Morphological and genetic variation in populations in a hybrid zone between Leucosceptrum japonicum and L. stellipilum (Lamiaceae) in the central Japanese mainland

Hybrid zones between two species can reflect the combined effects of gene flow, genetic novelty, selection, and isolation barriers. Leucosceptrum japonicum and L. stellipilum are two morphologically distinct species belonging to Lamiaceae, which are endemic to Japan with partially overlapping distributions. In the present study, morphological and genetic variation in 11 populations in a putative hybrid zone between L. japonicum and L. stellipilum were investigated. No introgression was observed in the sympatric parental populations based on the cpDNA haplotype. Morphological intermediacy, ITS additivity and reduced pollen fertility confirmed the occurrence of hybridization between L. japonicum and L. stellipilum. Hybrid individuals appear to be not only early hybrid generation, but also advanced hybrid generation and/or backcross hybrids. The morphology of the putative early hybrid generation was too variable to distinguish all these hybrids from the parental species. The factors that facilitated and maintained hybridization between L. japonicum and L. stellipilum are discussed.