Reduced incompatibility in the production of second generation hybrids between two Magnolia species revealed by Bayesian gene dispersal modeling.

PREMISE OF THE STUDY: Hybrid zones are areas where gene flow between related species is currently occurring, so information on the compatibility between related species and their hybrids is essential for predicting the dynamics of such zones generated by introgressive hybridization. In this study, we quantified the compatibility among Magnolia stellata, M. salicifolia, and their hybrids in a hybrid zone using gene dispersal modeling.

METHODS: After determining the genealogical classes of adult trees in the hybrid zone, the paternity of 574 open-pollinated seeds from 37 known maternal trees was analyzed with microsatellite markers. A neighborhood-based Bayesian gene dispersal model developed by us for estimating compatibility was then applied to the paternity data.

KEY RESULTS: When M. stellata or M. salicifolia were mothers, interspecific mating to produce F1 hybrids yielded significant incompatibility, but backcrossing with F1 hybrids did not. Furthermore, when F1 hybrids became mothers, no significant incompatibility resulted from backcrossing to parental species or intra-F1 mating to produce F2 hybrids. The estimated proportion of F1 hybrids in the outcrossed seeds (1.7%) in the hybrid zone was much lower than that in the adult trees (14.0%).

CONCLUSIONS: While it is difficult to obtain F1 hybrids, their low incompatibility makes it easy to produce advanced generation hybrids, once they have been successfully obtained. Although the production of F1 seeds is rare, heterosis and/or weak selection pressure in an empty niche between the parental species' niches may have contributed to the increased proportion of adult F1 hybrids in the hybrid zone.