Mating System and Sex Ratio

The sex ratio of the gametes is often related to a population's or species' mating system; that is, the pattern of union between male and female gametes as reflected in the frequency of self-fertilization versus outcrossing. Outcrossing hermaphrodites must transfer their male gametes (sperm or pollen) to other individuals to achieve fertilization, and many male gametes are lost due to the inefficiency of pollinators, wind, and water as pollen vectors. Outcrossing populations must therefore produce many more male gametes than the number of available female gametes (eggs or ovules). By contrast, the male gametes of self-fertilizing individuals are delivered to female gametes that are in close proximity; relative to outcrossers, selfers require fewer sperm per egg because there is higher probability that a given male gamete will have access to a female one. Local mate competition between male gametes results in lower male:female ratios in selfers relative to their outcrossing counterparts. As a result, the allocation of resources to traits associated with male versus female function in selfers is often observed to be much lower than that in closely related outcrossers.

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