Abstract: Andromonoecy is a rare sexual system in plants. The function of additional male flowers in andromonoecious species has been widely discussed; however, few studies have taken offspring fitness into account. In addition, little is known about the mechanisms that maintain andromonoecy in autogamous species. In this study, we compared morphology, pollinator preference, pollen production and export, siring ability, natural siring success, hundred seed dry weight, and seed germination rates between male and hermaphroditic flowers in an endangered autogamous andromonoecious species, Sagittaria guayanensis. Male flowers, which are larger than hermaphroditic flowers, required fewer resources to produce. Pollinators visited male flowers more frequently than they visited hermaphroditic flowers. In addition, pollen production and export were higher in male flowers. Hand pollination demonstrated that siring ability did not differ between flower type. However, the natural siring success of male flowers was triple that of hermaphroditic flowers. The seeds sired by male flowers performed better than those sired by hermaphroditic flowers, with greater dry weight and higher germination rate. In conclusion, male flowers may be superior pollen donors for outcrossing. The maintenance of andromonoecy in S. guayanensis may result from the better performance of male flowers in male function compared to that of hermaphroditic flowers.

Key words: Alismataceae, Andromonoecy, Autogamy, Male function, Offspring fitness, Sagittaria guayanensis