Road crossing in chimpanzees: A risky business

the Y. Likewise, X chromosomes spend 2/3 of their time in females, and should be selected to produce female-biased sex ratios. Hamilton marshaled the evidence supporting biased sex ratios caused by genes on the sex chromosome. Because autosomes continue to favor a balanced sex ratio, the important implication is that individuals do not always have indivisible interests — instead there can be conflict between contending sets of selfish genes. This particular bias will not occur in the haplodiploid Hymenoptera because sex is determined by ploidy rather than by special chromosomes, although maternally transmitted parasites can cause their preferred expected female biased sex ratios. But there is a spectacular example of a selfish sex ratio distorter in the parasitoid Nasonia vitripennis studied by Jack Werren. Called PSR for paternal sex ratio, it causes fathers to have excess sons, which ought to be puzzling because haploidiploid males cannot normally have sons (if the egg is fertilized, it becomes diploid and hence a daughter). It works because PSR is inherited on a B chromosome. B chromosomes are small inessential chromosomes that exist in one or more copies in some cells, They do not segregate neatly like autosomes (or A chromosomes) but copies do get transmitted in less regular fashion to offspring. When one or more copies of this B chromosome are transmitted via sperm, they cause the father's A chromosomes to condense into chromatin and be lost. What remains then, are the maternal autosomes, whose haploid state directs the development of a male, along with the successfully transmitted B chromosome. Models that incorporate the details of PSR's effects into sex ratio theory show that PSR can spread only when sex ratios are otherwise female-biased, but that it can spread at all is remarkable. It is an ultimate selfish element; in each generation, the B chromosome succeeds only by completely destroying its fellow traveler A chromosomes and joining a new set. Conclusion Düsing's basic insight into how sex ratios evolve has been developed into a wide-ranging and successful theory that predicts both the conventional 1:1 sex ratios and numerous extraordinary ones. A surprising amount of the evidence has come from haplodiploid Hymenopteran insects, though many of these effects can also be found in other organisms, even where chromosomal sex determination mechanisms would seem to lock them into 1:1 sex ratios. Most important, this seemingly arcane topic has played very prominent roles in the development …