Comparative phylogeography of co-distributed arboreal mites

Several factors such as history, gene flow or genetic drift shape the genetic structure of a species. Especially the climatic changes of the Quaternary had important impacts on recent phylogeographic and population genetic structures of organisms. So far, phylogeographic studies that focused on the Quaternary history, refugial areas and (re-)colonization routes in Europe have been clearly dominated by highly mobile taxa (seed of plants, vertebrates, flying insects). Comparative phylogeographic analyses focusing on distantly related but co-distributed species with proposed different dispersal abilities are an approved strategy to inference the effect of historical and/or contemporaneous processes driving species diversification. The present proposal is a pilot study that shall provide first insights into local- and large-scale phylogeographic patterns by comparing several co-distributed arboreal oribatid mite species with (presumed) different dispersal modes (phoretic versus non-phoretic behavior) in Europe. The use of traditional molecular techniques, plus modern next-generation sequencing (NGS) - a new application for population genetics and phylogeography in arachnids will allow for illuminating recent (gene flow, genetic drift) and past (bottlenecks) evolutionary processes as far as population contractions/expansion and colonization routes, which had important impacts on contemporary intraspecific diversity of species that overlap in time and space. The following three main hypotheses could be formulated and shall be tested in the framework of the project by the use of landscape genetic approaches, standard population genetics, clustering methods and correlation analyses: i) higher dispersal ability overlies the effect of historical processes on phylogeographic/ population genetic structure, ii) apparent low levels of host specificity and Palaearctic distributions are due to unrecognized cryptic diversity and iii) different underlying bio-geographical traits determined the varying responses of co-distributed species to Pleistocene glaciation events. The integration of species distribution models can further contribute to the exploration of the impact of global climate change on small organisms with different dispersal modes inhabiting specific habitats. Several direct and indirect effects (as e.g. climate change, habitat fragmentation, agricultural work, urbanization or deforestation) are responsible for shaping large-scale distribution of species. Given that most of the included study species are specialized to life on trees, insights into ecology, structure and dispersal modes/rates should serve as a basis for acquiring knowledge about the effects of human activities on micro- and mesofauna of specific habitats.

Bark beetles are well-known to everyone, especially due to their destructive effects in natural forests. Few people, however, know that there are some tiny, inconspicuous organisms which are really hooked on the beetles and this in the truest sense of the word. One of these was in the focus of the present project, namely the moss (or oribatid) mite with the scientific name "Paraleius leontonychus". The term "leontonychus" is a word of Latin derivation meaning "lion-like claw" and refers to strong, hook-like claws on the species legs which are adaptations to their special association with bark beetles. With these claws Paraleius actively attaches to the beetles (=the host) and uses them as a means of transport to change location. This behavior is called phoresy and is quite unusual for oribatids. In general, and especially because of the species' particular life and dispersal strategy, little was known about abundance, distribution range or possible host specificity of the study species. Our research project particularly focused on clarifying some of these issues. Further goals were the identification of other oribatids with phoretic behavior and elucidating the processes responsible for the current geographical distribution of different populations of the target species. In general, Paraleius spends its entire life cycle in the tunnels (galleries), which are fed in the bark by the beetles, where they find nutrition, reproduce and lay eggs. Of course, it is not only this particular oribatid mite species that occurs in the galleries. Investigating several beetle-infested bark samples revealed that few other oribatids can also be found on trees, suggesting that they represent a general oribatid mite community associated with bark beetles. Most of them, however, also occur in other substrates, like lichens, mosses or soil and might use the galleries only in search for food, mating partner, or shelter. Information on the dispersal mode of the investigated mites could be obtained from extensive studies of sampling material collected from pheromone traps attracting either the European spruce bark beetle or the sixtoothed spruce bark beetle. Accordingly, it became obvious that really no oribatid species, except Paraleius, actively uses bark beetles for transport. Molecular genetic analyses which operate on the detection of differences in the genetic makeup of individuals, however, yielded the most exciting findings, e.g. that (i) there can be several, mainly indistinguishable species hidden under one known - a phenomenon found in phoretic Paraleius, but also in other non-phoretic tree-living species, (ii) Paraleius species do not tend to be strictly host/beetle specific, or (iii) low dispersal ability, as assumed in oribatids, does not imply differentiation of the populations, also not of those that are geographically far away from each other.