GENETICS OF SEX RATIO VARIATION AMONG NATURAL POPULATIONS OF A DIOECIOUS PLANT.

The female-biased sex ratio in the genus Silene is the most widely documented example of sex ratio bias in plants. It has recently been shown that the genetic basis of sex ratio in this species involves a system of sex ratio distorters and restorers, but it is not known if these genetic elements are of fundamental importance in natural populations. I crossed plants from natural populations with known testers to examine the frequencies of sex ratio distorters and restorers in eight populations of S. alba, and to determine the extent that these elements are responsible for the sex ratio variation in nature. The genetic basis of sex ratio varied from one population to another, suggesting that sex ratio evolution occurs over a relatively small spatial scale and is asynchronous among local demes. The resulting variation in the frequency of sex ratio alleles among populations explained most of the sex ratio variation seen in nature, which suggests that the ecology and evolution of this trait in natural populations is governed primarily by underlying dynamics of selfish genetic elements. The possible causes and the evolutionary consequences of population structure at genes controlling sex ratio are discussed, as are the implications of these results for the overall importance of selfish genetic elements in natural populations.