Intragenomic Conflict and Sex Ratio Evolution - A primer.

So-called "selfish" genes distort Mendelian segregation to enhance their own transmission. This segregation distortion allows selfish elements to spread in populations, despite the fact that they are often harmful to the organisms in which they reside. Selfish genetic elements have long been speculated to be a potent evolutionary force, but their actual importance and frequency in natural systems has rarely been assessed. In recent years, the perceived importance of non-Mendelian elements has grown dramatically as an increasing number of experimental studies have established their influence on the evolution of biologically important traits such as sex ratio, reproductive isolation, dioecy and mate choice.

One common phenotypic manifestation of selfish elements in natural populations is a biased sex ratio. The classical example is X-linked meiotic drive in many Drosophila species, where an X-linked allele destroys Y-bearing sperm during gametogenesis, enhancing X-chromosome transmission and biasing the sex ratio. A similar system appears to exist in Silene. There is a wide variety of selfish elements that distort the sex ratio in natural populations; Y-chromosome meiotic drive, supernumerary chromosomes, cytoplasmic microorganisms and organelle genes (cytoplasmic male sterility in plants).

One reason these elements seem rare is that once they spread, the biased sex ratio creates selection for alleles at other loci that suppress their effects. This so-called “intragenomic conflict” has been found in nearly all systems involving sex ratio distortion, and could be important for the evolution of many aspects of sexual reproduction including the evolutionary stability of Mendelian segregation itself. The key to understanding sex ratio evolution in these systems is understanding how frequently these intragenomic conflicts exist, and how they are resolved in nature.

Department of Biology, PO Box 400328 University of Virginia, Charlottesville, VA 22904-4328
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