Evolutionary history and population structure in European Scutoverticidae

The mite family Scutoverticidae is classified within the Higher Oribatida (Circumdehiscentiae). More than 60 species of Scutoverticidae are known worldwide. In Central Europe four genera with nine species are described. These species live in soils with extremely changing conditions (flood plains or saline soils) or in dry and rocky sites in lowland areas as well as in the alpine zone. Most of these habitats are small and isolated from each other by greater distances. The evolutionary relationships of the Scutoverticidae and related families are highly uncertain. Species are not described in sufficient detail and most of their juvenile stages are not investigated morphologically. This lack of knowledge leads to an uncertain classification and to superfluous descriptions of new species, typically for many other oribatid families, too. Therefore, Central European Scutoverticidae are suitable as model group for a combined multidisciplinary investigation. In this context two main research questions need to be answered: 1) Are there morphological and genetic differences within and between geographically isolated populations of one species? Does gene flow exist among populations or are there cryptic species hidden within taxa showing broad ecological plasticity as Scutovertex minutus and S. sculptus? Thereby we intend to integrate morphological, postembryonic developmental and molecular genetic data (mtDNA sequences and AFLP-fingerprinting). 2) Does the genus Scutovertex represent one monophyletic entity and are the Scutoverticidae a monophylum or must this family be considered as para- or polyphyletic? The comparison of different species of Scutovertex and some other genera of the Scutoverticidae and of related families (e.g. Micreremidae, Passalozetidae, Cymbaeremaeidae, Unduloribatidae) by mitochondrial DNA sequences (COI), nuclear DNA sequences (28S rDNA) and morphological data as well as their phylogenetic analysis should contribute to a classification of the family based on a natural system of relationships.

In the course of the research project Central European species of the soil and moss dwelling oribatid mite family Scutoverticidae (Acari: Oribatida) were investigated dealing with two main goals: i) To evaluate species integrity with respect to genetic differentiation between populations and the possible existence of cryptic species within the target taxa S. minutus and S. sculptus. ii) To test the monophyly of the genus Scutovertex and the relationship of Scutoverticidae to other oribatid families with so called plicate nymphs. Detailed morphological, morphometric and molecular genetic analyses of the Scutovertex species (S. minutus, S. sculptus, S. alpinus, S. pannonicus, S. pilosetosus, S. pileatus n.sp., S. ianus n.sp., S. arenocolus n.sp. ) confirmed the high diversity within this genus in Europe. Analysis of sequences of the mitochondrial cytochrome oxidase I (COI) gene showed remarkable differences in mtDNA diversity estimates and the distribution of mitochondrial lineages between the two closely related species S. sculptus and S. minutus in Austria. Genetic diversity was considerably lower in S. minutus than in S. sculptus. A clear geographic sub-division into samples originating from north and south of the Central Alps became evident in S. minutus, whereas on this regional scale no phylogeographic structure was found in S. sculptus. The lack of sub- structure with regard to habitat types in S. sculptus suggests that this species can cope with a wide range of environmental conditions. Analyzing molecular genetic data from specimens originating from far distant localities (from Russia, Sweden to France) indicate that S. sculptus might represent a complex of several cryptic species exhibiting noticeable genetic, but hardly any morphological divergence. The Scutoverticidae are considered to represent an assemblage of distantly related but morphologically similar genera. Both, molecular genetic and morphological data revealed a paraphyletic genus Scutovertex, with S. pictus probably representing a distinct genus, and Lamellovertex, Exochocepheus, and Provertex were confirmed as members of the family. To estimate the systematic position of the Scutoverticidae 40 representatives of all five subdivisions of Brachypylina were included in the study. A molecular phylogeny was established on the basis of three nuclear markers (28S rDNA, ef-1alpha, hsp82) and the evolution of six morphological traits (scalps, plicate cuticle, sclerits, and centrodorsal setae in nymphs plus octotaxic system and pteromorphs in adults) was traced over the molecular phylogeny using parsimony, likelihood and Bayesian approaches. The molecular phylogeny indicated considerable discrepancies to the commonly used classification of the five "circumdehiscent" subdivisions, suggesting paraphyly of the three families: Scutoverticidae, Ameronothridae, and Cymbaeremaeidae. Ancestral state reconstructions of these six diagnostic characters and statistical evaluation of alternative phylogenetic hypotheses also partially rejected the current morphology-based classification and suggested multiple convergent evolution of some traits, e.g. the plication of juvenile stages.