Sex ratio, floral traits, and genetic variation of androdioecious Osmanthus fragrans L. (Oleaceae) and the implications for maintenance of high male frequency

Several examples of androdioecy appear to have evolved from dioecy and have low male frequency (< 0.5). However, the evolutionary pathway to androdioecy in Oleaceae may come from hermaphroditism. Osmanthus fragrans L. has a 1:1 sex ratio in nature populations. Significant differences are observed not only in flowering phenology but also in some floral traits between males and hermaphrodites. The protandry in the same population and the protogyny in the same plant may promote the xenogamy between genders. The majority of flower traits related with the pollen production are different between males and hermaphrodites. Males bear more flowering nodes, and more flowers per node, and larger anther in all three populations. This characteristic demonstrated that males have more male advantage than hermaphrodites. Population genetic structure of O. fragrans is genetically homogeneous at the species level, and most variations exist within a population. The percentage of variation among populations (13%) and between males and hermaphrodites (0%) is low. Moreover, genetic differentiation was very low between genders not only among populations but also in the same population. This genetic variation could be attributed to the occurrence of high levels of xenogamy between genders. Therefore, high male frequency and more male fitness advantage in males are the essential conditions for this mating system, which plays an important role during population reproduction and regeneration. The 1:1 sex ratio could be the result of integrative effects of sexual system, mating system, and reproductive success.