Skewed male reproductive success and pollen transfer in a small fragmented population of the heterodichogamous tree <Emphasis Type="Italic">Machilus thunbergii</Emphasis>

Heterodichogamy is defined as the presence of two flower morphs that exhibit the male and female functions at different times among individuals within a population. Heterodichogamy is regarded as an adaptation to promote outcrossing through enhanced inter-morph mating, together with a 1:1 morph ratio. However, in highly fragmented populations, the morph ratio may be more likely to be biased by stochastic events. In such a situation, individuals of a minority morph within a population are expected to have higher reproductive success than those of a majority morph, which may suffer from pollen shortages of the minority morph. In this paper, we evaluated mating patterns and male reproductive success in a highly fragmented population of Machilus thunbergii, a putative heterodichogamous evergreen laurel tree. Results of paternity analysis indicated that the selfing rate was not clearly different between the two morphs. In contrast, the proportion of intra-morph mating was higher in the majority-morph (MM) mother trees than in the minority-morph (MF) mother trees. Bayesian estimated male reproductive success indicated that male reproductive success was higher in minority-morph (MF) than in majority-morph (MM) mother trees. These findings indicate that (1) the majority morph mothers, suffering a shortage of the opposite morph pollen, could partly compensate for the reduced reproductive success by intra-morph mating rather than by selfing, and (2) negative-frequency dependent selection may be involved in the maintenance of the two morphs.