Evolutionary processes in oribatid mites at different scales in time as indicated by molecular markers

• Main Author: Schäfer, Marina
• Format: Others
• Language: English; en
• Published: 2009
• Online Access: http://tuprints.ulb.tu-darmstadt.de/1955/1/Schaefer_Dissertation_2009.pdf; Schäfer, Marina <http://tuprints.ulb.tu-darmstadt.de/view/person/Sch=E4fer=3AMarina=3A=3A.html> : Evolutionary processes in oribatid mites at different scales in time as indicated by molecular markers. Technische Universität, Darmstadt [Ph.D. Thesis], (2009)

Chelicerata is one of the oldest arthropod phylum, already represented by diverse forms in the Cambrian fossil record, and mites (Acari) are the most diverse representative of the ancient lineage of Chelicerata. Oribatid mites (Acari, Oribatida) are species rich ubiquitous soil arthropods playing an important role in decomposition processes. They are a phylogenetic old taxon, with the oldest fossils dating to the Early Devonian (~380 million years ago), that presumably originated on land. Peculiarly, parthenogenesis is common in oribatid mites and lineages probably radiated while being parthenogenetic. The long-term existence of lineages that reproduce without males contradicts theories about the advantage and maintenance of sexual reproduction and is extremely rare among animals. Less than 1% of all organisms reproduce without sex, whereas in oribatid mites about 10% of the species are parthenogenetic. Using molecular markers we investigated evolutionary processes in oribatid mites at three time-scales. (1) The age of oribatid mites was estimated using 18S rDNA sequences and a Bayesian molecular clock approach. Remarkably, the results suggest that the radiation of oribatid mites fills the gap in the fossil record between the Cambrian explosion (540 mya) and the earliest fossil records of terrestrial ecosystems (410 mya). Therefore, oribatid mites likely were among the earliest colonisers of land, using the interstitial as stepping stone to colonise terrestrial habitats. Presumably, early terrestrial food webs were formed by omnivorous and detritivorous arthropods, thereby facilitating the invasion of terrestrial habitats by later colonisers of higher trophic levels. The ancestral state reconstruction of reproductive modes showed that the colonisation of truly terrestrial soils by oribatid mites resulted in multiple losses of sexual reproduction in basal groups and that some lineages reproduce parthenogenetically for millions of years. (2) The last ice-age lasting from ~115,000-11,000 years ago had major impact on species and genetic diversity of European arthropods. Using a molecular marker with intermediate resolution of several million years, the mitochondrial cytochrome oxidase I (COI) gene, we investigated the impact of this major climatic influence on soil organisms. Variations in COI of two oribatid mite and two springtail (Hexapoda, Collembola) species were investigated on a pan-European spatial scale. Colonisation patterns and spatial refugia differed between mites and springtails, but genetic diversity was high in each of the species at small and large spatial scale with genetic distances being extraordinarily high (>18% p-distance). The results indicate that the genetic structure of present day soil animal populations reflect pre-Pleistocene colonisation and diversification events. This suggests that the major evolutionary forces that shaped the belowground system differ from those that shaped above-ground ecosystems. (3) Processes that drive speciation in soil are unknown and the widespread occurrence of parthenogenesis among soil-living organisms has received little attention. Microsatellites are molecular markers that enable to resolve the actual genetic structure of populations, to analyse recent evolutionary processes and to infer reproductive modes. Microsatellite markers were used to investigate the population structure and genetic diversity of one sexual and one parthenogenetic oribatid mite species that coexist in the same habitat. The results showed that genetic diversity is high in both species and that gene flow in the sexual species is sufficient to ascribe all individuals in the sampling area to a single population. Parthenogenetic reproduction could be confirmed in the putatively parthenogenetic species with the population being more strongly structured indicating lower mobility of this species. Environmental factors responsible for the patchy distribution of individuals in both species could not be identified. However, microsatellites proved to be promising tools to analyse the genetic constitution of oribatid mite populations allowing to estimate population structure, population size and gene flow as major driving factors of evolutionary processes.