Why do plants have biased sex ratios?

By Nathaniel Sharp, EEB

One of the most celebrated arguments in evolutionary biology is the idea that populations should consist of an even number of males and females. The reason is that when one sex is rare, there is an advantage to parents who produce more offspring of that sex, evening out the sex ratio. A one-to-one sex ratio should be the most stable strategy, and therefore the most common.

However, a surprising number of plants don’t seem to follow this rule; many species have more females than males. Traditionally, scientists have suspected that more females are produced because the Y chromosome—the male determining chromosome—has fewer genes than the X. As a result, when pollen grains arrive at a flower and start to grow down the stigma to fertilize ovules, X-bearing pollen grains outcompete Y-bearing ones.

But this explanation did not satisfy Josh Hough from the University of Toronto. He suspected that females should still evolve ways to ensure an even sex ratio in their offspring, such as reducing pollen competition on their stigmas. In a paper in Evolution, Hough developed mathematical models to predict the evolutionarily stable sex ratio, and found that it should evolve to be one-to-one, even with selection against Y-bearing pollen.

So why is sex ratio often biased in nature? Hough predicted that uneven sex ratios might be stable if pollen competition confers an evolutionary advantage, such as helping to weed out pollen bearing harmful mutations. After extending previous models to include selection against mutations, he found that biased sex ratios can indeed be evolutionarily stable.