The breeding system and seed production of a distylous perennial, Ophiorrhiza japonica, were analyzed on Iriomote Island, located near the southwestern end of the Ryukyu Islands. On Iriomote, O. japonica is monomorphic with a pin morph. Artificial pollination experiments confirmed self- and intramorph compatibility in the populations, but automatic self-pollination was not detected. Seed set in 12 sampling plots in two natural populations was constant and abundant, indicating high pollinator availability. Our results indicate that self compatibility can be assumed to have been important in the establishment of the monomorphic populations on Iriomote Island. The loss of distyly is not characterized by specialization in self-fertilization, but rather intramorph compatibility can be assumed to have been more important in maintenance of the pin-monomorphic population as a factor facilitating outbreeding
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