Mites as chelicerates: placement of Acariformes and Parasitiformes among Chelicerata and insights on Acariformes ingroup phylogeny

Phylogenetic inferences employing almost complete ribosomal sequences in a Bayesian framework are reported. At least two representatives from all euchelicerate orders (among them 72 are acariform mites) and numerous out-groups (Myriapoda, Crustacea, Hexapoda, Onycophora, and Priapulida) were sampled, summing up to 228 terminal taxa. Sequences were aligned using published rRNA secondary structures as guides. This approach allowed employing separate substitution models for stems and loops and identification of regions of ambiguous alignment. Although chelicerate phylogeny is essentially unsolved in our molecular analysis, the data strongly support diphyletic Acari, with Acariformes as a sister group of Solifugae (Poecilophysidea hypothesis). Parasitiformes was resolved as a monophylum, but its placement is sensitive to analyses’ parameters. Among Acariformes, some of the so-called Endeostigmata, which in recent classifications have been placed in Sarcoptiformes along Oribatida, are recovered as basal relative to a taxon including most of Acariformes (Trombidiformes + Sarcoptiformes). Desmonomatan oribatid mites were recovered as monophyletic and a sister group of Astigmata. Among Trombidiformes, Eleutherengona and Parasitengona were recovered monophyletic. On the other hand, Eupodina was not. Halacaridae is traditionally regarded as a Bdelloidea sister-group, but in our analyses it was recovered as a sister group of the diverse Parasitengonina. This placement was anticipated in exclusively morphological studies and is congruent with the presence in these groups of a reduced fixed digitus and scythe-like movable digit in chelicerae; reduction of coxal gland saccules and cheliceral protractor muscle arrangement. Labidostommatidae is recovered as the basal Trombidiformes, in agreement with their chelate chelicerae (plesiomorphic character state). Anystidae appears as diphiletic, with a more derivate position for Erythracarus which along with Adocaeculus (Caeculidae) is recovered as sister to Halacaridae +Parasintegonina, congruent to the presence in both clades of a thumb-claw process, a feature absent in Anystinae.