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Abstract

Because of its presumed high levels of dioecy (separate male and female plants), study of the native Hawaiian angiosperm flora has been important in development of many hypotheses about conditions favoring the evolution of dioecy. The importance of ecological correlates with dioecy has proven difficult to assess, however, because of lack of data on the origins of dioecy in the Hawaiian Islands. Clearly, these correlations are of greater interest in taxa where dioecy evolved in the Hawaiian Islands (autochthonous evolution of dioecy) than in taxa that are the result of dioecious colonists with subsequent speciation in the Hawaiian Islands. Because the Hawaiian flora is small and extremely isolated, colonists can be identified and their breeding systems hypothesized, thus allowing inferences on the origins of dioecy. Using current taxonomic information, the incidence of dioecy in native Hawaiian angiosperm species is 14.7%, lower than previous estimates, but still the highest of any known flora worldwide. Ten percent of the colonists were sexually dimorphic (dioecious, gynodioecious, polygamodioecious, and subdioecious), and over half (55.2%) of current dimorphic species are in lineages arising from dimorphic colonists, showing that dimorphism is high in part because colonists were dimorphic. Autochthonous evolution of dimorphism occurred in at least 12 lineages (e.g., hermaphroditic colonists of Bidens (Asteraceae), the Hawaiian Alsinoideae (Caryophyllaceae), and Hedyotis (Rubiaceae) led to species—rich lineages that include many dimorphic species). One—third (31.8%) of current dimorphic species are in lineages arising from monomorphic colonists. Dioecy in the Hawaiian Islands is a result of both dimorphic colonists as well as evolution of dioecy in Hawaiian lineages from hermaphroditic colonists. The high incidence of dimorphism is not because dimorphic colonists evolved more species per colonist than monomorphic colonists. Detailed studies of individual lineages are critical to elucidate causal factors in the evolution of dioecy.

Origins of Dioecy in the Hawaiian Flora

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