Evaluation of morphological traits in Trhypochthoniidae with focus on Allonothrus, and morphology-molecule conflict in classification and phylogeny of Nothrina (Acari: Oribatida)

Abstract

The oribatid mite family Trhypochthoniidae exhibit the greatest collective ecological valence among the families of the middle-derivative infraorder Nothrina. Our objectives were: to examine poorly-known aspects of their morphology, with a focus on Allonothrus and a redescription of adult A. tuxtlasensis; to compare ontogenetic development of leg setation among the genera; and to test congruence between molecular and morphological inferences about relationships within Trhypochthoniidae and among Nothrina in general. New data on leg setal ontogeny are presented for Allonothrus giganticus, Archegozetes longisetosus and Trhypochthonius sp., and aspects of leg setation in the seven genera are compared. Collectively their setation is unique among Nothrina, particularly due to losses of certain fundamental tarsal setae. In a broad analysis of oribatid mite tibiae I and II, we propose that ancestral seta cʺ is usually present on I and often present also on II, but in most instances it has been given the notation lʺ or vʺ by analogy. An unusual diversity of subcapitular structure in Trhypochthoniidae gives insight into evolutionary transitions and terminological problems. Overall, there is little congruence between morphology-based phylogenetic trees (or classifications) and published molecular trees, with the following conclusions being most salient. Most molecular studies do not support a monophyletic Malaconothroidea but neither do they present a consistent picture of relationships of the two included families. Monophyly of Trhypochthoniidae (sensu lato) was generally supported—rendering moot the separation of monogeneric families Allonothridae, Mucronothridae and Trhypochthoniellidae—but their sister-group was inconsistent. Malaconothridae, also consistently monophyletic, usually were recovered as more basal in Nothrina. Considering their unusual shared narrow ventrosejugal articulation and a significant set of uncommon regressive characters, it seems reasonable to retain the two families in Malaconothroidea, pending more compelling molecular data. The distribution of apomorphies in a set of 23 morphological characters does not support molecular trees of trhypochthoniid genera. Trhypochthoniellus and Afronothrus are the most apomorphic genera, Trhypochthonius the least. Twenty-four molecular phylogeny studies with relevant representation are collectively inconclusive as to whether Nothrina is a paraphyletic (as usually perceived) or monophyletic group. Surprisingly, Hermanniidae were consistently recovered as relatively basal or even outside Nothrina, not as the sister-group of Brachypylina. Nanhermanniidae were consistently associated with Crotoniidae, and their unique morphology may be paedomorphic. Allonothrus giganticus Haq, 1978 is considered a senior subjective synonym of Allonothrus pararusseolus Subías & Sarkar, 1982 from Tripura, India (new synonymy).