AOBPreview originally published online on February 19, 2009
Annals of Botany 2009 103(9):1385-1394; doi:10.1093/aob/mcp037
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This article appears in the following Annals of Botany issue: Special Issue: Plant-Pollinator Interactions [View the issue table of contents]
Flowering phenology influences seed production and outcrossing rate in populations of an alpine snowbed shrub, Phyllodoce aleutica: effects of pollinators and self-incompatibility
Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
* For correspondence. E-mail gaku{at}ees.hokudai.ac.jp
Received: 31 October 2008 Returned for revision: 15 December 2008 Accepted: 9 January 2009 Published electronically: 19 February 2009
Background and Aims: Because of differences in snowmelt time, the reproductive phenologies of alpine plants are highly variable among local populations, and there is large variation in seed set across populations. Temporal variation in pollinator availability during the season may be a major factor affecting not only seed production but also outcrossing rate of alpine plants.
Methods: Among local populations of Phyllodoce aleutica that experience different snowmelt regimes, flowering phenology, pollinator availability, seed-set rate, and outcrossing rate were compared with reference to the mating system (self-compatibility or heterospecific compatibility with a co-occurring congeneric species).
Key Results: Flowering occurred sequentially among populations reflecting snowmelt time from mid-July to late August. The visit frequency of bumble-bees increased substantially in late July when workers appeared. Both seed set and outcrossing rate increased as flowering season progressed. Although flowers were self-compatible and heterospecific compatible, the mixed-pollination experiment revealed that fertilization with conspecific, outcrossing pollen took priority over selfing and hybridization, indicating a cryptic self-incompatibility. In early snowmelt populations, seed production was pollen-limited and autogamous selfing was common. However, genetic analyses revealed that selfed progenies did not contribute to the maintenance of populations due to late-acting inbreeding depression.
Conclusions: Large variations in seed-set and outcrossing rates among populations were caused by the timing of pollinator availability during the season and the cryptic self-incompatibility of this species. Despite the intensive pollen limitation in part of the early season, reproductive assurance by autogamous selfing was not evident. Under fluctuating conditions of pollinator availability and flowering structures, P. aleutica maintained the genetic composition by conspecific outcrossing.
Key words: Alpine snowbed, autogamy, bumble-bee, cryptic self-incompatibility, flowering phenology, mixed pollination, outcrossing rate, Phyllodoce aleutica, pollination success, seasonality, self-pollination
Present address: Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo 156-8502, Japan
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