A hybrid zone dominated by fertile F₁s: Maintenance of species barriers in <em>Rhododendron</em>

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 F(1)s, and that there are many separate genets present. Hand pollination and germination experiments showed that R. x sochadzeae plants can produce viable seed of F(2)s or backcrosses in either direction. Furthermore, adult backcrosses have been observed in habitats atypical for R. x sochadzeae. From this, all non-F-1 hybrid derivatives appear to be eliminated in the hybrid zone at Tiryal Dag as a result of postgermination selection. This absence of post-F-1 hybrid derivatives apparently prevents introgression. This type of hybrid population is here termed an F-1-dominated hybrid zone (F(1)DZ), and also occurs in Encelia. The observed dominance of F(1)s within a narrow habitat range is best explained by habitat-mediated superiority of F(1)s over all other genotype classes. Therefore, habitat-mediated selection against the second hybrid generation might be preventing interspecific gene flow in R. x sochadzeae. F(1)DZ formation is postulated to require the formation of F(1)s in quantity, habitat-mediated superiority in F(1)s, and highly specific habitat conditions.