Summary: The Male Function Hypothesis
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Summary: The Male Function Hypothesis by Sebastian Molnar, Aug. 2000 Flowering plants typically produce large flower heads (i.e. inflorescences), and yet only produce a relatively small number of seeds. At least five hypotheses have been postulated for explaining the evolution of large inflorescence sizes and the co-occurrence of low seed set. These are 1) the male function hypothesis, 2) pollination limitation, 3) pollinator attraction, 4) selective abortion, and 5) bet hedging. The male function hypothesis states that competition to increase male fitness by pollen donation is responsible for large inflorescences. The pollination limitation hypothesis suggests that seed set is typically low due to a low amount of pollen transfer. The pollinator attraction hypothesis suggests that large inflorescences exist to attract pollinators. The selective abortion hypothesis states that many flowers enables a plant to abort some fruits in order to save and allocate resources to a small number of otherwise higher quality seeds. Finally, the bet hedging hypothesis alludes to the idea that there is unpredictable spatial and/or temporal variation in the environment, and therfore large flower heads allows for an advantage in rare favorable conditions (i.e. more flowers means more opportunity for survival). The male function hypothesis suggests that pollen delivery increases with flower number, while the fitness of the female function (seed set) decreases or levels off due to limited resources. Thus, selection for the male function through pollen competition is a possible mechanism for the evolution of large inflorescences. Campbell (1989) tested the male function hypothesis (i.e. that pollen donation increases more with flower number than does seed set) using dye analogs to track pollen in natural populations of Ipomopsis aggregata (which has perfect flowers, is self-incompatible, and is pollinated via hummingbirds). Campbell’s estimates of pollen donation, pollen receipt, and seed set suggests that the male function hypothesis is not an appropriate explanation for large floral displays in I. aggregata. Plants with more flowers either donated or received more pollen, however, both pollen donation and receipt increased linearly with little change in the relative amounts on a per flower basis. Campbell (1989) also found that seed set increased disproportionately with flower number and that there was an increase in functional femaleness with flower number -- not a decrease, as suggested by the male function hypothesis. Reference:
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