Introgressive hybridization and density of stellate hairs on the abaxial surface of leaves of Quercus acutissima and Quercus variabilis, around Iijima Town, Nagano Prefecture

Quercus acutissima and Quercus variabilis, both of which are Section Cerris, are assumed to hybridize with each other because individuals with intermediate morphology are sometimes observed in the field. However, whether or not these individuals are indeed hybrids of the two species is uncertain, because there has been no molecular study on hybridization between them. In this study, we sampled leaves of Section Cerris species around Iijima Town, Nagano Prefecture, where individuals with intermediate morphology have been observed with a high frequency (the so-called hybrid zone), and we attempted to clarify their genetic characteristics by using both chloroplast simple sequence repeat (SSR) and nuclear SSR analyses. We aimed to address the following questions; i) Do these two species share the same chloroplast haplotypes? ii) Is the nuclear DNA of hybridized individuals an admixture of the two species? iii) Is the degree of admixture of each individual is related with the morphological traits, specifically stellate hair density? We sampled leaves of 90 individuals in the hybrid zone as well as two Q. acutissima populations (Hachioji City and Hamamatsu City, total 60 samples) and one Q. variabilis population (Seto City, 42 samples) as reference populations. The results showed that all individuals in the hybrid zone and reference populations had the same haplotype, supporting the hypothesis of introgressive hybridization between these two species. All individuals in the hybrid zone were admixtures of genetic clusters of the two species, as revealed by structural analysis of the nuclear SSR data, also suggesting introgressive hybridization. The individuals with a higher density of stellate hairs on the abaxial surface of leaf showed lower a proportion of the Q. acutissima genetic cluster. This result suggests that the degree of admixture of each individual was related with morphological traits.論