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 between eastern Asia and eastern North America. We test a hypothesis that the persistence of this early spring flowering 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 hexandrum Royle) individuals over 2 years. To examine autogamy, seed set under different pollination treatments was compared. To clarify whether automatic self-pollination is achieved by movement of the pistil 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 cross-pollinated 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 stamens moving simultaneously to contact the stigma. Stigmatic pollen load increased little during the day time, 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 American mayapple, 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.
Highlights
The transition of the breeding system from outcrossing to selfing has been considered to be a widespread evolutionary trend in flowering plants (Stebbins 1957). Baker (1955)Published by Oxford University Press on behalf of the Annals of Botany Company
We address the following questions: (i) Are there potential pollinators for this nectarless early spring flowering herb in the Himalayan region? (ii) When and how does the autogamy take place? In particular, we try to quantify whether the selfing is caused by the pistil inclining, as previous studies suggested
Breeding system Seed sets of bagged flowers and openpollinated flowers were high and not significantly different from each other or from hand cross- or self-pollination treatments (Fig. 2). These results indicated that Himalayan mayapple was fully SC and capable of automatic self-pollination
Summary
The transition of the breeding system from outcrossing to selfing has been considered to be a widespread evolutionary trend in flowering plants (Stebbins 1957). Baker (1955). The transition of the breeding system from outcrossing to selfing has been considered to be a widespread evolutionary trend in flowering plants (Stebbins 1957). Xiong et al — Breeding system in mayapples or extremely unpredictable. He illustrated that a nectarless orchid (Ophrys apifera) could be favoured by deceit pollination by bees, but was capable of selffertilization by curving of the caudicles that delivered pollinia directly onto the stigma. Autonomous self-pollination has been classified into three types, termed ‘prior’, ‘competing’ and ‘delayed’, according to the timing of self-pollination relative to cross-pollination (Lloyd and Schoen 1992). In which self-pollination is delayed until the opportunity for outcrossing has passed, is considered to provide reproductive assurance (Lloyd 1992)
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