Observations on Hybridization of Three Species of North American Dermacentor Ticks

Abstract

Female Dermacentor andersoni Stiles confined on the host with male D. variabilis (Say) later produced infertile eggs, whereas the reciprocal cross sometimes yielded progeny of both sexes. Meiotic chromosome pairing, karyokinesis, and cytokinesis in some of these hybrid males suggest various degrees of chromosome homology. Certain morphological characters of the hybrids appeared intermediate while others resembled one parent species more than the other. Hybridization between D. andersoni X D. occidentalis Marx produced no progeny when male D. andersoni mated female D. occidentalis, but the reciprocal cross yielded offspring. Dermacentor variabilis is common east of the Rocky Mountains in the United States, in parts of California, Oregon, Idaho, Mexico, and Canada. D. occidentalis is prevalent from central Oregon to lower California. D. andersoni occurs in the Rocky Mountain States, western North Dakota, South Dakota, and Nebraska, south to New Mexico, California, and parts of Canada (British Columbia, Alberta, and Saskatchewan). The ranges of these taxa overlap in certain areas, yet there are some differences in apparent preferences for bioclimatic zones (Rapp, 1960; Wilkinson, 1967). Although peak activity of adults of one species might be out of phase with another (i.e., peak of D. variabilis activity is often a month later than that of D. andersoni), both species might simultaneously parasitize the more widely traveling hosts, such as ungulates and carnivores. On the host, there may be differences in preferred site of attachment as observed with D. andersoni and D. albipictus (Packard) on deer (Wilkinson, 1970), but Dermacentor males actively seek out receptive females (Berger et al., 1971) and could encounter individuals of both species. Thus there is some practical relevance to studReceived for publication 31 August 1971. * Acari: Ixodidae. t Supported in part by Public Health Service Research Grants AI-06169 and AI-09556 from the NIAID (principal investigator, James H. Oliver, Jr.). t Callaway Professor, Department of Biology, Georgia Southern College, Statesboro, Georgia 30458. ? Research Scientist, Research Station, Canada Department of Agriculture, Lethbridge, Alberta. II Sanitarian Director, Retired, U. S. Public Health Service, Rocky Mountain Laboratory, Hamilton, Montana 59840. ies of the outcome of crosses among these important disease vectors. MATERIALS AND METHODS Ticks were fed on rabbits, guinea pigs, and sheep by confining them in capsules on clipped areas of the hosts or in ear bags on rabbits (Bailey, 1960). Details of geographic origin of the species and particulars of each experiment are presented with the results of each experiment. Each of us conducted experiments unaware of similar experiments by the others. Experiments were conducted at Hamilton, Montana; Lawrence, Kansas; Berkeley, California; and Kamloops, British Columbia, and span a period from 1954 to the present. Preparation of tissues for cytological studies consisted of removing males from hosts 3, 4, 5, and 6 days after initial attachment and placing them in modified Carnoy's fixative for approximately 24 hr, before storing in a refrigerator. Subsequently, testes were removed, stained with acetocarmine, and typical squash preparations made.