M. Charalambous

Four new cytotypes of the onchocerciasis vector blackfly Simulium guianense in Brazil

Abstract. Simulium (Trichodagmia) guianense is an important Amazonian vector of onchocerciasis. Examination of the polytene chromosome banding patterns of larvae from five sites in Brazil revealed the occurrence of four cytotypes, designated A, B, C and D. The chromosomal standard, Simulium guianense A, occurred at two localities in Goias State (on the Rio Tocantins and Rio Mucambao) where it was the only cytotype. The other three sites examined yielded one different cytotype from each: B from Rio Oyapoque in Amapa State, C from Rio Tocantins in Maranhao State, and D from Rio Xingu in Para State. All cytotypes differed by at least two fixed inversions, but a sex determining system was not evident in any cytotype. As the cytotypes have been found allopatrically it is not certain that they represent sibling species; sampling of sympatric populations would resolve this. During certain times of the year, voracious anthropophagy by S.guianense sensu lato occurs at the localities sampled for cytotypes A, C and D (biting data are not available for the cytotype B locality). In some other areas, however, S.guianense s.l. is entirely zoophilic. Further studies are needed, therefore, to elucidate the biting habits, vectorial capacity, geographic distribution and taxonomic status of these four, and perhaps additional, cytotypes comprising the S.guianense complex.

Cytological and isoenzyme analysis of the Bucay and Quevedo cytotypes of the onchocerciasis vector Simulium exiguum (Diptera: Simuliidae) in Ecuador

Four cytotypes of Simulium exiguum occur in Ecuador, where this morphospecies is the primary vector of onchocerciasis. In this paper, we give the first full description of the banding pattern of the larval polytene chromosomes of the Quevedo cytotype and assess its degree of genetic separation from the Bucay cytotype. Both cytotypes differ from the chromosomal standard sequence (of the Cayapa cytotype) by the fixed inversions IIL-5 and IIL-6. The Quevedo cytotype additionally differs from the standard and Bucay cytotypes by possessing a differentiated X chromosome, which is indicated by the inversion IIS-A. As the degree of reproductive isolation between the Bucay and Quevedo cytotypes has not yet been established, they must be regarded as intraspecific variants of the same species. In fact, isoenzyme characterizations showed that the Bucay and Quevedo cytotypes are differentiated only to the extent expected of incipient species or geographical populations. Moreover, the sibling species status previously given to the Bucay cytotype needs be reassessed, there being inadequate analysis from areas in Ecuador where Bucay occurs in sympatry with the standard Cayapa cytotype. No isoenzyme electromorphs were discovered that identified all or most adult females of any one (cytotypepure) collection.

Distribution and taxonomic status of chromosomal forms of the onchocerciasis vector Simulium exiguum in central Ecuador

The public health importance of onchocerciasis in Ecuador has continued to grow since this disease was discovered in the Esmeraldas Province by Carvajal & Zerega (1980). A survey conducted in 1989 demonstrated not only that prevalence of the disease was increasing in Esmeraldas, but also the movement of infected people from Esmeraldas was spreading the disease to new areas where the biting densities of potential vector Simuliidae are high enough to establish new foci of infection (Guderian & Shelley, 1992). Sirnuliurn exiguurn Roubaud sensu lato has been incriminated as the main vector of Onchocerca volvulus Leuckart in the principal focus of onchocerciasis in Esmeraldas (Shelley & Arzube, 1985). Larval polytene chromosome analysis of S.exiguurn s.1. from several localities has revealed it to be a species complex. At present four chromosomal forms (cytotypes) have been uncovered: Aguarico, Bucay, Cayapa and Quevedo (Procunier et a/., 1985; Procunier, 1989). Aguarico and Cayapa are thought to be distinct species (Procunier et al., 1985). Although S.exiguurn s.l . is known to be widespread in Ecuador, occurring on both sides of the Andes between altitudes of SO and 2000m (Shelley et al., 1989), the distribution of each of the four cytotypes is not well documented. The purpose of this paper is to map the distribution of chromosomal forms of S.exiguurn in central Ecuador on the western side of the Andes, a region to/ through which many people infected with 0.volvulus are migrating. Eleven collections were made in September 1991 at sites where S.exiguurn s.1. has been recorded before (Charalambous et al., 1993) and at sites where it was likely

Fitness trade-offs and the maintenance of alternative male morphs in the bulb mite (Rhizoglyphus robini)

Alternative reproductive phenotypes (ARPs) occur across a wide range of taxa. Most ARPs are conditionally expressed in response to a cue, for example body size, that reliably correlates with the status of the environment: individuals below the (body size) threshold then develop into one morph, and individuals above the threshold develop into the alternative morph. The environmental threshold model provides a theoretical framework to understand the evolution and maintenance of such ARPs, yet no study has examined the underlying fitness functions that are necessary to realize this. Here, we empirically examined fitness functions for the two male morphs of the bulb mite (Rhizoglyphus robini). Fitness functions were derived in relation to male size for solitary males and in relation to female size under competition. In both cases, the fitness functions of the two morphs intersected, and the resulting fitness trade‐offs may play a role in the maintenance of this male dimorphism. We furthermore found that competition was strongest between males of the same morph, suggesting that fitness trade‐off in relation to male size may persist under competition. Our results are a first step towards unravelling fitness functions of ARPs that are environmentally cued threshold traits, which is essential for understanding their maintenance and in explaining the response to selection against alternative morphs.

The potential for dispersal of onchocerciasis in Ecuador in relation to the distribution of the vector Simulium exiguum (Diptera:Simuliidae).

The future dispersal of onchocerciasis in Ecuador is dependent on the distribution of cytotypes of the vector species complex Simulium exiguum. Over the last 14 years, collections of larvae have been made from over 25 rivers, between 80-1600 m altitude, from various sites on both sides of the Andes. Analysis of larval polytene chromosomes was used to determine the distributions of each cytotype. On the western side of the Andes, the Cayapa cytotype (the only cytotype directly incriminated as a vector) has a distribution from Santo Domingo de los Colorados northwards. The Quevedo and Bucay cytotypes occur from Santo Domingo de los Colorados southwards. On the eastern side of the Andes, the Aguarico cytotype occurs in the Rio Aguarico and a new cytotype is present in the tributaries of the Rio Napo. Whether the disease will spread south of Santo Domingo and on the eastern side of the Andes depends on vector capacity of the cytotypes and the dispersal patterns of individuals infected with onchocerciasis. At present the Aguarico, Bucay and Quevedo cytotypes are known to be efficient hosts, but their biting preferences and biting densities have not yet been evaluated.

Cytogenetical analysis of the Simulium damnosum complex (Diptera: Simuliidae) in Guinea Bissau

Abstract. In a 3‐year study during 1990–92, larval collections of the Simulium damnosum complex from the River Corubal system in Guinea Bissau revealed that the only sibling species present were S. sirbanum and the Konkouré form of S. konkourense, but not S. damnosum s.s. which had been found at some of the localities (Saltinho and Cusselinta) in a previous survey by Quillévéréet at. (1981). Their differential distributions were seasonally consistent between years. S. sirbanum was concentrated in the upper reaches of the Corubal, moving downstream at the end of the dry season to exploit new breeding grounds, whereas S. konkourense seemed to be concentrated in the lower reaches and moved upstream in the middle of the dry season.

Isolation by distance and a chromosomal cline in the Cayapa cytospecies of Simulium exiguum, the vector of human onchocerciasis in Ecuador.

The population genetic structure of the Cayapa cytospecies of Simulium exiguum, the vector of onchocerciasis, was analysed using allozyme frequency and chromosomal inversion polymorphism data from 6 and 15 populations respectively, collected in Ecuador. Eight allozyme loci were scored. No unique allozyme markers were found enabling us to identify biting adults of the vector from the non-vector Bucay cytotype. Mannose-phosphate isomerase (Mpi) contributed largely to the significant heterogeneity in gene frequency among populations of the Cayapa cytospecies and also to the overall population structuring (FST=0.015 ± 0.014) which fitted the isolation by distance model. However, heterozygote deficits were recorded for Mpi in four of the six populations, which could indicate that selection is acting at this locus but this hypothesis will require further convincing evidence. Furthermore the significant population structuring of allozymes was not evident when Mpi was omitted from the analysis. All inversion polymorphisms (IIS-B, IIS-F, IIL-A and IIL-B) were in Hardy-Weinberg equilibrium, showed significant heterogeneity between populations and revealed the occurrence of an altitudinal cline in inversion IIS-B frequency. The inversion polymorphisms revealed a significant degree of population structuring (FST=0.083 ± 0.027), which can be explained by the isolation by distance model. A UPGMA cluster analysis revealed the relatively remote, high altitude Rio Mira populations to be the most genetically distinct.