Asymmetric introgression between Magnolia stellata and M. salicifolia at a site where the two species grow sympatrically

Abstract

In order to understand the ongoing evolutionary relationships between species, it is important to elucidate patterns of natural hybridization. In the zone where two species are sympatrically distributed, we examined 274 individuals of Magnolia stellata, Magnolia salicifolia, and their putative hybrids by means of 16 nuclear and three chloroplast microsatellite markers. Hybrid classes of individuals were estimated by admixture analyses. Morphological traits were also investigated for 64 of the 274 individuals. Admixture analyses revealed that 66 of the 274 individuals were classified as hybrids, comprising 17 F1 and 19 F2 individuals, 27 backcrosses to M. salicifolia, and 3 individuals of unknown origin. Morphological data from the 64 individuals agreed well with their genetic admixture rates. Spatial locations of F1 and F2 hybrids at the study site were intermediate between the two purebred species, indicating that the site preferences of hybrids are intermediate. The occurrences of F2 and backcross hybrids indicate that F1 hybrids are fertile. The chloroplast DNA haplotypes of all F1 hybrids corresponded to those detected in M. salicifolia, so that maternal parents of the F1 hybrids were all M. salicifolia. Furthermore, no hybrid individuals derived from a backcross to M. stellata were detected. These results suggest that the direction of hybridization and the subsequent introgression have been quite asymmetric and that the introgression occurred from M. stellata into M. salicifolia.