RESUMO
Larvae of the Simulium metallicum complex (Diptera: Simuliidae) were collected from three foci of human onchocerciasis in Mexico. Specimens were separated into five different cytotypes, identified by morphological characteristics (head pattern and body colour) or polytene chromosome features. Differences were found between foci in the cytotype composition of the S. metallicum complex. Nearly all specimens were cytotype A in the Chamula (97%) and Soconusco (86%) foci. In the Oaxaca focus, however, cytotype I predominated (63%) with 14% cytotype A. Cytotype I comprised only 6% of specimens in the Soconusco focus and was very rare in the Chamula focus. Cytotypes B and H occurred only rarely in all three foci. Cytotype X was found only in Oaxaca. Environmental variables were measured at the collection sites and canonical correspondence analysis (CCA) was used to investigate the associations of cytotype distributions with the following factors: stream size, amount of shade, water clarity, pH, temperature and altitude. Members of the S. metallicum species complex were found to be differentially distributed according to stream conditions and there was a significant correlation between their distributions and the environmental variables. The most important factor in canonical axis 1 of the CCA was pH (t-value -4.38) with temperature (-2.48) and altitude (2.19) having some influence, but the other variables were unimportant. In the second canonical axis, pH (-3.52) was the only variable having a significant effect. Thus, cytotype A was associated with high pH, high temperature and low altitude; B was associated with similar sites but with higher temperatures and at lower average elevations. Cytotype H was found at the centres of the ranges of these variables, I at sites with the lowest temperatures and highest altitudes and X in rivers with the lowest pH.
Assuntos
Simuliidae , Animais , Demografia , Meio Ambiente , Humanos , Larva , México , Simuliidae/classificaçãoRESUMO
Larval collections of Simulium ochraceum Walker (Diptera: Simuliidae) were made in a variety of streams in the three onchocerciasis foci in Mexico and identified by cytotaxonomic criteria, based on the banding pattern of polytene chromosomes in larval salivary gland nucleii. S. ochraceum cytotype A was found in the Soconusco focus, cytotype B in the Oaxaca focus and cytotype C (not previously recorded in Mexico) in the Chamula focus. Attempts to analyse chromosomes of adult specimens were unsuccessful. A preliminary study of the cuticular hydrocarbons of adult specimens by gas liquid chromatography provided evidence that this technique may be suitable for separating the cytotypes.