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Evolution of gynodioecy and dioecy: theoretical issues and the case of Bidens sandvicensis and B. cervicata Schultz, Stewart T.


Numerical models are shown to allow the spread of males into equilibrium nucleo-cytoplasmic populations, resulting in the evolution of subdioecy or dioecy. Conditions for the evolution of subdioecy from two classes of nucleo-cytoplasmic gynodioecy are found to be similar to, and in some cases less stringent than, those required for evolution of sub dioecy from nuclear gynodioecy. Conditions for the evolution to dioecy from nucleo-cytoplasmic gynodioecy are likewise similar to those from nuclear gynodioecy, except that for someparameter sets, dioecy is not possible from nucleo-cytoplasmic gynodioecy due to the persistence of acytoplasmic polymorphism that allows continued production of hermaphrodites. Sex ratios in gynodioecious, endemic, Kauaian Bidens sandvicensis and B. cervicata are strongly correlated with elevation, aspect, and out crossing rate as estimated from analysis of electrophoretic genotypes at the phosphglucoisomerase and shikimate dehydrogenase loci. Female frequencies at four population centers averaged 0.33, 0.27, 0.14,and 0.18 from 1989 to 1992, and 1989 measured out crossing rates at these centers were0.50, 0.55, 0.75 and 0.71. In Bidens sandvicensis, significant inbreeding depression occurs in seed set (0.2 to0.5), germination (0.40), first-year survivorship in the field on Kauai (0.42), probability of first-year flowering (0.58) and number of heads produced per flowering plant (0.68). Total inbreeding depression thus exceeds 0.9. Performance of hermaphrodites relative to females was lower than one in seed set per head (0.7), and in quality of open-pollinated progeny: germination (0.73), first-year survivorship in the field (0.78), probability of flowering (0.95), and number of heads produced per flowering plant (0.80), although only the first three components were significantly different. Inbreeding depression in hermaphrodites thus appears sufficient to maintain females in populations of Bidens sandvicensis,and is responsible for at least 75% of the over threefold fitness of females relative to hermaphrodites. These results indicate that inbreeding depression can profoundly influence the evolution of floral structure and mating system on isolated oceanic islands.

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