Where Eriophyoidea (Acariformes) Belong in the Tree of Life

SIMPLE SUMMARY: Eriophyoidea (gall mites) are a group of four-legged, vermiform plant feeders that are among the most economically damaging groups of mites. Gall mites (although not all of them cause galls) were thought to belong within Trombidiformes, which represents a very large group of mites that includes other plant-feeding taxa, such as spider mites (Tetranychidae) and flat mites (Tenuipalpidae). Most recent phylogenetic analyses show no support for this taxonomic assignment. Instead, gall mites are almost certain to be closely affiliated with Nematalycidae, a group of vermiform mites that exclusively live in the soil. The small number of analyses that support the placement of gall mites within Trombidiformes are compromised by a lack of data, being based on only a few genes and/or excluding critical taxa. The majority of analyses support the placement of gall mites outside Trombidiformes. ABSTRACT: Over the past century and a half, the taxonomic placement of Eriophyoidea has been in flux. For much of this period, this group has been treated as a subtaxon within Trombidiformes. However, the vast majority of recent phylogenetic analyses, including almost all phylogenomic analyses, place this group outside Trombidiformes. The few studies that still place Eriophyoidea within Trombidiformes are likely to be biased by incomplete taxon/gene sampling, long branch attraction, the omission of RNA secondary structure in sequence alignment, and the inclusion of hypervariable expansion–contraction rRNA regions. Based on the agreement among a number of independent analyses that use a range of different datasets (morphology; multiple genes; mitochondrial/whole genomes), Eriophyoidea are almost certain to be closely related to Nematalycidae, a family of vermiform mites within Endeostigmata, a basal acariform grade. Much of the morphological evidence in support of this relationship was apparent after the discovery of Nematalycidae in the middle of the 20th century. However, this evidence has largely been disregarded until very recently, perhaps because of overconfidence in the placement of Eriophyoidea within Trombidiformes. Here, we briefly review and identify a number of biases, both molecular- and morphology-based, that can lead to erroneous reconstructions of the position of Eriophyoidea in the tree of life.