Unisexual organisms have three major pathways by which to adjust the sex ratio of their populations in response to environmental clues: zygotic sex determination, sex change in postzygotic life stages (including adult life stages), and sex-dependent mortality. The general pattern that emerges from empirical (largely descriptive) studies on plants is that sex ratio in unisexual organisms becomes more male-biased with decreasing resource availability. This appears to be largely independent of the specific type of resource. In dioecious plants, a large number of studies demonstrate that individual plants alter their sex expression in response to changing environmental conditions over time. Specifically, populations become male-biased under stressful conditions. The majority of these changes occur in response to decreasing resource availability, such as water. Cosexual (monoecious) plants, in contrast, have the potential to adjust within-individual sex ratios (and thereby also sex ratios at the population level) in response to spatial or temporal environmental changes, but relatively little is known about the environmental triggers for such sex ratio adjustments.
Studies of hermaphroditic plants report results that are similar to those observed in dioecious plants: a stronger male-bias is associated with low resource levels. Theoretical model predicts that selection will favor female-biased sex ratios in favorable habitats because of the higher cost of female function and the higher likelihood of establishment in the proven, high-quality maternal site. However, this hypothesis of'parental habitat selection' can be opposed by the 'asymmetric gamete exchange' hypothesis that predicts a female-biased ratio in unfavorable sites. This opposing model suggests that the gamete with the higher mobility (the pollen in the case of plants) will leave the site of highest production toward less-favorable sites. In the less-favorable sites, the influx of males will create a male-biased sex ratio; thus, selection will favor maternal plants that overproduce female gametes in these low-quality sites, and create female-biased sex ratio in the less reproductive sites.
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