Polymorphism of the thrombostasin gene in the horn fly (Haematobia irritans) revealed in a cDNA library and in genomic DNA

Thrombostasin (TS) is a newly described thrombin-inhibiting protein isolate d from the saliva of the horn fly (Haematobia irritans), a blood-sucking ec toparasite of cattle. This report provides a detailed characterization of t he TS gene and the first analysis of the allelic complexity of a gene for a n anti-hemostatic protein from a blood-feeding insect. Multiple point mutat ions at fixed positions in the TS gene were identified in a cDNA library pr epared from mRNA isolated from horn fly salivary glands. When translated, t he variant mRNAs would specify five biochemically active peptides that diff er in molecular weight, isoelectric point and predicted secondary structure . Allelic variation with the same mutation pattern was revealed in the geno mes of individual flies collected in the field and sampled from a long-stan ding laboratory colony. Approximately 60% of flies examined carried heteroz ygous alleles, including five additional alleles not found in the cDNA libr ary. Comparative analysis of the allelic mutations and the predicted effect s on secondary structures of the active proteins produced suggest that the TS gene may be undergoing evolutionary selection.