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 isolated from the saliva of the horn fly (Haematobia irritans), a blood-sucking ectoparasite of cattle. This report provides a detailed characterization of the TS gene and the first analysis of the allelic complexity of a gene for an anti-hemostatic protein from a blood-feeding insect. Multiple point mutations at fixed positions in the TS gene were identified in a cDNA library prepared from mRNA isolated from horn fly salivary glands. When translated, the variant mRNAs would specify five biochemically active peptides that differ in molecular weight, isoelectric point and predicted secondary structure. Allelic variation with the same mutation pattern was revealed in the genomes of individual flies collected in the field and sampled from a long-standing laboratory colony. Approximately 60% of flies examined carried heterozygous alleles, including five additional alleles not found in the cDNA library. Comparative analysis of the allelic mutations and the predicted effects on secondary structures of the active proteins produced suggest that the TS gene may be undergoing evolutionary selection.