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Hybridization among sympatric species of Rhododendron (Ericaceae) in Turkey: morphological and molecular evidence¹

¹ School of Environmental and Evolutionary Biology, University of St Andrews, St Andrews, Fife KY16 9TH, UK; and ² Royal Botanic Garden, Inverleith Row, Edinburgh EH20 3LR, UK

Rhododendron (Ericaceae) is a large genus in which barriers to hybridization are especially weak, but many species are maintained in sympatry. Hybridization among four species of Rhododendron subsect. Pontica, which occur in sympatry in Turkey, was investigated. Material of R. ponticum, R. smirnovii, R. ungernii, and R. caucasicum and their putative hybrids was collected from the wild. Based on morphology, chloroplast DNA and nuclear ribosomal DNA restriction fragment length polymorphism (RFLP) profiles, each accession was identified as a species or hybrid combination. Five of the six possible hybrid combinations among the four species were detected. Rhododendron ponticum x R. smirnovii was represented by a single individual and R. caucasicum x R. smirnovii by one small group of hybrid plants. The combinations R. ponticum x R. ungernii and R. ungernii x R. smirnovii showed evidence of frequent backcrossing, while R. ponticum x R. caucasicum appeared unusual in that an intermediate hybrid type was abundant, whereas hybrids with phenotypes approaching either parent were rare. Possible explanations of this latter situation are discussed. The results suggest that natural hybridization among Rhododendron species is common and that ecological factors are important in maintaining integrity when species occur in sympatry.

Key Words: diploid hybrid speciation • ecological isolation • hybridization • introgression • Rhododendron • RFLP

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