Evolution of Dioecy in Schiedea (Caryophyllaceae: Alsinoideae) in the Hawaiian Islands: Biogeographical and Ecological Factors

Abstract

Breeding systems in Schiedea and Alsinidendron (Caryophyllaceae: Alsinoideae) were characterized in order to determine whether the dioecy that occurs in Schiedea evolved in situ in the Hawaiian Islands. The occurrence of hermaphroditism in 14 of the 22 species of Schiedea, as well as outgroup comparison, indicate that dioecy is a derived breeding system. Species diversity and endemism are greatest on the older Hawaiian Islands, suggesting that these islands were colonized first. Diclinous breeding systems are more common on the older islands, probably because of the greater length of time available for the evolutionary transition from hermaphroditism to dicliny. Dicliny appears to reduce the probability of inter-island colonizations; among extant species those with hermaphroditic breeding systems are more likely to occur on more than a single island. Based on distributional patterns, it appears likely that dicliny has evolved at least three and possibly six times in Schiedea. Species occurring in dry areas are likely to have evolved from wet or diverse mesic forest ancestors. As species of Schiedea shifted to dry habitats, the evolution of dicliny appears to have been favored, perhaps by loss of pollinators and subsequent increased selfing rates. Under such conditions, the expression of inbreeding depression may have favored unisexual individuals and the evolution of dioecy.