Abstract
Approximately 80 % of angiosperm species produce hermaphroditic flowers, which face the problem of male-male sexual interference (one or more anthers gets in the way of disseminating pollen from other anthers) or male-female sexual interference (the pistil interferes with disseminating pollen from the anthers by preventing the anther from touching a pollinator, or the anther prevents pollinator from depositing outcross pollen on the stigma). Slow stamen movement in hermaphrodite flowers has been interpreted as an adaptation for reducing male-male sexual interference. Using slow stamen movement in Lychnis cognata (Caryophyllaceae), this study presents new evidence that this phenomenon can reduce both male-male and male-female sexual interference. Ten stamens in L. cognata flowers vertically elongated their filaments in two batches and displayed similar patterns in pollen dispensing. More importantly, 10 stamens bend out of the floral centre by curving the filament also in 2 batches and pollen grains located at the flower centre displayed the highest viability. Thus, three stages of stamen movement can be identified, comprising two male stages (M1 and M2) and one female stage (F). We found that the main pollinator for L. cognata, Bhutanitis yulongensis (Papilionodae) generally preferred M1 flowers. Manipulation experiments show that vertical stamen movement enabled the anthers to dehisce at different times to prolong the presentation of pollen grains. Horizontal movement of the stamen decreased both male-male and male-female interference. However, vertical stamen movement had a minor role in increasing amount of pollen received by the stigma. This study provides the first direct experimental evidence of concurrent male-male and male-female interference in a flower. We suggest that the selection pressure to reduce such interference might be a strong force in floral evolution. We also propose that other selective pressure, including pollen dispensing mechanisms, pollen longevity, pollinator behaviour and weather, might contribute to floral evolution.
Highlights
Stamen movement refers to situations in which stamens move under their own volition or when they move in response to stimulation by external triggers such as pollinators (Ren 2010)
Our results indicated that Bhutanitis yulongensis was the major pollinator of L. cognata
This study provides the first evidence that slow stamen movement in L. cognata reflects an adaptation to present pollen gradually to pollinators and, more importantly, decreases concurrent male–male and male–female sexual interference in flowers
Summary
Stamen movement refers to situations in which stamens move under their own volition or when they move in response to stimulation by external triggers such as pollinators (Ren 2010). Four main types of stamen movement are recognized (Ren 2010): stimulated Slow stamen movement is widespread and is characterized by changes in stamen position, the degree of dichogamy during flowering, and the movement of the anther to a specific area that allows it to directly deliver pollen to recipient stigmas for self-fertilization (Ren 2010)
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