DNA polymorphisms and subpopulations in Babesia bovis.

Independent isolates of Babesia bovis differ by only a limited number of polypeptides, some of which may be important as host protective antigens. Avirulent derivatives of these parasites also differ from their virulent counterparts in only a few polypeptides. To identify genes encoding such polypeptides we have isolated cDNA clones corresponding to poly(A)+ RNAs that are expressed only in certain isolates. For this purpose a cDNA clone library was constructed from poly(A)+ RNA of the K-avirulent isolate (KA). These clones were screened by colony hybridization using [32P]cDNA complementary to poly(A)+ RNA from KA and from virulent isolates, in order to identify clones that selectively hybridize to one cDNA probe. Hybridization of DNA from three clones, designated pK4, pK5 and pK6 to poly(A)+ RNA from various isolates revealed different and complex patterns. The gene represented by clone pK5 appeared to be transcribed predominantly in avirulent parasites. Analysis of genomic DNA by the Southern procedure enabled each isolate to be distinguished and suggested that most isolates are comprised of a heterogeneous mixture of subpopulations. Analysis of genomic DNA from parasites obtained after passage of KA through the tick vector (Boophilus microplus) suggested that a subpopulation was being selected that more closely resembled KV than KA.