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Trente ans de lutte contre l’onchocercose en Afrique de l’Ouest. Traitements larvicides et protection de l’environnement

 | 
Laurent Yaméogo
, 
Christian Levêque
, 
Jean-Marc Hougard

Références des articles présentés sur le cédérom / Papers references presented on CD-ROM

Cytotaxonomic revision of the Simulium sanctipauli subcomplex (Diptera: Simuliidae) in Guinea and the adjacent countries including descriptions of two new species

D.A. Boakye, R.J. Post, F.W. Mosha, D.P. Surtees et R.H.A. Baker

Résumé

The Simulium sanctipauli Vajime & Dunbar subcomplex of the West African S. damnosum Theobald complex is cytotaxonomically revised for the western part of the Onchocerciasis Control Programme area. The subcomplex is defined and a chromosomal key provided for the identification of the sibling species and forms recognized. Two sibling species are newly described, S. leonense Boakye, Post & Mosha (Sierra Leone) and S. konkourense Boakye, Post, Mosha & Quillévéré (Guinea and Sierra Leone). Detailed chromosomal data are provided as warranty for the conclusions about the specific or infraspecific status of the taxa recognized.

Texte intégral

Introduction

1Human onchocerciasis is a severely debilitating blinding disease caused by infection with the parasite Onchocerca volvulus (Nematoda: Filarioidea). In West Africa it is transmitted exclusively by members of the Simulium (Edwardsellum) damnosum Theobald complex (Diptera: Simuliidae). The sibling species within this complex have been distinguished by chromosomal (cytotaxonomic) features, and their accurate identification remains difficult. Nevertheless it is known that they can show profound differences in their epidemiological importance (Post & Boakye, 1992) and much effort has been invested in the taxonomy of the complex. However, the taxonomic description of the complex remains largely dependant on larval chromosome data, which has sometimes proved difficult to interpret. This is especially true for the S. sanctipauli Vajime & Dunbar subcomplex, and the present work was undertaken to clarify this confused subcomplex.

2The S. damnosum complex in West Africa consists of eight named cytospecies, described on the basis of the interspecific inversion differences in the banding sequences of the larval silk gland polytene chromosomes (Vajime & Dunbar, 1975). Simulium sanctipauli and S. soubrense Vajime & Dunbar, which are both vectors of human onchocerciasis, share a host of floating inversions but were distinguished from each other by the inversion IIL-7. However, this inversion was subsequently found to be polymorphie extensively across West Africa (Quillévéré, 1975; Quillévéré et al., 1982; Meredith et al., 1983), and therefore was not suitable as a spedes-diagnostic inversion. More recently Post (1986) used two previously unrecognized fixed inversions (IL-A and IIL-A) to distinguish three spedes, namely S. sanctipauli, S. soubrense and S. soubrense ′B′. ′Beffa′ form (Meredith et al., 1983) was considered as an intraspedfic variant within S. soubrense, and forme ′Konkouré′ (Quillévéré et al., 1982) remained unassigned to any particular species within the S. sanctipauli subcomplex (Post, 1986).

3Previous distribution maps of S. sanctipauli and S. soubrense based solely on IIL-7 (e.g. Garms & Vajime, 1975; Quillévéré et al., 1982) cannot easily be redrawn according to the new identification criteria proposed by Post (1986). However, in the Onchocerdasis Control Programme (OCP) Western Zone specimens identified since June 1984 from Guinea and surrounding countries were karyotyped-for IL-A and IIL-A as well as IIL-7 and other inversions. A reclassification of the subcomplex in this area has been possible through the analysis of karyotype data from these old records, and from recent chromosome analysis carried out from various localities both within and outside the OCP Western Zone.

Materials and methods

4Larvae were fixed in Carnoy’s solution during breeding site prospections in Senegal, Guinea, Mali, Sierra Leone and Liberia (table 1). Guinea Bissau was not prospected, and Côte d’ivoire is considered outside the scope of this paper. Most collections were fixed in cold Camoy’s solution by OCP staff, and subsequently stored at 4°C. Specimens were identified morphologically as belonging to the S. damnosum complex according to Crosskey (1973). Larval silk gland polytene chromosomes were usually prepared according to a modification of Dunbar’s (1972) method.

5The larval abdomen was split open in Camoy’s fixative along the mid-ventral line, and larvae were rinsed three times in distilled water. Larval bodies were blotted dry and hydrolysed in 3.5% HCl (Analar grade) at 60–65°C for seven minutes. After hydrolysis larvae were again rinsed three times in distilled water, blotted dry and stained in Feulgen for one to three hours. Larvae were sexed according to the shape of the gonadial rudiment and the silk glands mounted in lacto-acetic orcein. When required, preparations were subsequently made permanent by careful removal of the cover slip without freezing, drying both cover slip and slide in absolute ethanol and remounting in Euparol.

6Inversions were recorded and species identified with reference to the chromosome maps published by Post (1986), who illustrates most of the inversions found in this study. However, a few new inversions and chromosome map revision of old inversions, are described below, and illustrated in figures 5 to 16. In practice most larvae were examined only for the diagnostic inversions for identification, although a small number of larvae from each sample were also fully karyotyped. In some samples all larvae were fully karyotyped to obtain estimates of the frequencies of polymorphic inversions. Data were analysed in two ways. Firstly by classical analysis of geographic and karyotypic (Hardy-Weinberg) distribution of individual inversions, and secondly for S. konkourense sp. n. by multivariate analysis of the sample frequencies of all inversions considered simultaneously. Two multivariate analyses were used. Firstly, principle component analysis of the dispersion (variance/covariance) matrix (Seber, 1984), using the programme SAS PROC PRINCOMP. This is an ordination technique that seeks to account for the maximum variance in a multivariate data set into a small number of dimensions which can be represented graphically. Secondly, the programme SAS PROC CLUSTER was used to derive a minimum spanning tree from the similarity matrix between samples (Gower & Ross, 1969). The minimum spanning tree connects all samples (plotted on a map or an ordination) according to their chromosomal similarity, with no loops and minimum overall length.

Fig. 1. The distribution of members of the Simulium sanctipauli subcomplex in Guinea and the surrounding countries.

Simulium sanctipauli subcomplex

7The S. sanctipauli subcomplex was previously distinguished, cytologically, from the rest of the S. damnosum complex by the fixed inversions IL-P.Q, IIL-4.6 and IIIL-4.17.2 (reviewed by Post, 1986). However in the present study we have found inversions IIL-6 and IIIL-2 to be polymorphie in samples from the Fouta Djallon mountains (table 1), and a careful reanalysis of the breakpoints of the widespread inversions IIIL-C1-C2 (previously reported by Post (1986) from S. sanctipauli, S. soubrense and S. soubrense ′B′) has shown that they are identical to IIIL-4.17. Hence chromosome sequences reported by Post (1986) as IIIL-4.17.2.C1.C2 can no longer be considered diagnostic. Furthermore, it is now clear that the IL is more complex than either Vajime & Dunbar (1975) or Post (1986) considered. During the course of this study it was noticed that the sequence IL-P.Q. B.C illustrated by Post (1986) was identical to the sequence IL-1 in S. damnosum sensu stricto, and that a hybrid between S. sanctipauli and S. sirbanum Vajime & Dunbar (Boakye & Mosha, 1988) appeared to be doubly, not triply, heterozygous in IL. Careful reexamination of the breakpoints of speciesdiagnostic IL sequences showed that a single fixed inversion normally occurred in the S. sanctipauli subcomplex, and not IL-P.Q as described by Post (1986). However, the single inversion had different breakpoints to those illustrated by Vajime & Dunbar (1975) for IL-6, and were apparently identical to those illustrated by Post (1986) for IL-B. Post’s (1986) inversion IL-C was found to be identical to IL-1, which was previously believed to be fixed in S. damnosum sensu stricto and S. sirbanum. In summary, the S. sanctipauli subcomplex-is normally distinguished from the standard IL chromosome sequence (as seen in S. squamosum (Enderlein)) by a single inversion IL-B, and not IL-6 or IL-P.Q, which were apparently mistaken for IL-B by Vajime & Dunbar (1975) and Post (1986), respectively. However, IL-B is not fixed, but polymorphic within the S. sanctipauli subcomplex, with IL-1, which Post (1986) mistook for a new inversion, IL-C.

Fig. 2. Idiogrammatic representation of the chromosomal inversions found in members of the Simulium sanctipauli subcomplex in Guinea and the surrounding countries. Inversions plotted to the left were found fixed, and those plotted to the right were floating. The inversion breakpoints have been plotted relative to standard S. squamosum.

Fig. 3. Principal component plot of samples of Simulium konkourense sp. n. based on the correlation matrix of inversion frequencies, with minimum spanning tree superimposed. The sample numbers are as in figure 4.

8In the present study identification of the S. sanctipauli subcomplex was based on homozygosity for the fixed inversion IIL-4. Members of the subcomplex were found in samples from Guinea, Mali, Sierra Leone and Liberia, but not Senegal (see table 1).

Cytological key for the identification of larvae of the Simulium sanctipauli subcomplex from Guinea, Mali and Sierra Leone

91 Inversion IIL-4 homozygous S. sanctipauli subcomplex 2

10- Inversion IIL-4 absent other species of S. damnosum complex

112 Inversion IIL-A homozygous S. sanctipauli

12- Inversion IIL-A absent 3

133 Inversion IL-A homozygous S. leonense sp. n.

14- Inversion IL-A absent 4

154 Inversion IIIL-24 absent S. konkourense sp. n.

16- Inversion IIIL-24 homozygous.S. soubrense ′Chutes Milo′ form.

Simulium (Edwardsellum) sanctipauli Vajime & Dunbar sensu stricto

17Inversion IIL-A (described as diagnostic of S. sanctipauli sensu stricto by Post (1986) was never found heterozygously in this study, even in six samples where both homozygotes occurred together (table 1), thus confirming its diagnostic value and the distinct specific status of S. sanctipauli.

Fig. 4. Distribution map of Simulium konkourense sp. n. showing the genetic relatedness between samples according to the minimum spanning tree derived from the inversion frequency similarity matrix. The numbered samples are as in table 1, and the others have been taken from data published by Post (1986).

18Simulium sanctipauli was only found in the eastern part of the study area, in the upper Sassandra, Sankarani, Baoulé and Bagoe river basins (table 1, fig. 1). However, it is already known from a few sites in south-eastern Sierra Leone (Post & Crosskey, 1985) and north-westem Liberia (Guzelhan & Garms, 1987).

19All specimens karyotyped were found to be homozygous for inversions IL-B, IS-A, IIL-4.6.A and IIIL-2 relative to S. squamosum standards. However, as a resuit of the small sample sizes obtained for S. sanctipauli, it was only possible to obtain a reliable estimate of the polymorphie inversion frequencies for some samples, from the Baoulé river basin (samples 295, 296, 298 and 292). No s.ex linkage was observed among the polymorphie inversions, which are indicated on the idiogram (fig. 2), and unweighted means of sample frequencies were 0.967 for IIIL-4.17.B and 0.017 for IIIL-I.

Simulium (Edwardsellum) leonense Boakye, Post & Mosha sp. n.

20Simulium soubrense ′B′ of Post, 1986 (204–205), and subsequent citations.

21Diagnostic features. Recognized on the basis of larval silk gland polytene chromosomes. Sibling species of the Simulium sanctipauli subcomplex (defined above by inversion IIL-4 homozygous) and distinguished from other members of the subcomplex by inversion IL-A present homozygously.

Fig. 5. Full karyotype of Simulium leonense sp. n. from R. Tabe near Baiima, collected 8 December 1980. IL-B.A/B.A, IS-A/A, IIL-4.6.7.D/4.6.7.D, IIS-7/7, IIIL-4/4.17.2, and IIIL-st/st. NO= Organiser. Ba=ring of Balbiani. The chromosomes are numbered at their centromeres.

Table 2. Simulium leonense summary inversion frequencies by river basin.

Table 2. Simulium leonense summary inversion frequencies by river basin.

Notes: Frequencies are unweighted means of sample frequencies from samples, 251,252,253,254,260, 262, 263, 264, 269 and 273. Sample sizes are usually slightly smaller than indicated in table 1 because it was often not possible to score all inversions in all larvae.

Table 3. Sex linkage in Simulium soubrense chutes milo form.

Table 3. Sex linkage in Simulium soubrense chutes milo form.

Notes: Data compiled from samples 34, 44, 48, 59, 60, 63, 72, 73, 81, 87, 90, 91, 94, 95, 96, 111, 114, 121, 123, 125, 127, 130, 139, 142, 145, 146, 151,173, 183, 187, 189, 199, 207, 281 and 284.

22Material examined. Holotype larval polytene chromosome preparation number 0712811–4 with associated larval body, SIERRA LEONE: River Gbangbaia, 1.5 km downstream of Mokasi (07°59, N/12°25’W), 07.xii.1981, collected by RJ Post and deposited in The Natural History Museum, London (NHM). Other material (excluded from the type series): larval bodies with associated chromosome preparations, collected simultaneously with holotype (NHM). Some pùpae collected simultaneously with the larval material, and some pinned adult females collected on human bait at Mokasi, are probably S. leonense sp. n. but cannot be unequivocally associated. They are in NHM.

Table 4. Simulium konkourense summary inversion frequencies by river system.

Table 4. Simulium konkourense summary inversion frequencies by river system.

Notes: Frequencies are unweighted means of sample frequencies from samples 70, 72, 73, 75, 123, 209, 214, 216, 218, 224, 226, 228, 229, 231, 234, 239, 241, 243, 246, 247, 249, 275, 236 and 237. Sample sizes are usually slightly smaller than indicated in table 1 because it was not always possible to score all inversions in all larvae.

23Distribution. Widespread in major rivers throughout Sierra Leone. Known also from Farmington river in Liberia (Guzelhan & Garms, 1987), and the Kolenté river headwater in south-western Guinea (table 1, fig. 1).

24Discussion. Inversion IL-A (described as diagnostic of S. soubrense ′B′ by Post, 1986) was found heterozygously in only one larva although 324 were found to be homozygous (table 1), thus confirming its diagnostic value and the distinct specific status of S. soubrense ′B′. This species was originally described by Post (1986), but not formally named because of its unknown cytotaxonomic relationship with forme ′Konkouré′ (Quillévéré et al., 1982). It now seems clear that forme ′Konkouré′ belongs to a newly recognized species (S. konkourense sp. n.-see below) which does not interbreed with S. soubrense ′B′, even in six samples where they occurred together (table 1). Thus we name as S. leonense sp. n. the species originally described under the provisional name S. soubrense ’B’ by Post (1986).

25The pattern of polymorphic inversions was found to be largely similar to that already described, with sex linkage of IIIL-4.17, (Post (1986) as S. soubrense ′B′ and IIIL-C1.C2). However it has now been possible to determine the sequence of the inversion IIL-complex2 listed by Post (1986). Homozygotes have not been seen before, but it has now been determined that heterozygotes have the karyotype IIL-4.X/4.6.7.D. Polymorphie inversions are indicated on the idiogram (fig. 2) and frequencies are summarized by river basin in table 2. Besides IL-A specimens karyotyped for this study were homozygous IL-B, IIS-7, IIL-4 and IIIL-2, relative to S. squamosum standards.

Simulium (Edwardsellum) soubrense Vajime & Dunbar ’Chutes Milo’ form

26Larvae of the S. sanctipauli subcomplex not possessing either inversion IL-A or IIL-A were defined as S. soubrense by Post (1986). Amongst such specimens examined in the present study inversions IIL-7.D were polymorphie but very rarely found heterozygously even in samples from the rivers Balé, Milo, Niandan and Kouya where both homozygotes (IIL-4.6/4.6 and IIL-4.6.7.D/4.6.7.D) occurred together (table 1). This observation clearly indicates a high degree of reproductive isolation between two homozygous cytotypes (informally referred to as ′Chutes Milo′ and ′Menankaya′ forms after the localities on the river Milo where they were first recognized), and this conclusion is supported by a consideration of the other unlinked inversions, particularly IS-A, IIIL-24 and IIIL-B.

Table 5. Sex linkage in Simulium konkouré forme konkouré

Table 5. Sex linkage in Simulium konkouré forme konkouré

Notes: Data compiled from samples 236, 237, 239, 241, 243, 246, 247, 249, 209, 214, 218, 224, 226, 228, 229, 231 and 234.

Fig. 6. Long arm of chromosome I of Simulium konkourense sp. n. (IL-B/1) from R. Mongo near Musaia collected on 17 December 1981.

27Inversion IIIL-24 was never found heterozygously, and homozygotes (IIIL-24/24) were nearly always found associated with IIL-4.6/4.6 (′Chutes Milo′ form) and never with IIL-4.6.7.D/4.6.7.D (′Menankaya′ form). The single known exception was a larva homozygous IIIL-24/24 and heterozygous IIL-4.6/4.6.7.D. However, this specimen could not have been a hybrid between the two cytotypes or it would have been doubly heterozygous. Similarly inversion IS-A was always found homozygously (IS-A/A) with IIL-4.6/4.6, but IS-A was polymorphie at lower frequencies and usually absent from larvae homozygous IIL-4.6.7.D/4.6.7.D (table 4). Inversion IIILB was never found associated with homozygotes IIL-4.6.7.D/4.6.7.D, but it was sex-linked in homozygotes IIL-4.6/4.6 such that most males were found heterozygous IIIL-B/st and most females did not possess the inversion (table 3).

28The virtual absence of heterozygotes (table 1) and the strong gametic-phase imbalance observed between unlinked inversions was found consistently between samples over a wide geographic area and time period. This can only be the resuit of ′Chutes Milo′ and ′Menankaya′ forms representing two distinct sibling species.

Fig. 7–10. Chromosome I: 7, long arm of Simulium konkourense (IL-1/1) from R. Mongo near Musaia collected on 17 December 1981; 8, long arm of S. leonense sp. n. (IL-B.A/B.A) from R. Taia at Mogbamu collected on 25 November 1980; 9, short arm of S. konkourense (IS-st/st) from R. Bagbe at Yifin collected on 18 December 1981; 10, short arm of S. sanctipauli (IS-A/A) from R. Sassandra at Soubre collected on 26 October 1984.

29Populations of S. soubrense further east in Côte d’ivoire and Ghana are most like ′Chutes Milo′. form, with IS-A usually fixed, IL-7.D usually absent and IIIL-24 present, although polymorphic at variable frequencies, rather than fixed (unpublished data). This is also true of Vajime & Dunbar’s original type specimens from the river Leraba (see Post, 1986). The exceptions to this trend are mostly found amongst samples from Togo and Nigeria, which show a somewhat lower frequency of ISA and higher frequency of IIL-7.D. However, this genetic divergence is not surprising because these are populations of the ′Beffa′ form of S. soubrense which is already described as a distinct cytotype (Meredith et al, 1983; Post, 1986). Hence ′Chutes Milo′ form seems to be typical S. soubrense, almost identical to the type material, and found throughout West Africa from Guinea eastwards where it merges into ′Beffa′ form. However, the sex linkage of IIIL-B indicates that ′Chutes Milo′ populations are, by definition, genetically differentiated from other S. soubrense populations, and therefore it seems that ′Chutes Milo′ should be considered to be a geographic race within S. soubrense. The spectrum of fixed and polymorphie inversions observed within S. soubrense ′Chutes Milo′ form is illustrated on the idiogram (fig. 2). All larvae were homozygous for inversions IS-A, IL-B, IIL-4.6 and IIL-4.17.2.24. Only two polymorphie inversions were recorded from S. soubrense ’Chutes Milo’ form from Guinea. Inversion IIIL-B which is sex-linked (table 3) and IIIL-I which was only recorded from Chutes Milo at an average frequency of 0.033.

30In spite of a few larvae found to be heterozygous for IIL-7.D (table 1) there is no evidence that these larvae are actually hybrids. Hybrids would normally be expected to be heterozygous for IIIL-24 and often IS-A as well as IIL-7.D, but in those spedmens heterozygous for IIL-7.D, which could be fully karyotyped, chromosome arms IIIL and IS were not heterozygous. Hence no hybrids were found amongst 210 larvae fully karyotyped from sympatric samples, and therefore the rate of hybridization is less than 0.005.

Table 6. Inversion frequencies in Simulium konkourense

Table 6. Inversion frequencies in Simulium konkourense

1Sample numbering corresponds to table 1, except P1–5 which are from Post (1986); M= ′Menankaya′ form, and K-Forme ′Konkoure′; 2sample sizes do not always correspond exactly with table 1 because it was not always possible to observe all chromosome arms in all specimens; 3see post (1986).

Fig. 11–13. Chromosome II: 11, short arm of Simulium leonense (IIS-7/7) from R. Taia at Mogbamu collected on 9 December 1980; 12, long arm of S. leonense sp. n. (IIL-4.6.7.D/4.6.7.D) from R. Tabe at Baiima collected on 8 December 1980; 13, long arm of S. konkourense sp. n. (IIL-4.X/4.X) from R. Koumba at Sidipo collected on 11 November 1986.

Fig. 14. Long arm of chromosome II from Simulium konkourense sp. n. (IIL-4.6.7.D/4.X) collected from R. Koumba at Sidipo on 11 November 1986.

31If ’Chutes Milo’ form is a geographic race within S. soubrense, it follows that ’Menankaya’ form must be part of a new species, which is described below and named as S. konkourense sp. n.

32In Liberia Kashan & Garms (1987), and Guzelhan & Garms (1987) described-two distinct cytotypes within S. soubrense (sensu Post, 1986). These two forms, which show restricted interbreeding, are known as the ′Farmington ′ form and the ′St Paul′ form, but it is not clear how they are related, taxonomically, to S. soubrense ’Chutes Milo’ form and S. konkourense sp. n. If inversion 2L-7 is considered on its own, then the ′St Paul′ form appears most similar to S. konkourense sp. n. and the ′Farmington′ form appears similar to S. soubrense ′Chutes Milo′ form. However, other inversions do not fit this pattern, and a clear description of their relationship requires a wider geographic spread of samples to determine their cytogenetic interactions in the contact zones between forms.

Simulium (Edwardsellum) konkourense Boakye, Post, Mosha & Quillévéré sp. n.

33Forme ′Konkouré′ of Quillévéré et al. 1982 (306–307), and subsequent citations.

34Diagnostic features. Recognized on the basis of larval silk gland polytene chromosomes. Sibling species of the S. sanctipauli subcomplex (defined above by inversion IIL-4 homozygous), and distinguished from other sibling species by the absence of inversions IIL-A, IL-A and IIIL-24.

35Material examined. Holotype larval polytene chromosome preparation number 2911813–3 with associated larval body, SIERRA LEONE: River Rokel at Bumbuna (09°03’N/11°44’W), 29.xi.1981, collected by RJ Post and deposited in The Natural History Museum, London. Other material (excluded from the type series): larval bodies with associated chromosome preparations, collected simultaneously with holotype (NHM). Some pupae collected simultaneously with the larval material, and some pinned adults reared from the pupae, probably include S. konkourense but cannot be unequivocally associated. They are in The National History Museum.

36Distribution. Widely distributed in major rivers in the highlands, from the Fouta Djallon mountains in Guinea south-eastwards along the Niger basin watershed between Guinea and Sierra Leone and Liberia (table 1, fig. 1).

37Discussion. Quillévéré (Quillévéré et al., 1982) was the first to notice cytotaxonomic heterogeneity within thé S. sanctipauli subcomplex from Guinea, and the recognition of S. konkourense sp. n. has been developed from his original criteria for the identification of forme ′Konkouré′. Whilst the name of his form-is not available in formai taxonomie nomenclature we feel that it is appropriate to use it as the stem for the name of our-new species, and in view of this Dr D. Quillévéré has agreed to be coauthor of the species name S. konkourense.

38Simulium konkourense sp. n. is characterized by the absence of inversions IL-A, IIS-A and IIIL-24, and inversion IS A is polymorphie at low frequency. These features distinguish S. konkourense sp. n. from S. sanctipauli, S. soubrense ’Chutes Milo’ form and S. leonense sp. n. The fixed and. polymorphie inversions are shown diagrammatically on the idiogram (fig. 2) and frequencies are summarized by river basin in table 4.

39However, the western Guinea populations of S. konkourense sp. n. do show some unique chromosomal features (table 6) and have already been described as a distinct geographic variant named forme ′Konkouré′ (Quillévéré et al., 1982). The geographic pattern of variation of the individual inversions (table 4) is not uniform, but multivariate analysis of inversion sample frequencies (table 6) by principal components analysis and minimum spanning tree (figs 3 and 4) suggests that the overall pattern of variation follows a stepped cline. The groups of populations correspond to, firstly the extreme westerly samples from the Koumba and Konkouré river basins, secondly the Bafing river basin and thirdly the samples from south-eastern Guinea and Sierra Leone. These three groups are geographically separated and examination of table 6 shows that there are no fixed inversion differences or other evidence of separate specific status. Indeed the overall pattern of variation seems to be typical of an intraspecific stepped cline (Endler, 1980). The two ends of this cline are genetieally substantially different, and are best considered as separate geographic races (′Menankaya′ form and forme ′Konkouré′) within S. konkourense.

Fig. 15–16. Chromosome III: 15 Simulium sanctipauli sp. n. (IIIL-4.17.2.B/4.17.2.B) from R. Comoe at Mbaso on 29 July 1980; 16, S. leonense sp. n. (IIIS-st/st, IIIL-2/2) from R. Tabe at Baiima on 8 December 1980.

40Within S. konkourense sp. n. the most characteristic feature of forme ′Konkouré′ is the complex rearrangement on chromosome arm IIL already described as diagnostic by Quillévéré et al. (1982). Exact banding pattern analysis has shown that the complex heterozygotes consist of two sequences IIL-4.6.7.D/4.X of which the inversion IIL-X is unique, and within the whole complex rearrangement, can on its own be considered to distinguish forme ′Konkouré′ from ′Menankaya′ form. The use of IIL-X for the identification of forme ′Konkouré′ within S. konkourense sp. n. is based on Quillévéré’s original criteria (Quillévéré et al., 1982). The inversion IIL-X was found only in the chromosome sequence I1L-4.X, which was polymorphic with IIL-4.6.7.D. Thus populations in which IIL-X occurs (either as a homozygote IIL-4.X/4.X, or heterozygote IIIL-4.X/4.6.7.D) are identified as forme ′Konkouré′ whilst other populations (which are fixed IIL-4.6.7.D/4.6.7.D) are defined as ′Menankaya′ form. These criteria for identification of forme ′Konkouré′ and ′Menankaya′ form are based on the cytogenetic characteristics of the population, as are the criteria for the identification of other geographic races within the S. sanctipauli subcomplex, such as ’Beffa’ form (Meredith et al., 1983) and ’Djodji’ form (Surtees et al., 1988).

41Amongst the floating inversions there was found to be geographic variation in the sex chromosome System of S. konkourense sp. n., following the multivariate cytogenetic stepped cline described above. In the ′Menankaya′ form of S. konkourense sp. n. from Sierra Leone and southeastern Guinea no sex-linked inversions were observed, whereas further west (S. konkourense sp. n. forme ′Konkouré′) IIIL sequences were sex linked. In the Bafing river basin the most common X and Y chromosomes were IIIL-2 and IIIL-4.17.2, respectively (table 5), which is identical to the sex chromosome System in S. leonense sp. n. (Post, 1986, as S. soubrense ′B′). In the Konkouré and Koumba river basins the most common Y chromosome is still IIIL-4.17.2., but the most common X chromosome sequence is IIIL-X (table 5), which is a new inversion (fig. 7) unknown outside the Fouta Djallon area.

Acknowledgements

42We wish to thank all OCP staff and national teams, too numerous to mention, for the collection of much of the material on which this study was based. We are particularly indebted to Y. Coulibaly and A. Sib for invaluable technical assistance. Also Drs B. Philippon, D. Quillévéré, R.W. Crosskey, R. Garms and H. Townson for valuable comments or discussion, and lastly we thank Dr E.M. Samba (Director OCP) for permission to publish.

Bibliographie

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Dunbar, R.W. (1972) Polytene chromosome preparations from tropical Simuliidae. World Health Organization mimeographed document WHO/ONCHO/72.95.

Endler, J.A (1980) Geographic variation, speciation and clines. Princeton, Princeton University Press.

Garms, R. & Vajime, GG. (1975) On the ecology and distribution of the species of the Simulium damnosum complex in different bioclimatic zones of Liberia and Guinea. Tropenmedizin und Parasitologie 26,375–380.

Gower, J.C. & Ross, G.J.S. (1969) Minimum spanning trees and single linkage cluster analysis. Applied Statistics 18,54–64.

Guzelhan, C. & Garms, R. (1987) Cytotaxonomy of the Farmington form of Simulium soubrense in Liberia. Tropical Medicine and Parasitology 38, 345.

Kashan, A. & Garms, R. (1987) Cytotaxonomy of the Simulium sanctipauli sub-complex in Liberia. Tropical Medicine and Parasitology 38,289–293.

Meredith, S.E.O., Cheke, R.A. & Garms, R. (1983) Variation and distribution of forms of Simulium soubrense and S. sanctipauli in West Africa. Annals of Tropical Medicine and Parasitology 77, 627–640.

Post, R.J. (1986) The cytotaxonomy of Simulium sanctipauli and Simulium soubrense (Diptera: Simuliidae). Genetica 69,191–207.

Post, R.J. & Boakye, D.A. (1992) Vector taxonomy and the control of human onchocerciasis in West Africa. Proceedings of the Section Experimental and Applied Entomology of the Netherlands Entomological Society (N.E.V) 3,105–109.

Post, R.J. & Crosskey, R.W. (1985) The distribution of the Simulium damnosum complex in Sierra Leone and its relation to onchocerciasis. Annals of Tropical Medicine and Parasitology 79,169–194.

Quillévéré, D. (1975) Etude du complexe Simulium damnosum en Afrique de l’Ouest I. Technique d’étude. Identification des cytotypes. Cahiers ORSTOM, Série Entomologie Médicale et Parasitologie 13,85–98.

Quillévéré, D., Guillet, P. & Sechan, Y. (1982) La répartition géographique des espèces du complexe Simulium damnosum dans la zone du projet Sénégambie (ICP/MPD/007). Cahiers ORSTOM, Série Entomologie Médicale et Parasitologie 19, (1981), 303–311.

Seber, G.A.F. (1984) Multivariate observations. New York, Wiley.

Surtces, D.P., Fiasorgbor, G., Post, R.J. & Weber, E.A. (1988) The cytotaxonomy of the Djodji form of Simulium sanctipauli (Diptera: Simuliidae). Tropical Medicine and Parasitology 39,120–122.

Vajime, C.G. & Dunbar, R.W. (1975) Chromosomal identification of eight species of the subgenus Edwardsellum near and including Simulium (Edwardsellum) damnosum Theobald (Diptera: Simuliidae). Tropenmedizin und Parasitologie 26, 111–138.

Table des illustrations

Légende Fig. 1. The distribution of members of the Simulium sanctipauli subcomplex in Guinea and the surrounding countries.
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Légende Fig. 2. Idiogrammatic representation of the chromosomal inversions found in members of the Simulium sanctipauli subcomplex in Guinea and the surrounding countries. Inversions plotted to the left were found fixed, and those plotted to the right were floating. The inversion breakpoints have been plotted relative to standard S. squamosum.
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Légende Fig. 3. Principal component plot of samples of Simulium konkourense sp. n. based on the correlation matrix of inversion frequencies, with minimum spanning tree superimposed. The sample numbers are as in figure 4.
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Légende Fig. 4. Distribution map of Simulium konkourense sp. n. showing the genetic relatedness between samples according to the minimum spanning tree derived from the inversion frequency similarity matrix. The numbered samples are as in table 1, and the others have been taken from data published by Post (1986).
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Légende Fig. 5. Full karyotype of Simulium leonense sp. n. from R. Tabe near Baiima, collected 8 December 1980. IL-B.A/B.A, IS-A/A, IIL-4.6.7.D/4.6.7.D, IIS-7/7, IIIL-4/4.17.2, and IIIL-st/st. NO= Organiser. Ba=ring of Balbiani. The chromosomes are numbered at their centromeres.
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Titre Table 2. Simulium leonense summary inversion frequencies by river basin.
Légende Notes: Frequencies are unweighted means of sample frequencies from samples, 251,252,253,254,260, 262, 263, 264, 269 and 273. Sample sizes are usually slightly smaller than indicated in table 1 because it was often not possible to score all inversions in all larvae.
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Titre Table 3. Sex linkage in Simulium soubrense chutes milo form.
Légende Notes: Data compiled from samples 34, 44, 48, 59, 60, 63, 72, 73, 81, 87, 90, 91, 94, 95, 96, 111, 114, 121, 123, 125, 127, 130, 139, 142, 145, 146, 151,173, 183, 187, 189, 199, 207, 281 and 284.
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Titre Table 4. Simulium konkourense summary inversion frequencies by river system.
Légende Notes: Frequencies are unweighted means of sample frequencies from samples 70, 72, 73, 75, 123, 209, 214, 216, 218, 224, 226, 228, 229, 231, 234, 239, 241, 243, 246, 247, 249, 275, 236 and 237. Sample sizes are usually slightly smaller than indicated in table 1 because it was not always possible to score all inversions in all larvae.
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Titre Table 5. Sex linkage in Simulium konkouré forme konkouré
Légende Notes: Data compiled from samples 236, 237, 239, 241, 243, 246, 247, 249, 209, 214, 218, 224, 226, 228, 229, 231 and 234.
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Légende Fig. 6. Long arm of chromosome I of Simulium konkourense sp. n. (IL-B/1) from R. Mongo near Musaia collected on 17 December 1981.
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Légende Fig. 7–10. Chromosome I: 7, long arm of Simulium konkourense (IL-1/1) from R. Mongo near Musaia collected on 17 December 1981; 8, long arm of S. leonense sp. n. (IL-B.A/B.A) from R. Taia at Mogbamu collected on 25 November 1980; 9, short arm of S. konkourense (IS-st/st) from R. Bagbe at Yifin collected on 18 December 1981; 10, short arm of S. sanctipauli (IS-A/A) from R. Sassandra at Soubre collected on 26 October 1984.
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Titre Table 6. Inversion frequencies in Simulium konkourense
Légende 1Sample numbering corresponds to table 1, except P1–5 which are from Post (1986); M= ′Menankaya′ form, and K-Forme ′Konkoure′; 2sample sizes do not always correspond exactly with table 1 because it was not always possible to observe all chromosome arms in all specimens; 3see post (1986).
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Légende Fig. 11–13. Chromosome II: 11, short arm of Simulium leonense (IIS-7/7) from R. Taia at Mogbamu collected on 9 December 1980; 12, long arm of S. leonense sp. n. (IIL-4.6.7.D/4.6.7.D) from R. Tabe at Baiima collected on 8 December 1980; 13, long arm of S. konkourense sp. n. (IIL-4.X/4.X) from R. Koumba at Sidipo collected on 11 November 1986.
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Légende Fig. 14. Long arm of chromosome II from Simulium konkourense sp. n. (IIL-4.6.7.D/4.X) collected from R. Koumba at Sidipo on 11 November 1986.
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Légende Fig. 15–16. Chromosome III: 15 Simulium sanctipauli sp. n. (IIIL-4.17.2.B/4.17.2.B) from R. Comoe at Mbaso on 29 July 1980; 16, S. leonense sp. n. (IIIS-st/st, IIIL-2/2) from R. Tabe at Baiima on 8 December 1980.
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Auteurs

Onchocerciasis Control Programme, World Health Organization, Ouagadougou, Burkina Faso

Department of Medical Entomology, Liverpool School of Tropical Medicine, Liverpool, UK
Animal Taxonomy Section, Department of Entomology, University of Wageningen, P.O. Box 8031, 6700 EH Wageningen, Netherlands.

Onchocerciasis Control Programme, World Health Organization, Ouagadougou, Burkina Faso

Department of Medical Entomology, Liverpool School of Tropical Medicine, Liverpool, UK

Onchocerciasis Control Programme, World Health Organization, Ouagadougou, Burkina Faso

© IRD Éditions, 2003

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