeast Turkey, Rhododendron x sochadzeae (R. ponticum x caucasicum) forms large populations in which neither segregation nor backcrossing occur, in habitats intermediate between those of its parents. Using single-copy species-specific random amplified polymorphic DNA and inter simple sequence repeat markers, it was determined that most or all R. x sochadzeae plants are F1s, and that there are many separate genets present. Hand pollination and germination experiments showed that R. x sochadzeae plants can produce viable seed of F2s or backcrosses in either direction. Furthermore, adult backcrosses have been observed in habitats atypical for R. x sochadzeae. From this, all non-F1 hybrid derivatives appear to be eliminated in the hybrid zone at Tiryal Dag as a result of postgermination selection. This absence of post-F1 hybrid derivatives apparently prevents introgression. This type of hybrid population is here termed an F1-dominated hybrid zone (F1DZ), and also occurs in Encelia. The observed dominance of F1s within a narrow habitat range is best explained by habitat-mediated superiority of F1s over all other genotype classes. Therefore, habitat-mediated selection against the second hybrid generation might be preventing interspecific gene flow in R. x sochadzeae. F1DZ formation is postulated to require the formation of F1s in quantity, habitat-mediated superiority in F1s, and highly specific habitat conditions.
杂交区域内发生的一些过程维持了同域或邻域可育杂交物种之间的隔离屏障。虽然内在选择对杂种适合度的影响已为人熟知，但对外在适合度变异的了解却较少。在土耳其东北部的蒂里亚尔山，杂交杜鹃（Rhododendron x sochadzeae，由ponticum杜鹃和caucasicum杜鹃杂交而成）在介于其亲本栖息地之间的生境中形成了大量种群，其中既没有分离现象也没有回交现象。利用单拷贝物种特异性随机扩增多态性DNA和简单序列重复区间标记，确定了大多数或所有杂交杜鹃植株都是F1代，并且存在许多不同的基因株。人工授粉和发芽实验表明，杂交杜鹃植株能够产生F2代或双向回交的可育种子。此外，在杂交杜鹃非典型的栖息地中也观察到了成年回交植株。由此可见，由于发芽后选择的结果，所有非F1代杂种衍生物似乎在蒂里亚尔山的杂交区域中被淘汰了。这种F1代后杂种衍生物的缺失显然阻止了基因渗入。这种类型的杂交种群在这里被称为F1代主导的杂交区域（F1DZ），在Encelia属中也有出现。在狭窄的栖息地范围内观察到的F1代优势现象，最好的解释是F1代在栖息地介导下优于所有其他基因型类别。因此，栖息地介导的对第二代杂种的选择可能正在阻止杂交杜鹃的种间基因流动。据推测，F1DZ的形成需要大量F1代的形成、F1代在栖息地介导下的优势以及高度特定的栖息地条件。
