Analysis of Genetic Variation in Ribosomal DNA Internal Transcribed Spacer and the NADH Dehydrogenase Subunit 4 Mitochondrial Genes of the Onchocerciasis Vector Simulium ochraceum

Onchocerciasis is a serious disease vectored by black flies in the genus Simulium that are infected with the filarial parasite Onchocerca volvulus. In the Americas, black flies of the Simulium ochraceum s.l. species complex are important vectors of this parasite. Cytological studies have suggested that this species complex consists of at least three cytotypes that inhabit distinct habitats. In this study, the NADH dehydrogenase subunit four (ND4) and internal transcribed spacer (ITS) of the ribosomal RNA gene cluster were used to explore the degree of genetic diversity among S. ochraceum s.l. populations found in the three O. volvulus foci in Mexico. Both sequence regions were found to exhibit intra- and interpopulation variation. Four different ND4 alleles were found among the populations examined. Similarly, variation was noted in the ITS domain sequences within and among populations. Variation within the ITS sequence was primarily confined to a complex microsatellite locus. Four ITS length variants were observed, two of which were only seen in flies collected from the onchocerciasis focus in northern Chiapas. These data suggest that the ND4 and ITS sequences may prove to be useful markers for exploring interactions within and among the S. ochraceum s.l. populations in Mexico.