Karyotypes and Sex Determination in Some Dermanyssid Mites (Acarina: Mesostigmata)1

Abstract

The number, gross morphology, and size of the chromosomes were determined for 3 species of dermanyssid mites. Early embryonic somatic tissue was studied using the aceto-orecin or aceto-carmine squash technique. Dermanyssits gallinae (De Geer), the chicken mite, had a haploid number of 3 and a diploid number of 6 chromosomes. The 2 isobrachial chromosomes were approximately 7.0 and 6.8μ and the heterobrachial one was about 5.4μ long. One secondary constriction was observed and it was situated on the short arm of the heterobrachial chromosome. The 2 macronyssine species had smaller, but a greater number of chromosomes than did D. gallinae. Ornithonyssus sylviarum (Canestrini & Fanzago), the northern fowl mite, had haploid and diploid numbers, respectively, of 9 and 18 cephalobrachial (or diffusecentric) chromosomes and these were 1.0 and 1.8μ long. The tropical rat mite, Ornithonyssus bacoti (Hirst), had haploid and diploid chromosome numbers of 8 and i6, respectively, and the chromosomes were 1.9 to 3.7μ long. Seven of these appeared to be cephalobrachial (or diffusecentric), whereas the eighth appeared isobrachial (or diffusecentric) and was approximately twice the length of the shortest chromosome. It is not certain that the 2 macronyssine species have monocentric chromosomes since no centromeres were seen, therefore the designations of cephalo- and isobrachial chromosomes are made tentatively in O. sylviarum and O. bacoti . Close agreement between the results of rearing experiments and of chromosome analyses of eggs indicated that sex determination in the 3 species was of the haplo-diploid type. Virgin females, in the 2 macronyssine species, laid only haploid eggs and produced male offspring only. Virgin female D. gallinae did not oviposit. Mated females of all 3 species laid haploid and diploid eggs in a 1:1 ratio and produced progeny of both sexes. It was suggested that D. gallinae and the macronyssine species might not be confamilial.