Karyotypes and sex determination in two species of laelapid mites (Acari: Gamasida)

The chromosome number and morphology of two species of the family Laelapidae have been determined: Hypoaspis aculeifer Canestrini, 1887 has 9() and 18() chromosomes and Cosmolaelaps miles Berlese, 1914 7() and 14(). Both karyotypes have monokinetic chromosomes and show obvious similarity. The longest chromosome of both species consists of a normal and a heterochromatic arm. The two laelapid mites prove to be arrhenotokous, as unfertilized females lay eggs from which only males arise. A theory has been postulated that within the Gamasida an evolutionary line towards arrhenotoky is present.Arguments supporting an independent evolution of sex determining system in the two acarine taxa Actinotrichida and Anactinotrichida are discussed.