Mating system and population genetic structure of an androdioecious tree, Fraxinus lanuginosa Koidz. (Oleaceae) in northern Japan

Models for the maintenance of androdioecy have suggested that selfing of hermaphrodites decreases the frequency of males in a population (the ‘male frequency’). To test this hypothesis, we used electrophoretic allozyme methods to study the mating system and population genetics of an androdioecious tree, Fraxinus lanuginosa, which exhibits large variations in male frequency among subpopulations in central Hokkaido (northern Japan). We estimated the outcrossing rates by using seeds assayed at three polymorphic loci, and found that the multilocus outcrossing rate (tm) increased with increasing male frequency (q) (tm = 0.69 to 0.99, q = 0.11 to 0.59). Fixation indices (Fj) estimated from these tm values ranged from 0.006 to 0.185, and were significantly greater than zero in plots with a low male frequency (q ≤ 0.35), but not in plots with intermediate or high male frequencies (q ≥ 0.47). However, fixation indices at the late life stage (Fa) were not significantly different from zero at five loci (Fa = 0.056 to 0.101, q = 0.11 to 0.61), and the Fa values were lower than the Fj values in several of the plots where both values were measured. These results indicate that inbreeding depression substantially decreases the pro- portion of selfed progeny in partially self-fertilising subpopulations. A theoretical model for the maintenance of androdioecy showed expected male frequencies significantly lower than the observed values in plots with high male frequency (q = 0.59 to 0.61), although the differences between the expected and observed values of male frequencies were not significant in plots with intermediate or low male-frequencies (q = 0.11 to 0.47). In this paper, the factors are discussed that affect variations in the male frequency among subpopulations of F. lanuginosa based on these findings.