Genetic structure of interspecific hybridization between the long corolla tube species Isodon longitubus and its short corolla tube congener I. inflexus

Abstract Interspecific hybridization between the short corolla tube species Isodon inflexus and the long corolla tube species I. longitubus was analyzed using genetic and morphometric markers. Bayesian clustering analysis using microsatellites revealed that plants in the contact zone consisted of two I. inflexus individuals, 33 I. longitubus individuals, and 13 hybrid individuals (F2-like and backcrosses to both I. inflexus and I. longitubus). Using the sequences of the psbA-trnH inter genic spacer in chloroplast DNA, three haplotypes were found among pure I. inflexus, while four haplotypes were found among pure I. longitubus. Most hybrid individuals had a haplotype found only in I. inflexus, suggesting that the initial F1 might have been formed by hybridization with I. inflexus as the maternal parent, and that in later hybrid formation, hybrids or I. inflexus tended to serve as the maternal parent. Although strong prezygotic isolation mechanisms contribute to preventing hybridization between the species, human habitat disturbance might have created the contact zone. Although mature hybrids in the contact zone contained equal proportions of backcrosses to both I. inflexus and I. longitubus, seedlings comprised more individuals originating from backcrosses to I. longitubus. The dominance of backcrosses to I. longitubus was probably caused by the quantitative difference between the parental species in the contact zone. In the contact zone, signs of contemporary hybridization in the nuclear genome might have been diluted by repeated backcrossing. The present study could illustrate the process of unidirectional introgression leading to chloroplast capture, a phenomenon frequently observed in Isodon species in Japan.