Abstract
Genome compositions of the morphological crosses called Akakuro-matsu (Pinus×densi-thunbergii UYEKI) which were regarded as the hybrids between P. densiflora SIEB. et Zucc. and P. thunbergii PARL. were investigated using DNA molecular markers, and compared with the genome composition of P. densiflora and P. thunbergii. As the result of chloroplast DNA analysis using RFLP, chloroplast genome type separated into P. densiflora type and P. thunbergii type at 1. 8 of resin duct index (RDI). Nuclear DNA analysis using RAPD, revealed that crosses with 1.4-1.8 RDI possessed the genomes of the two species, and they were hybrids between P. densiflora and P. thunbergii. The frequency of hybridization, however, was lower than it has been considered, and it was clarified that most of morphological crosses were P. densiflora. All of these natural hybrids had P. densiflora's chloroplast genome. Since chloroplast DNA is inherited from pollen parent in conifers, the hybrids cause only in the combination of P. thunbergii×P. densiflora. This indicates that the reproductive isolation exists between the two Pinus species which had been considered to be nearly related phylogenetically.
