Evolution of haplodiploidy in dermanyssine mites (Acari: Mesostigmata)

Haplodiploidy, a widespread phenomenon in which males are haploid and females are diploid, can be caused by a number of different underlying genetic systems. In the most common of these, arrhenotoky, males arise from unfertilized eggs, whereas females arise from fertilized eggs. In another system, pseudoarrhenotoky, males arise from fertilized eggs, but they eliminate the paternal genome at some point prior to spermatogenesis, with the consequence that they do not pass this genome to their offspring. In 1931 Schrader and Hughes-Schrader suggested that arrhenotoky arises through a series of stages involving pseudoarrhenotokous systems such as those found in many scale insects (Homoptera: Coccoidea), however, their hypothesis has been largely ignored. We have used a phylogenetic analysis of 751 base pairs of 28S rDNA from a group of mites (Mesostigmata: Dermanyssina) that contains arrhenotokous, pseudoarrhenotokous, and ancestrally diplodiploid members to test this hypothesis. Neighbor-joining, maximum-parsimony, and maximum-likelihood methods all indicate that the arrhenotokous members of this group form a clade that arose from a pseudoarrhenotokous ancestor, rather than directly from a diplodiploid one. This provides unequivocal support for the hypothesis of Schrader and Hughes-Schrader. The wider implications of this result for the evolution of uniparental genetic systems are discussed.