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Hybrids between the acid-loving species Rhododendron ferrugineum and the basic soil species Rhododendron hirsutum occur on soils of intermediate pH in the European Alps. Material from two hybrid zones approximately 500 m apart, and also nearby populations of each parent species, was surveyed for presence/absence of 31 random amplified polymorphic DNA markers that distinguish parents. Based on morphological assessment, the material comprised 51 putative hybrids, 18 putative R. ferrugineum individuals and 26 putative R. hirsutum plants. RAPD data were analysed using a Bayesian approach implemented by the program newhybrids, and also by principal coordinates analysis. The identity of all R. ferrugineum plants examined was confirmed; however, of the putative R. hirsutum individuals examined, two were certainly and 11 possibly hybrid derivatives. Among all hybrid derivatives examined, about half were designated as F1s or a similar class, otherwise backcrosses to R. hirsutum appeared to be common whereas other hybrid classes were rare and backcrosses to R. ferrugineum possibly absent. Despite this, artificially generated seed of F2 class and backcrosses in each direction showed greater viability than one parent (R. hirsutum). Introgression from R. ferrugineum was also detected in a population that from morphology appeared to contain only R. hirsutum. Hence, the direction of backcrossing might be highly asymmetric within hybrid zones, causing unidirectional gene flow from R. ferrugineum into R. hirsutum. Conversely, the rarity of backcrosses to R. ferrugineum, F2s and later hybrid generations, which might be due to phenology effects and habitat-mediated selection, could play a part in restricting gene flow towards R. ferrugineum.

In alpine ecosystems, microscale variation in snowmelt timing often causes different flowering phenology of the same plant species and seasonal changes in pollinator activity. We compared the variations in insect visitation, pollen dispersal, mating patterns, and sexual reproduction of Rhododendron aureum early and late in the flowering season using five microsatellites. Insects visiting the flowers were rare early in the flowering season (mid-June), when major pollinators were bumblebee queens and flies. In contrast, frequent visitations by bumblebee workers were observed late in the season (late July). Two-generation analysis of pollen pool structure demonstrated that quality of pollen-mediated gene flow was more diverse late in the season in parallel with the high pollinator activity. The effective number of pollen donors per fruit (N(ep)) increased late in the season (N(ep) = 2.2-2.7 early, 3.4-4.4 late). However, both the outcrossing rate (t(m)) and seed-set ratio per fruit were smaller late in the season (t(m) = 0.89 and 0.71, seed-set ratio = 0.52 and 0.18, early and late in the season, respectively). In addition, biparental inbreeding occurred only late in the season. We conclude that R. aureum shows contrasting patterns of pollen movement and seed production between early and late season: in early season, seed production can be high but genetically less diverse and, during late season, be reduced, possibly due to higher inbreeding and inbreeding depression, but have greater genetic diversity. Thus, more pollinator activity does not always mean more pollen movement.

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.