Cytological evidence for automictic thelytoky in parthenogenetic oribatid mites (Acari, Oribatida): Synaptonemal complexes confirm meiosis in Archegozetes longisetosus

Diplo-diploid parthenogenesis (thelytoky) is a widespread phenomenon in the mite taxon Sarcoptiformes, and is unusually frequent in the suborder Oribatida, where it characterizes almost 10% of extant species, including whole genera and families. Based on molecular and cytological data, terminal fusion automixis with an inverted meiotic sequence based on holokinetic chromosomes has been suggested as the reproductive mode in these mites. However, unequivocal structural evidence for meiosis is missing. The model organism Archegozetes longisetosus, a thelytokous member of the parthenogenetic family Trhypochthoniidae, was studied to gain ultrastructural insight in oocyte progression and meiotic processes. In this study, ovarian nuclear organization of its tritonymphal instar was examined by transmission electron microscopy (TEM). Numerous synaptonemal complexes were observed in the ovary, unequivocally confirming automixis (meiotic thelytoky) in oribatid mites for the first time. No recombination nodules were found. The nuclei are transcriptionally active in late prophase. Inverted meiosis is discussed as a result of the spatial arrangement of chromatid segregation.