Summary

Some mite species are male-diploid while others are haplodiploid, with haploid males arising from unfertilized eggs (arrhenotoky) or from fertilized eggs via the elimination of the paternal genome at some stage before or during spermatogenesis (pseudo-arrhenotoky). Arrhenotoky confers the advantage to the female of controlling the offspring sex ratio by controlling the fertilization process. It is now well established that sex ratio control is also possible under pseudo-arrhenotoky. However, it is still not known how diplodiploidy affects the possibilities for sex ratio control.

Shifts in the offspring sex ratio have been demonstrated in relation to density, food availability and mating delays. This ability to control the sex ratio can be an adaptive trait when population mating structure varies. Theory based on single-generation mating groups predicts a female bias and can give qualitatively correct predictions, but in several cases sex ratios are more female biased than these predictions. It is argued that mites often show complex population mating structures, such as local multigeneration populations that are themselves subdivided into single-generation mating groups. This creates selection at various hierarchical levels and it is shown theoretically that these additional selection levels can create a stronger female bias in the offspring sex ratio. Before accepting this explanation, more critical tests are needed under different population mating structures. Mites are ideal objects for such studies because their population mating structures vary greatly.