2009 Volume 60 Issue 2 Pages 61-70
Morinda umbellata subsp. boninensis (Rubiaceae), a perennial climber endemic to the Bonin (Ogasawara) Islands, was examined in the field to determine its sex expression and reproductive nature. Natural populations comprised hermaphroditic and staminate individuals. Hermaphroditic flowers had four or five stamens and a style with two stigmatic lobes; staminate flowers had four or five stamens and lacked a style and stigmatic lobes. There was no significant difference in stainability of the pollen with aniline blue in the anthers of either type of flower. Both morphs successfully resulted in seed set in artificial cross pollination experiments. Observations of living plants and bagging and crossing experiments revealed that M. umbellata subsp. boninensis is functionally androdioecious. The stigmas of hermaphroditic flowers protrude beyond the corolla tube and are positioned above the anthers, whereas anthers of the staminate flowers protrude beyond the corolla tube. This indicates that stigma and anther heights appear to be reciprocal between hermaphroditic and staminate flowers. Pollen differs significantly in size between the two sexual morphs. Based on these floral features, we consider the sexual system in subsp. boninesis may be derived from distylous hermaphroditism.
