Abstract:In order to understand the evolution of dioecy and pollination modes in the genus Mallotus, the sexual system and pollination biology of M. apelta were studied in wild populations from Guangdong and Guangxi Provinces in southern China. The results showed that all the investigated populations contained female (F), male (M) and bisexual (B) individuals, and the average sex ratio was 1:0.66:0.18 (F:M:B). There were no significant differences in base diameter of trunk among three sexual individuals. Although the pollen of male flowers from pure male individuals and that from bisexual individuals had the same germination rate, histochemical and micro-morphological characters, the single male flower from male individuals produced 1.26-1.63 times pollen grains more than those of bisexual individuals. Female individuals shared the same ovule numbers per flower, seeds per fruit and fruit setting per inflorescence with bisexual individuals. The control crossing (pollen came from males) significantly increased the fruit setting of pure females (96.67%) comparing to bisexual individuals (76.00%) (P < 0.001), indicating higher potential seed fertility in female individuals than that in bisexual individuals. Insects (fly, beetle) were observed to visit male and bisexual but not female individuals in the field, indicating that insects made no contribution to the fruit production of female individuals. More than 30% of the netted female flowers set fruits, while all the bagged female flowers failed to set fruits. Therefore, these indicate that M. apelta is a typical subdioecious species. The results also demonstrate that the male and female individuals have higher fitness than bisexual individuals in populations. Females are pollinated by wind, while bisexual individuals show a combination of entomophilous and anemophilyous characteristics. More comprehensive investigation of the sexual system and pollination mode in the species and its congeners may shed light on our understanding of the evolution of dioecy and pollination modes in the genus Mallotus.
