No abstract

A cytotaxonomic study of the medically important insect vector, Simulium damnosum s.l., revealed the presence of seven and possibly eight distinct taxa from central and northeastern Tanzania. Larval salivary gland polytene chromosome maps are presented for the first time for five cytotypes and one sibling species, which include the Nkusi form, the Sanje form, the Kisiwani form, Ketaketa C1 and C2, and the Kibwezi form. Inversion disequilibrium in males of the Kibwezi form indicate population substructuring is occurring and that this population may be in the early stages of speciation. Adults of the sibling species S. damnosum Kibwezi form and cytotype of the S. damnosum Nkusi form were identified using Malpighian tubule polytene chromosomes. The taxonomic status of the populations under study are discussed in relation to previously published papers and unpublished reports. Dimorphisms for centromere band enhancement occur on all three polytene chromosomes of the complement. The same centromere band can be polymorphic, sex linked, fixed, or lost in various cytotypes. In constructing a partial phylogeny, a hypothetical intermediate is proposed to account for the diverse fate of these centromere band dimorphisms and other inversion polymorphisms in different members of this nearly pan African complex. This pattern of chromosome restructuring is consistent with that seen for other species complexes within the Simuliidae.

Eleven members of the Simulium metallicum complex from Central America and South America are described using the polytene chromosome banding pattern of S. metallicum A as standard. The members are B from Mexico and Guatemala, C from Colombia, D and E from Venezuela, F from Panama, G from Costa Rica, H from Mexico, Guatemala, and Panama, I from Mexico and Guatemala, and J and K from Panama. A cytophylogeny separates the members into three lineages. H, I, J, and K form one lineage, separated from the standard by eight rearrangement steps. D and E form a second lineage on the basis of six fixed inversions in common. Three floating inversions characterize the common progenitor of C and G, which together form a third lineage. B is independently derived from the standard and separated from it by seven rearrangement steps. F is very close to A. Six cytotypes (A, B, H, I, J, and K) appear cytologically to be reproductively isolated and are considered sibling species. One cytotype, E, is a sibling species by virtue of cytological criteria, and possible ecological and physiological factors. The remaining four members (C, D, F, and G) are cytotypes requiring additional evidence to confirm sibling species status. Sibling A is provisionally considered to represent S. metallicum s.s.; sibling H may be S. horacioi Okazawa &Onishi. The status of members within the complex as vectors of Onchocerca volvulus is discussed.

This paper presents an overview of advances in cytological research on the biosystematics of vector simuliid complexes in the areas of identification, age grading, and the evolution of resistance in relation to the epidemiology and control of human onchocerciasis. Systematic theory is discussed and relevant examples are given to show its application in predicting and resolving current problems in species identification for New World and Old World vector complexes. These complexes include Simulium damnosum s.l. and S. neavei s.l. from Africa and S. exiguum s.l., S. metallicum s.l., S. ochraceum s.l., and S. oyapockense s.l. from Latin America. The evolution of resistance in S. damnosum s.l. and the need for future molecular research as part of resistance management strategies are discussed.

The ribosomal DNA (rDNA) of Rhagoletis pomonella was localized within both salivary gland polytene chromosomes and somatic cell mitotic chromosomes by in situ hybridization using the heterospecific Drosophila melanogaster rDNA clone, Dm238. In situ hybridization analysis of polytene nuclei showed that R. pomonella rDNA is located in the nucleolus and adjacent granular network of chromosome 1. The site of origin of rDNA is within this isomorphic granular network. The preservation of nucleolar ultrastructure in some polytene chromosome preparations allowed light microscope localization of R. pomonella rDNA to the apparent periphery of fibrillar centres within fibrillar complexes. In somatic cell nuclei, DM238 hybridized to the nucleolus organizing region (NOR) located on chromosome 1 at the site of the secondary constriction. The frequency distribution of heteromorphisms for rDNA content, differential appearance of secondary constrictions, non-pairing of the NOR and differences in homologue lengths suggests that the structural differentiation of this region in chromosome 1 is sex linked. This the first published description of the salivary gland polytene chromosomes from R. pomonella, and we include a tentative karyotype description, polytene chromosome maps and comments on their suitability for banding and molecular analysis.