The sexually linked Mpi locus is presumably involved in imidothiazole resistance in Oesophagostomum dentatum parasites

Information about genetic changes during the selection process could indica te mechanisms underlying the spread of resistance to anthelmintic drugs. Fo r clarification of the role of the Mpi locus encoding mannose-phosphate iso merase enzyme in determining resistance, genotyping of Oesophagostomum dent atum strains was performed using an isoelectrofocusing technique. In levami sole- and pyrantel-selected strains the allele associated with resistance h as probably been found. Significant values for genetic differentiation betw een treated and untreated strains of common origin were recorded by F-st in dices (theta = 0.078; P = 0.0008). The specific genomic makeup of a flubend azole-resistant strain, which did not correlate with that of the remaining isolates, might be ascribed to a different action of the anthelmintic or di fferent environmental conditions under which resistance against this drug a rose. The absence of heterozygotes in male populations indicated an XX/X0 s ystem of sex determination for the Mpi locus, thus providing a greater pote ntial for the development of resistance. A possible involvement of alleles linked with mannose-phosphate isomerase in alterations of membrane receptor s that can be associated with resistance against imidothiazole-based drugs is discussed.