Unusual patterns of hybridization involving a narrow endemic Rhododendron species (Ericaceae) in Yunnan, China

Abstract

Premise of the study: One potential threat to rare species is genetic swamping caused by hybridization, but few studies have quantified this threat. Rhododendron cyanocarpum is a narrow endemic species that occurs sympatrically with potentially in-terfertile congeners throughout its range within Yunnan, China. We searched the entire distribution of R. cyanocarpum for hybrids and examined the patterns of hybridization to assess potential threat from hybridization. Methods: In a comprehensive field survey, we detected only one instance of hybridization involving R. cyanocarpum, with R. delavayi, at Huadianba near Dali. Material of both species and putative hybrids was examined using morphology, chloroplast DNA, nuclear ribosomal DNA, and Bayesian analysis of AFLP profiles. Key results: Of 10 putative hybrids, two were F1' s and at least seven were F2' s. Four backcrosses to R. delavayi were detected among material with R. delavayi -like morphology within the hybrid zone. Backcrosses to R. cyanocarpum were not detected. Therefore F2' s outnumbered all other classes within the hybrid zone, a situation not previously confirmed for plants and ex-tremely rare generally. Hybridization was asymmetrical, with R. delavayi as the maternal parent in all but one of the hybrids detected. Conclusions: Although natural hybridization is common in Rhododendron, it is rare in R. cyanocarpum and is apparently not accompanied by backcrossing toward R. cyanocarpum. Hence, there is no immediate risk of genetic swamping, unless habitat disturbance increases and changes the patterns of hybridization. Our study is the first to report a plant hybrid zone dominated by F2 hybrids. This pattern might contribute to species barrier maintenance. © 2010 Botanical Society of America.