Parthenogenesis in Oribatid Mites (Acari, Oribatida): Evolution Without Sex

Abstract

Oribatid mites (Acari, Oribatida) are an extraordinarily old and speciose group of chelicerate arthropods that probably originated in Silurian times. A high number (∼10%) of oribatid mite species reproduces via parthenogenesis, presumably by terminal fusion automixis with holokinetic chromosomes and an inverted sequence of meiotic divisions. Several of the old taxa of oribatid mites likely have radiated while being parthenogenetic. Many species of those parthenogenetic clusters are morphologically distinct – this distinctness contrasts with high genetic variance, as has been confirmed by molecular studies, e.g. for Platynothrus peltifer and species of the genus Tectocepheus. Platynothrus peltifer comprises at least seven distinct molecular lineages which are geographically separated and may be recognized as cryptic species. Stable isotope ratios (15N/14N and 13C/12C) of oribatid mite species indicate that they occupy distinct trophic niches; however, the exact nature of these niches is unknown. One of the few microhabitats colonized by specific oribatid mite species is the bark of trees. The tree-inhabiting genus Crotonia re-evolved sexual reproduction from parthenogenetic ancestors, potentially while colonizing trees. Understanding the high degree of parthenogenetic reproduction in soil living oribatid mites allows the dissection of the functional role and evolution of sexual reproduction, and the factors responsible for the long-term survival and radiation of parthenogenetic species.