Cytological studies on the specific distinctness of the ovine and bovine 'Strains' of the Nematode Haemonchus Contortus (Rudolphi) cobb (NematodaL Trichostrongylidae)
The studies reported here follow the conclusions of Roberts, Turner, and McKevett (1954) that the ovine and bovine "strains" of Haemonchus contortus (Rudolphi 1803) Cobb 1898 are distinct species. The cytology of the two forms has been investigated and it was found that the chromosome number for each form was 2n = 11 (B), 12 (@). The autosomes of each form measure 3¼ in length and, whereas the X-chromosomes of the worms from sheep are similar in size to the autosomes, the X-chromsomes of the worms from cattle attain a size of 8¼. Fertile hybrid females were obtained in a cross-breeding experiment and were also seen in a natural, mixed infestation. These, however, appeared in only small numbers and, furthermore, as they were never seen in animals with pure infestations as judged by the type of larva, it seemed evident that some fertility barrier is present. Some discussion is given to the host specificity of the two forms and further evidence is brought forward to support previous conclusions that some degree of host specificity is present. It is concluded that the restricted degree of interbreeding encountered, together with considerations of host specificity, support the claims of Roberts, Turner, and McKevett (1954) that the two forms are separate species.