Pollinator scarcity drives the shift to delayed selfing in Himalayan mayapple Podophyllum hexandrum (Berberidaceae)

Abstract

Recent molecular phylogenetics have indicated that American mayapple (mainly self-incompatible, SI) and Himalayan mayapple, which was considered to be self-compatible (SC), are sister species with disjunct distribution betweeneastern Asiaand eastern NorthAmerica.We testahypothesis that the persistence of this early springflowering herb in the Himalayan region is attributable to the transition from SI to SC, the capacity for selfing in an unpredictable pollination environment. Pollinator observations were conducted in an alpine meadow with hundreds of Himalayan mayapple (Podophyllum hexandrumRoyle) individuals over 2 years. To examine autogamy, seed set under different pollination treatmentswascompared. To clarify whetherautomatic self-pollination is achieved bymovementof thepistil as a previous study suggested, we measured incline angles of the pistil and observed flower movement during anthesis using video. Floral visitors to the nectarless flowers were very rare, but solitary bees and honeybees could be potential pollinators. Seed set of bagged flowers was not significantly different from that of open-pollinated, self- or crosspollinated flowers. However, removal of petals or stamens lowered seed yield. The angles of inclination of pistils did not change during the process of pollination. Automatic self-pollination was facilitated by petals closing and stamensmoving simultaneously to contact the stigma. Stigmatic pollen load increased little during the daytime, in contrast to a sharp increase when the flowers closed during the night-time. These observations indicated that Himalayan mayapple was SC and delayed self-pollination was facilitated by the movement of petals rather than the pistil. Compared with SI Americanmayapple, no obvious inbreeding depression in SC Himalayan mayapple may contribute its existence in the uplifting zone. A scarcity of pollinators may have driven the shift to delayed selfing in P. hexandrum. © The Authors 2013.