Evolution of haplodiploidy in dermanyssine mites (Acari : Mesostigmata)

Haplodiploidy, a widespread phenomenon in which males are haploid and femal es are diploid, can be caused by a number of different underlying genetic s ystems. In the most common of these, arrhenotoky, males arise from unfertil ized eggs, whereas females arise from fertilized eggs. In another system, p seudoarrhenotoky, males arise from fertilized eggs, but they eliminate the paternal genome at some point prior to spermatogenesis, with the consequenc e that they do not pass this genome to their offspring. In 1931 Schrader an d Hughes-Schrader suggested that arrhenotoky arises through a series of sta ges 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 rDN A from a group of mites (Mesostigmata: Dermanyssina) that contains arrhenot okous, pseudoarrhenotokous, and ancestrally diplodiploid members to test th is hypothesis. Neighbor-joining, maximum-parsimony, and maximum-likelihood methods all indicate that the arrhenotokous members of this group form a cl ade that arose from a pseudoarrhenotokous ancestor, rather than directly fr om 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.