EVOLUTION OF UNISEXUALITY IN THE HAWAIIAN FLORA - A TEST OF MICROEVOLUTIONARY THEORY

The evolution of separate sexes as a means of avoiding self-fertilizat ion requires the controversial coexistence of large inbreeding depress ion and high selfing rate in the ancestral hermaphrodite population. F itness components of adult females and hermaphrodites in nature, of th eir open-pollinated progeny, and of experimental selfs and outcrosses onto hermaphrodites were compared in endemic Hawaiian Bidens sandvicen sis, all of whose known populations are gynodioecious, consisting of a mixture of females and hermaphrodites. Multilocus selfing rates of he rmaphrodites were also estimated, and sex morph ratio monitored over f our seasons in three populations of B. sandvicensis and one population of gynodioecious B. cervicata. Total mean inbreeding depression in se ed set (in the glasshouse), germination rate (in an open-air nursery o n Kauai), and first year survivorship and fecundity in the field were estimated as 0.94 (SE 0.04), and occurred primarily in drought months. Lower survivorship and fecundity of selfs were partially explained by their consistently smaller size. Open-pollinated seed of females had significantly lower germination rate, proportion flowering, and fecund ity than outcrossed progeny of hermaphrodites, suggesting moderate bip arental inbreeding in females and a lack of any non-outcrossing advant age to progeny of females. In all fitness components, open-pollinated progeny of hermaphrodites were inferior to those of females and to out crosses, and in most components were superior to selfs. Total performa nce of open-pollinated progeny of females relative to those of hermaph rodites was calculated as 2.3 (SE = 0.4), but since inflorescences of females also set 20% to 50% more seed than those of hermaphrodites, th eir total relative ovule success was estimated as 3.2 (SE = 0.5). If i nheritance of male sterility is nuclear, this superiority is sufficien t to maintain females in frequencies over 20% in populations, whose ac tual frequencies ranged from 14% to 33%. In four populations, selfing rates of hermaphrodites, assayed in seedlings, were 0.50, 0.45, 0.25, and 0.30, but since substantial inbreeding depression occurred prior t o germination, the mean selfing rate of hermaphrodite ovules exceeded 0.57. female frequencies were significantly higher in the two populati ons with higher hermaphrodite selfing rate. These results suggest that inbreeding depression can exert a profound influence on the mating sy stem of self-compatible plants on Hawaii and perhaps other oceanic isl ands, and can be sufficiently strong to electively favor the eliminati on of the male function.