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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

A pictorial guide to the chromosomal identification of members of the Simulium damnosum Theobald complex in West Africa with particular reference to the Onchocerciasis Control Programme Area

Daniel Adjei Boakye

Résumé

Simulium damnosum Theobald is made up of a complex of sibling species. Nine species are described in the area covered by the World Health Organisation’s Onchocerciasis Control Programme in West Africa (OCP). These are, S. squamosum, S. yahense, S. canctipauli, S. soubrense, S. leonense, S. konkourense, S. damnosum s.s., S. sirbanum and S. dieguerense. All of them are vectors of human onchocerciasis, albeit to different capacities. Reliable species determination presently depends on larval cytotaxonomic criteria. The diagnostic chromosomal inversions and other micromorphological characters used in the identification of the species found in the OCP area are presented with figures for their recognition, some distributional information is also given.

Texte intégral

Introduction

1Simulium damnosum Theobald the main vectors of human onchocerciasis in Africa has been found to be a complex of sibling species based on the analysis of the banding sequence on the polytene chromosomes from the larval salivary glands (Dunbar, 1966; Vajime and Dunbar. 1975; Quillévéré, 1975). Eight species were formally described from West Africa: S. squamosum Enderlein, S. yahense, S. sanctipauli, S. soubrense, S. damnosum s.s., S. sirbanum, S. sudanense and S. dieguerense (Vajime and Dunbar. 1975). Out of these the specific status of S. sudanense was not unequivocally accepted (Bedo, 1977) and recently, Vajime (1989) has synonymised it with S. sirbanum. S. sanctipauli and S. soubrense have been reclassified under a single sub-complex S. sanctipauli s.l. (Post, 1986). The S. sanctipauli sub-complex comprises of four main species S. sanctipauli, S. soubrense, S. leonense and S. konkourense and their forms (Boakye et al.. 1993). All the West African species are vectors of onchocerciasis but with varying vectorial capacities hence any control based on selective larvicidal treatments require reliable species identification.

Accepted 15 October 1992

2Methods of identifying adults have been beset with many difficulties (Dang and Peterson, 1980; Garnis et al., 1982) such that larval cytotaxonomy remains the most reliable means of identification. However, since the pioneering work of Vajime and Dunbar (1975) and Quillévéré (1975), nothing comprehensive has been published. Other authors have concentrated only on the variations encountered in specific groups and reclassifying some of the inversions of earlier works (Quilévéré et al., 1982; Meredith et al., 1983; Post. 1982.1986; Boakye and Mosha, 1988; Surtees et al., 1988; Boakye et al., 1993). This array of information is scattered in different journals and makes it difficult for students and beginners in S. damnosum s.l. cytotaxonomy to correctly identify diagnostic inversions for the different cytospecies. It has also resulted in the proliferation of different modes of naming inversions and their interpretation. Furthermore, information has been collected over the years in the Onchocerciasis Control Programme in West Africa (OCP) which has not been published.

3This presentation collates information from various publications and new data on the various inversions used in the identification of the different cytotypes of the S. damnosum s.1. in West Africa with particular reference to the OCP area. Pictures of larval polytene chromosomes showing the important inversions in the different cytospecies are presented to serve as a training guide for S. damnosum s. 1. cytotaxonomy.

The chromosomes

4The members of the S. damnosum complex have a chromosome complement of three pairs (n = 3). From the longest and decreasing in size, they are numbered; one (I), two (II) and three (III) each with distinct long (L) and short (S) arms based on the position of the centromere (Fig. 1 a). From the tip of the short arm of chromosome one (IS) to the tip of the long arm of chromosome three (IIIL) is referred to as the total complement length (TCL) and given a value of one hundred. Each of the three chromosomes is arbitrarily subdivided into units based on its proportion relative to the total complement length. Chromosome I is 42% TCL and hence divided into 42 sections numbered 1 – 42; Chromosome II is 30% TCL. divided into 30 sections (43–72) and chromosome III 28% TCL (73–100) (Vajime and Dunbar. 1975).

Fig. 1 a Idiogramatic representation of the chromosome complement of members of the S. damnosum s.l. in West Africa.
Fig. 1 b Diagramatic representation of an inversion.

Fig. 2 Cytospecies of the S. damnosum complex in the OCP area.

Fig. 3 Full chromosome complément of S. squamosum showing the ectopic pairing of the centromeres. B = Ring of Balbiani. db = double bulb. PB = para-Balbiani. b = blister. C = centromere.

5The giant chromosomes show morphological detail in their banding patterns suitable for use in identification. Members of the S. damnosum species complex recorded so far have essentially the same chromosomal bands but these can be rearranged in fixed inversions, complement of floating inversions, sex determining Systems and micromorphological characters such as band dimorphisms.

Inversions

6These are chromosomal mutations characterized by the reversai of a chromosome segment (Ayala and Kiger, 1984) (Fig. 1b). In S. damnosum s.l. cytotaxonomy, the inversions are given numbers, for example, 1IL-7 (Vajime and Dunbar, 1975) or letters, IIL-A (Post, 1986) based on an arbitrarily chosen standard sequence. The chromosomal sequence of S. squamosum is taken as the standard for members of the S. damnosum complex in West Africa.

7Inversions may occur in a homozygous or heterozygous condition. The latter is more easily seen due to either a local non-pairing or reversed loops of the normally paired double structure (e.g. IL/12, Fig. 7). Inversions are termed fixed or floating. Fixed inversions are inter-specific and diagnostic to a particular species. Therefore, all the members of the same species have a particular fixed inversion. Floating inversions are intraspecific being absent, present homozygously or heterozygously in different individuals of the same species.

Figs. 4–8 Chromosome 1 (S. squamosum/S. yahense). (4) Standard sequence for members of the S. damnosum s.l. in west Africa. S-1/1, IL-3/3. (5) Long arm of chromosome I; IL-3/3. (6) Long arm of chromosome I; IL-3.12/3.12. (7) Long arm of chromosome I; IL-3/3.12. (8) Long arm of chromosome I; 3/3.18. C = centromere. NO = nucleolar organiser.

Figs. 9–10 Chromosome IS of S. squamosum showing IS-23 and altered centromere region.

Chromosomal markers

8The morphology of the chromosomes presents characters useful in determining

  1. the various chromosomes (I, II and III)
  2. the long and short arms of each chromosome and
  3. different inversion sequences.

9Whilst some markers such as the centromeres are obvious, others require a knowledge of the subdivisions of a particular chromosome. A very important observation is that a chromosome may be stretched out, bent over itself or contracted during preparation thus giving the impression that the inversion markers differ on chromosomes from different cells.

1. Chromosome I

10This is very-often easily identified by the length It is the longest chromosome but does not always appear as such due to preparation procedures. The long and short arms are of almost equal length. Another character of use is a large bulbular area about halfway between the long and short arms within which is located the centromere (C) (Fig. 3). The short arm (IS) is differentiated by the presence of a nucleolar organiser (NO). Generally, the chromosome appears to be weak or broken at the NO (Fig. 3). The bands forming the sub-division number 12 on IS (Fig. 4) and numbers 34–35 and 38–39 on IL (Fig. 5) are good landmarks for recogmzing most of the inversions on chromosome I.

Chromosome II

11An easily identifiable centromere separates the chromosome into a definite short and long arm-acrocentric chromosome. On the short arm is located a pufiy area usually in the form of two superinposed circles. These are shown on Fig. 3; the ring of Balbiani (B) and an euchromatic section termed double bubble (db). Also on the short arm but very close to the centromere are two prominent dark bands (segment 54, Fig. 11).

12The long arm (IIL) contains a marker which is very important or routine species identification-the para-Balbiani (PB) in the sixty-first segment (Fig. 11). Also important in identifying the different cytotypes are the series of bands comprising the segments 64, 65, 66 and 67. Most of the inversions-on IIL include these bands. Other landmarks are the segments 70 and 72.

13The short arm of chromosome II shows very little variation in inversions between the various cytospecies in the S. damnosum complex.–It is therefore-not very useful in-routine identifications.

3. Chromosome III

14This is also acrocentric with an easily iden tifîahle centromere that separates the short and longarms. A recognisable pufiy euchromatic region termed the blister (b) is found towards the end of short arm (Fig. 3). Important segments are; 86–87 and 94–95–96 (Fig. 16). Similar to the second chromosome, the short arm of chromosome III shows little variation, appearing uniform in all of the cytospecies.

Description of cytospecies

15The members of the S. damnosum complex described below are shown in Fig. 2. They all share the fixed inversions IS-1 and IL-3. The designation of forest and savanna is only an indication of the areas where the cytotypes are usually found. The descriptions given below should not be considered as definitive for each species along its entire distributional range. As more samples are examined, new inversions continue to be recorded as either fixed or polymorphic in various populations. On the figures, only break-points of the rare inversions are given and not the actual inversions except where they happened to occur on the same chromosome with a more common inversion. When only break-points are given, they are denoted by broken lines.

Figs. 11–14 Chromosome II of S.squamosum/S.yahense (11). Standard sequence of Chromosome II for West.African S. damnosum s.l. as found in S. squamosum. (12) Chromosome II of S. yahense; IIL-18/18, IIS/6. (13) Long arm of S. yahense, IIL/18. (14) Long arm of S. squamosum; IIL/42. B = Ring of Balbiani, db = double bulb. PB = para-Balbiani. C = centromere.

Fig. 15 Long arm of chromosome II of S. yahense heterozygous for IIL-61 (IIL-18/18.61). C = centromere.

Simulium squamosum Enderlein

16This is the chosen standard for the West African members of the S. damnosum complex. It is the closest of this group to the East African forms from which it is differentiated by the inversions IS-1 and IL-3 (Fig. 4 and 5). IL-3 has been found to be polymorphic in some populations of this species in Nigeria and Guinea (Boakye, unpubUshed; Dunbar, unpublished information to Dr. S. E. O. Meredith).

17Chromosomes I, II and III of S. squamosum have been found to be joined together at their centromeres (ectopic pairing) in most areas (Fig. 3). Vajime and Dunbar (1975) reported-sex determination associated with an alteration of the centromere region of chromosome.I in Cameroon and Burkina Faso. However, in Guinea. Sierra Leone and Western Côte d’Ivoire, sex determination is related to a band dimorphism of the centromere of chromosome III (SCIIL Fig. 17). About 80% of males show this band polymorphism while the majority of females lack it In certain populations of this species identified from Ghana, the band dimorphism is present simultenously with an inversion designated IIIL-41 (Fig. 18). Meredith (unpublished report to the WHO/OCP, 1987) also reported different sex chromosomes in S. squamosum from Central Africa. Thus sex determination is associated with different chromosomes in different populations.

18Common inversions recorded in this species are; IS-2, IS-6, (Fig. 4); IS-23 (Figs. 9 and 10), IL-6 (Fig. 5), IL-12 (Figs. 6 and 7), IIS-6 (Figs. 11 and 12), IIL-18 (Figs. 12 and 13), IIL-41 (Fig. 14), IIIL-28, IIIL-22 (Fig. 16), IIIL-82 (Fig. 19) and 1IIL-42 (Fig. 20). The centromeres for the second and third chromosomes are much prominent in size in this species and S. yahense than for the other members of the complex in West Africa. The proportions of floating inversions are not uniform along the entire distributional range of the species.

19S. squamosum can be found in the forest zone breeding in small rivers. However, it has occasionally been identified from some medium to big rivers in the savanna zone.

Simulium yahense Vajime and Dunbar

20This species is found breeding in small rivers in the forest zone. It is rarely found in the savanna except in forested and mountainous enclaves. Its distribution is therefore pacthy across the OCP area. Recently, its distribution has been found to be extending in Southern Ghana due to the insecticidal control of the other cytospecies by the OCP (Fiasorgbor et al., 1992).

21This is the closest of the West African S. damnosum s.l. to the chosen standard S. squamosum. The two species do not show any fixed inversion differences (Quillévéré, 1975) casting some initial doubt as to their separate specific identity. However, their specific status has been confirmed by iso-enzyme electrophoresis (Meredith and Townson, 1981; Garms and Zillmann, 1984, Thompson et al., 1990).

22Chromosomally, S. yahense is distinguished from S. squamosum by the inversion IIL-18. About 97% of the females and 54% of males were found homozygous for the inversion IIL-18 (i.e. IIL-18/18) and the remaining 3% females and 46% males were IIL/18 (i.e. heterozygous; IIL-st/18) (Vajime and Dunbar, 1975). The homozygous and heterozygous conditions of IIL-18 are shown in Fig. 12 and 13. This situation has been found to differ in various populations. In Togo, Benin, Ghana and parts of the Côte d’Ivoire, both males and females are essentially IIL-18 homozygotes. In the Fouta Djallon areas of Guinea, males are 100% IIL-st/18 and females IIL-18/18. In Eastern Guinea and Sierra Leone, some populations are found with IIL/18 occurring equally in males and females. This creates an identification problem when this species is found sympatric with S. squamosum since the latter has IIL-18 as a floating inversion.

23Other inversions found in S. yahense populations are; IS-11 (Fig. 4), IL-12 (Figs. 6 and 7), IL-15, IL-18 (Figs.5 and 8), IIS-6 (Figs. 11 and 2), IIL-40 (Fig. 12), IIL-61 (Fig. 15), SCffl (Fig. 17), IIIL-22, IIIL-43 and IIIL-28 (Fig. 16). Some populations also show ectopic pairing of the centromeres of the three chromosomes (Fig. 3).

Figs. 16–20 Chromosome III of S. sqamosum/S. yahense. (16) Standard sequençe of Chromosome III for West African S. damnosum s.l. as found in S. squamosum/S. yahense. (17) band dimorphism of contromere of chromosome III; SCI. (18) Chromosome III of S. squamosum showing band dimorphism of the centromere and a heterozygous inversion-IIIDL/41. (19) Chromosome III of S. squamosum; IIIL/82. (20) Chromosome III of S. squamosum; IIIL/42. b = blister. C = centromere.

Figs. 21–24 Chromosome I of S. sanctipauli subcomplex. (21) Chromosome I with IS-A and IL-B homozygous; IS-A/A, IL-B/B. (22) Short arm showing IS-A heterozygous. (23) Short arm with IS-21 heterouzygous. (24) Short arm with IS-24. C = centromere.

Figs. 25–26 (25) Full karyotype of S. leonense (S. soubrense B); IS-A/A, IL-B.A/B.A, IIS-7/7. IIL-4.6.D.7/4.6.D.7. IIIL-2/2.17.4. (26) Chromosome I of S. sanctipauli s.l.; IL-B/B. B = Ring of Balbiani. db = double bulb. PB = para-Balbiani. b = blister. C = centromere. NO = nucleolar organiser.

Figs. 27–29 Chromosome II of S. sanctipauli s.l. (27) Chromosome II of S. sanctipauli: IIL-4.6A/4.6.A. (28) Chromosome II of S. sanctipauli; IIL-4.6.A.7/4.6.A.7. (29) Chromosome II of S. sanctipauli; IIL-4.6.A/4.6.A.7. B = Ring of Balbiani, db = double bulb. PB = para-Balbiani. C = centromere.

Fig. 30 Chromosome II of S. sanctipauli IIS/6b, IIL-4.6.A/4.6.A. B = Ring of Balbiani. db = double bulb. PB = para-Balbiani. C = centromere

Simulium sanctipauli s.l.

24The descriptions of S. sanctipauli and S. soubrense (Vajime and Dunbar, 1975) have been revised by Post (1986) and Boakye et al. (1993). While formerly thought to consist of two cytospecies, Boakye et al. (1993) now describe four species under this sub-complex. These are; Simulium sanctipauli, Simulium soubrense, Simulium leonense and Simulium konkourense.

25In the initial description of S. sanctipauli and S. soubrense, the two species were separated from S. squamosum by the Inversions IL-6, IIL-4.6 and the triple inversion IIIL-2.17.4. They were separated from each other by the inversion IIL-7 (Vajime and Dunbar, 1975). Quillévéré (1975) was the first to raise objection to the separation of the two species due to the presence of high proportions of IIL-7 heterozygotes in some populations in the Côte d’Ivoire. Similar observations were made in Togo (Meredith et al., 1983). Post (1986) in his revision maintained the inversions IIL-4.6 and IIIL-2.17.4 as diagnostic for the sub-complex but reported that the breakpoints of the inversion IL-6 were wrongly demarcated and that a double inversion designated IL-PQ was rather present. He further described new inversions that could be used to separate the two cytospecies from each other. Boakye et al. (1993), State that the break-points for IL-6 is incorrect but maintain that a single inversion difference and not the double inversion IL-PQ normally occurred in S. sanctipauli s.l. This inversion is apparently similar to IL-B of Post (1986) and it is therefore designated as such. However, the inversion IL-B as described by Post (1986) does not exist just as the inversions IIIL-C1.C2-The new breakpoints of IL-B are shown on Fig. 21, 25 and 26.

26Furthermore, various populations in Guinea, Sierra Leone, Liberia and Côte d’Ivoire show polymorphisms for the inversions IL-B (Figs. 21 and 25), IIL-6 (Figs. 2732) and IIIL-2.17 + 4 (Fig. 38). Thus, these inversions are not fixed for all members of the sub-complex. In view of these fmdings, the only fixed inversion diagnostic for the members of S. sanctipauli s.l. is IIL-4 (Boakye et al., 1993). Each member in addition to this has some fixed inversions, which differentiate it from the other species of the sub-complex.

Simulium sanctipauli s.s. Vajime and Dunbar (sensu Boakye et al., 1993)

27This species is differentiated from the other members by the presence of the intraspecifïc inversion IIL-A (Post, 1986) which is shown on Figures 27–30. Other fixed inversions are; IIL-6 and IIIL-2. Floating inversions recorded are IS-A (Fig.s21 and 22), IS-24 (Fig. 24), IL-1 (Fig. 46), IIL-7 (Figs. 28 and 29), IIIL-B (Figs. 38–40), IIIL-4 and IIIL-17. The rest are; IS-21, IS-5 and IS-20 (Fig. 23), IS-F (Fig. 21), IIS-7 and IIS-A (Fig. 32), IIL-S (Fig. 30). IIIL-M and IIIL-A (Fig. 37). Some individuals also show a micromorphological alteration of the centromeres of chromosome II or chromosome III.

28A geographical variant of this species designated as S. sanctipauli Djodji form was found in Togo and Ghana. This form has no fixed inversion differences from other populations of S. sanctipauli except that sex determination is associated with the inversion IS-2L (Fig. 23), wrongly labeled as IS-α (Surtees, 1986). Males of the Djodji form are heterozygous for the inversion and it is absent in females (Surtees et aL, 1988). The diagnostic character relates to a population and therefore is not useful for identifying individuals.

29S. sanctipauli has been recorded predominantly from the Côte d’Ivoire and Southern Ghana. Occasionally, it has been identified from rivers in Guinea. Sierra Leone and Liberia. One record of this species is known from Mali. Since the extension of larviciding to rivers in the south of Togo and Ghana in 1988, the Djodji form has not been found in larval samples from the area (OCP, unpublished reports).

Simulium leonense Boakye, Post and Mosha (originally described by Post, 1986, as S. soubrense B)

30This species is characterized by the inversions IL-A (Fig. 25), IIS-7 (Fig. 32) and IIIL-2 (Figs. 37 and 38). There are eight floating inversions; IS-A (Figs. 21 and 22), IIL-6, IIL-7, IIL-D, and IIL-X (Figs. 32–34), IIIL-4 and IIIL-17 (Figs. 35–40). Sex-linkage is found to be associated with IIIL- 4.17 in most populations. Rare inversions recorded in this species are IS-M (Fig. 21), IS-N and IS-C (Fig. 26), I L-T (Fig. 25), IIS-A, IIL-W and IIL-B (Fig. 32), IIIL-K and IIIL-D (Fig. 37) and IIIL-I (Fig. 41).

31Distribution of this cytospecies is limited mostly to Sierra Leone and its borders with Guinea. However, records of its presence in Liberia exist (Güzelhan and Garms, 1987).

Figs. 31–34 Chromosome II of S. sanctipauli s.l. (31) Chromosome II of S. soubrense’, IIL-4.6/4.6. (32) Chromosome II typical for S. leonense. S. konkourense Menenkaya form and S. soubrense St. Paul form; IIS-7/7, IIL-4.6.D.7/4.6.D.7. (33) Chromosome II of S. konkourense Konkouré form; IIL-4.X/4.X. (34) Chromosome II of S. konkourense Konkouré form; IIL-4.X/4.6.D.7. B = Ring of Balbiani. db = double bulb. Pb = para-Balbiani. C = centromere.

Figs. 35–37 Chromosome III of S. sanctipauli s.l. (35) Chromosome III homozygous for IIIL-5 (IIIL-2.17.4.5/2.17.4.5). (36) Chromosome III with IIIL-17 and IIIL-4heterozygous; IIIS-st/st, IIIL-2/2.17.4. (37) Chromosome III found commonly in all members of the S. sanctipauli subcomplex; IIIL-2.17.4/2.17.4. C = centromere.

Simulium sourense Vajime and Dunbar (sensu Boakye et al., 1993)

32The original classification of this species was reviewed by Post (1986). He recognised all members of the S. sanctipauli sub-complex lacking the inversions 1L-A and IIL-A as S. soubrense. Recent observations have however_ shown that there are lots of fixed inversion differences between various populations which are sometimes found breeding in sympatric situations. Hence some have been raised to specific status and others, are at present, considered as geographic variants designated as forms. Those regarded as different species are included in the S. konkourense (Boakye et al., 1993; see below). The various forms described under S. soubrense are; Beffa form from Togo and Benin (Meredith et al., 1983), Farmington and St. Paul forms from Liberia (Kashan and Garnis, 1987) and Chutes Milo form (Boakye et al., 1993).

33Chromosomally, all these forms and others not formally designated as such but belonging to S. soubrense lack_the inversions IL-A and IIL-A. Inversions IIL-6 and IIIL-2 are fixed in ail populations. The following common inversions are recorded floating in some populations but fixed in others; IS-A. IS-J, IS-G, IS-H, 1L-B, IIS-7, IIL-D, IIL-7, IIIL-4, IIIL-17, IIIL-5, IIIL-24, IIIL-B, IIIL-26, IIIL-E, IIIL-25 and IIIL-I. these inversions are shown on Figs. 21–41.

Figs. 38–41 Chromosome III of S. sanctipauli s.l. (38) Long arm of chromosome III homozygous for IIIL-2.17.B.4. (39) Long arm of Chromosome III; IIIL-2.17.B.4.24/2.17.4.24. (40) Chromosome III; IIIS-st/st, IIIL-2.17.B.4/2.17.4. (41) Asynapsis of homologous arms of Chromosome MIL due to an inclusion (arrow). C = centromere.

Figs. 42–45 (42) Chromosome I recorded for S. damnosum s.s.l. S. sirbanum: IS-2.3/2.3, IL-1/1.2, IL-1 not shown). (43) Chromosome I of S. damnosum s.s/S. sirbanum with IS-3 heterozygous; IS-2/2.3. (44) Chromosome I of S. dieguerense: IS-2/2, IL-35/35. (45) Altered centromere region of Chromosome I (arrow) found in-S. dieguerense males. C = centromere. NO = nucleolar organiser.

Figs.46–48 Chromosome I of S. damnosum s.s./S. sirbanum. (46) Long arm of Chromosome I typical of S. damnosum s.s./S. sirba-num; IL-3.1/3.1. (47) Long arm heterozygous for IL-2. (48) Chromosome I homozygous for IL-2 and heterozygous for IS-2 and IS-3 (IS/2.3, IL-2/2). C = centromere.

Figs. 49–52 (49) Chromosome II with standard sequence for IIS and heterozygous for inversions on the long arm typical of both S. damnosum s.s. and S. sirbanum; IIS-st/st, III-C/C.8.3. (50) Chromosome II with the inversions IIL-C/C.8 common to S damnosum s.s. males. (51) short arm of chromosome II of S. sirbanum heterozygous for IIS-45. (52) Short arm of Chromosome II of S. sirbanum heterozygous for IIS-8. B = Balbiani ring, db = double bulb. C = centromere, PB = Para-Balbiani.

Beffa form

34Populations of this form show a distinctive sex-linkage associated with an inversion designated IIS-6b (Fig. 30) (Meredith et al., 1983). Males are heterozygous for the inversion which is absent in females. Floating inversions include IS-A, IS-J, IS-G, IS-H, IL-B, IIL-D, IIL-7, IIIL-4, IIIL-17 and IIIL-24.

Chutes Milo form

35Chutes Milo form seems to be typical S. soubrense, almost identical to Vajime and Dunbar’s Type material from the River Leraba (Boakye et al., 1993). IS-A, 1IL-6, IIIL-4, IIIL-17 and IIIL-24 are fixed. The inversions IIS-7, IIL-7, IIL-D and IIIL-5 are absent. Specific to the Chutes Milo form but different from other populations in Côte d’Ivoire and Ghana that share similar inversions, is the association of IIIL-B with sex determination.

Farmington and St. Paul forms

36The Farmington form is characterised by the lack of any sex-linked inversion and absence of IIL-D. Polymorphic inversions recorded are IIS-7, IIL-7, IIIL-4, IIIL-17, IIIL-5 and IIIL-26.

37The St. Paul form is identifîed by the fixation of IIL-D.7, IIIL-4.17 and sex-linkage to IIIL-5. Float ing inversions are IL-1 and IIS-7.

Simulium konkourense Boakye, Post, Mosha and Quillévéré

38This species includes two cytotypes; Konkouré form (Quillévéré et al., 1982) and Menankaya form (Boakye et al., 1993) formerly considered as forms of S. soubrense because of the absence of the inversions IL-A and IIL-A. These two forms show clinal variation in the proportion of. floating inversions along their distributional range with two extremely different chromosomaL forms at the ends. Populations of the two forma are reproductively isolated from the Chutes Milo form in the areas of sympatry confirming their specific identity from that species. Populations considered as Menankaya form have a fixation of the inversion IIL-D.7 and those considered Konkouré have larvae with the inversion IIL-4.X/4.X or IIL-4.X/4.6.D.7. Hence the criteria used can be applied to individual larvae and not necessarily populations. Common polymorphie inversions reported for this species are; IS-A, IL-1, IIS-7, IIIL-4, IIIL-2, IIIL-17 and IIIL-5. Rare inversions are IS-J, IS-X, IS-N, IL-T, IL-X, IL-D, IIIS-H, IIIL-X, IIIL-Y, IIIL-E, IIIL-Z, IIIL-G and IIIL-I.

Simulium damnosum s.s. Theobald (sensu Vajime and Dunbar)

39S. damnosum s.s. was cytotaxonomically diagnosed by Vajime and Dunbar (1975), as being distinct from S. squamosum by three independent inversions; IL-1, IIL-3 and IIIL-2. A re-examination of these inversions by Quillévéré (1975) revealed that the chromosome IIL inversion difference was not a single and simple one but rather a complex rearrangement. Post (1982) confirmed this finding and showed that some four different inversions were involved which he grouped together and termed IIL-C (Fig. 53). Sex-determination is related to IIL-8 in most populations of this species. Most males were found with the inversion IIL-C/C.8 (Fig. 50) and females IIL-C/ (Fig. 53).

40Recently, Vajime and Gregory (1990) have reported that the predominant cytotype in populations described as S.damnosum s.s. (Vajime and dunbar, 1975) in the OCP area is different from the type species identified from Uganda and should therefore be called Volta form (Dunbar and Vajime, 1981). Cytotypes considered similar to the type material were called Nile form (Dunbar and Vajime, 1981) but have been re-designated as S. damnosum s.s. with sex-linkage to the inversions IS-3.2. The predominant cytotype in the OCP area is therefore considered a separate taxon but without any formai specific name. Fiasorgbor et al. (pers. comm.) suggests the two cytotypes as sub-species of a single species S. damnosum s.s. Descriptions of inversions given in this text refer to both forms.

41Fixed inversions are IL-1 (Fig. 46), IIL-C and IIIL-2 (Fig. 57). Routine identification is based on the presence of IIL-C. Common polymorphic inversions in this species are; IS-2, IS-3 (Figs. 42, 43 and 48), IL-2 (Figs. 42, 47 and 48), IIL-8, IIL-3, IIL-2, IIL-15 (Figs. 49, 50 and 5356), IIIL-6 and IIIL-7 (Figs. 5861). Some rare inversions are IS-18, IS-22, IL-13, IIL-35 and IIL-15.

42This species is found mainly in the transition zone between the forest and the savanna. It has been recorded from all the eleven countries of the OCP. It is the predominant cytotype of the savanna group found in the forest areas of Ghana, Togo, Benin and Côte d’Ivoire.

Simulium sirbanum Vajime and Dunbar

43Two cytotypes originally considered as different species, S. sirbanum and S. sudanense, by Vajime and Dunbar (1975) are now included in this species (Vajime, 1989). The individuals of this species are separated from S. squamosum by the fixed inversions IL-1, HL-C.8 and IIIL-2. Thus, it differs from S. damnosum s.s. by fixation of HL-8. Sex-determination is related to IS-3 in both cytotypes. In one cytotype (former S. sirbanum), females do not possess the inversion whilst females of the other cytotype are found homozygous for the inversion. Most males of both forms are heterozygous (IS\3) and therefore indistinguishable (Vajime and Dunbar, 1975; Vajime, 1989).

44Routinely, this species is diagnosed by the sequence IIL-C.8/C.8, IIL-C.8.3/C.8.3 or IIL-C.8/C.8.3. Floating inversions observed are; IS-2, IS-3 (Figs. 42, 43 and 48), IL-2, IL-13 (Figs. 42, 47 and 48), IIS-45 (Fig. 51), IIS-8 (Fig. 52), IIL-3, IIL-15, IIL-35 (Figs.49 and 53–56), IIIL-6 and IIIL-7 (Figs. 58–61), IIIL-27 (Fig. 57). Inversions such as IL-13, IIS-45, IIS-8, IIL-15, IIL-35 and IIIL-27 were not commonly observed.

45Simulium sirbanum is the most widely distributed and important vector of human onchocerciasis of the S. damnosum s.l. in the OCP area. It has been been reeorded from all the river basins in the area although it occurs predominantly irr the savanna zones. The presence of this species in the forest areas is usually during the dry season. However, deforestation has resulted in savanna enclaves in the forest zone which do harbour S. sirbanum populations all year round.

Figs. 53–56 (53) Chromosome II of S. damnosum s.s.; IIS-st/st, III-C/C. (54) Chromosome II of S. sirbanum/S. dieguerense: IlS-st/st, IIL-C.8/C.8. (55) Chromosome II of S. sirbanum; IlS-st/st, IIL-C.8.3/C.8.3. (56) Chromosome II of S. sirbanum: IlS-st/st. IIL-C.8./C.8.64. B = Balbiani ring, db = double bulb. C = centromere. PB = Para-Balbiani.

Fig. 57–59 (57) Chromosome III of S. damnosum s.s./S. sirbanum/S. dieguerense, IIIS-st/st, IIIL-2/2. (58) Chromosome III of S. damnosum s.s./S. sirbanum; IHS-st/st, IIIL-2.6.7/2.6.7. (59) Chromosome III of S. damnosum s.s./S. sirbanum; IHS-st/st, IIIL-2/2.6.7. C = centromere.

Figs. 60–61 Chromosome III of S.-damnosum s.s./S. sirbanum; (60) IIIS-st/st, IIIL-2.7/2.7. (61) IIIL-2/2.7. C = centromere.

Simulium dieguerense Vajime and Dunbar

46Vajime and Dunbar (1975) described this species as having four inversion differences from S. squamosum. These were IS-2, IL-12, 1IL-3.8 and IIIL-2. Post (1982) revised the diagnostic criteria replacing IL-12 and IIL-3 by IL-35 (Fig. 44) and IIL-C respectively. In this species IL-12 and IIL-3 are absent. Later, Boakye and Mosha (1988) reported sex-linkage to an altered segment of the centromere region of chromosome I (Fig. 45) similar to that reported for S. squamosum (Vajime and Dunbar. 1975). About 90% of males and 17% of females have the altered segment.

47Routine identifications based on only IIL-C.8 could resuit m mis-identifications with S. sirbanum since these inversions are common to both species. Floating inversions found in this species are IIL-68 (Fig. 54) and IIIL-28 (Fig. 16).

48Simulium dieguerense has a limited distribution. It has been recorded breeding mostly on the Bakoye and Bafing river basins in Mali and Guinea. Occasionally, it is found breeding in other rivers in Guinea and Sierra Leone.

Acknowledgements

49I wish to thank all members of the Vector Control Unit. OCP. who provided material for cytotaxonomic identification during the period of this undertaking. I am indebted to A. Sib, Y. Coulibaly. S. Naniougou and B. Bougsere whose training as cytotaxonomy technicians resulted in the idea for this pictorial guide. The comments and suggestions of Drs. S. E. O. Meredith and R. J. Post were invaluable. Mr. G. Schoon helped in the development and printing of the photographs. Finally, 1 acknowledge the encouragement of Dr. D. Quillévéré, Chief VCU, Dr. B. Philippon and Dr. C. Back who actually helped with the drawing of Figure 2. Permission to have this published was kindly given by Dr. E. Samba, Director, OCP, Dr. E. Samba also requested the WHO/OCP to pay for the printing of the manuscript. Finally the author benifitted from a WHO/TDR support during the writing of this manuscript.

Bibliographie

References

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Bedo, D. G.: Cytogenetics and evolution of Simulium omatipes Skuse (Diptera: Simuliidae). 1. sibling speciation. Chromosoma (Berlin) 64 (1977) 37–65

Boakye, D. A., F. W. Mosha The distribution and chromosome polymorphism of Simulium dieguerense (Diptera: Simuliidae). Trop. Med. Parasitol. 39 (1988) 117–119

Boakye. D. A., B. J. Post. F. W. Mosha. D. P. Surtees. R. H. A. Baker: Cytotaxonomic revision of the Simulium sanctipauli subcomplex in Guinea and the adjacent countries including descriptions of two new species. Bull. Ent. Res 83 (1993)

Dang. P. T., B. V. Peterson: Pictorial keys to the main species groupa within the Simulium damnosum Theobald complex occurring in West Africa. Tropenmed. Parasitol. 31 (1980) 117–120

Dunbar. R. W.: Four sibling species included in Simulium damnosum Theobald (Diptera: Simuliidae) from Uganda. Nature 209 (1966) 597–599

Dunbar. R. W., C. G Vajime: Cytotaxonomy of the Simulium damnosum complex. In: Laird, M. (Ed.): Blackflies: The future for biological methods in integrated control. Academic Press, London (1981) 31–43

Fiasorgbor. G. K., S. A. Sowah. D. A. Boakye. G. Zerbo: Distribution of Simulium yahense after larviciding activitiesin the Southern extension of the Onchocerciasis Control Programme. Trop. Med. Parasitol. 43 (1992) 72–73

Garms, R. R. A. Cheke. C. G. Vajime. S. Sowah: The occurrence and movements of different members of the Simulium damnosum complex in Togo and Benin. Z. Angew. Zool. 69 (1982) 219–236

Garms. R., U. Zillmann. Morphological identification of Simulium sanctipauli and S. yahense in Liberia and comparison of results with those of enzyme electrophoresis. Tropenmed. Parasit. 35 (1984) 217–220

Güzelhan. C., R Garms: Cytotaxonomy of the Farmington form of Simulium soubrense in Liberia. Trop. Med. Parasitol. 38 (1987) 345

Kashan. A., R. Garms. Cytotaxonomy of the Simulium sanctipauli sub-complex in Liberia. Drop. Med. Parasitol. 38 (1987) 289 293

Meredith. S. E. O., H. Townson: Enzymes for species identification in the Simulium damnosum complex from West Africa. Tropenmed. Parasit. 32 (1981) 123–129

Meredith. S. E. O., R. A. Cheke. R. Garms: Variation and distribution of forms of Simulium soubrense and S. sanctipauli in West Africa. Ann. Trop. Méd. Parasitol. 77 (1983) 627–640

Post. R. J.: The cytotaxonomy of Simulium (Edwardsellum) dieguerense Vajime and Dunbar 1975. Tropenmed. Parasit. 33 (1982) 37–39

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

Quillévéré.-D.: Etude du complexe Simulium damnosum en Afrique du l’Ouest. I. Technique d’étude. Identification des cytotypes. Cahiers O.R.S.T.O.M., série Ent. méd. Parasitol. 13 (1975) 87–100

Quillévéré. D.. P. Guillet. Y. Sechan: La répartition géographique des èspeces du complexe Simulium damnosum dans la zone du projet Sénégambie (ICP/MPD 007). Cahiers O.R.S.T.O.M., série Ent. méd. Parasitol. 19 (1982) 303–309

Surtees. D. P.: A new cytotype within Simulium sanctipauli (Diptera: Simuliidae) from Togo. Trans. Roy. Soc. Trop. Med. Hyg. 80 (1986) 343

Surtees. D. P., G. Fiasorgbor. R. J. Post. E. A. Weber: The cytotaxonomy of the Djodji form of Simulium sanctipauli (Diptera: Simuliidae). Trop. Med. Parasitol. 39 (1988) 120–122

Thompson. M. C., A. Davies. J. B. Davies: A new PGM electromorph diagnostic for S. squamosum from Sierra Leoné and Togo but not found in S. squamosum from Cameroun. Acta Leidensia 59 (1990) 303–305

Vajime. C. G.: Cytotaxonomy of Sirba form populations-of the Simulium damnosum complex in West Africa: amendments to sex chromosomes and sibling status. Trop. Med. Parasitol. 40 (1989) 464–467

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

Vajime. C. G.. W. G. Gregory: Species complex of vectors and epidemiology. Acta Leidensia 59 (1990) 235–252

Table des illustrations

Légende Fig. 1 a Idiogramatic representation of the chromosome complement of members of the S. damnosum s.l. in West Africa.Fig. 1 b Diagramatic representation of an inversion.
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Légende Fig. 2 Cytospecies of the S. damnosum complex in the OCP area.
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Légende Fig. 3 Full chromosome complément of S. squamosum showing the ectopic pairing of the centromeres. B = Ring of Balbiani. db = double bulb. PB = para-Balbiani. b = blister. C = centromere.
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Légende Figs. 4–8 Chromosome 1 (S. squamosum/S. yahense). (4) Standard sequence for members of the S. damnosum s.l. in west Africa. S-1/1, IL-3/3. (5) Long arm of chromosome I; IL-3/3. (6) Long arm of chromosome I; IL-3.12/3.12. (7) Long arm of chromosome I; IL-3/3.12. (8) Long arm of chromosome I; 3/3.18. C = centromere. NO = nucleolar organiser.
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Légende Figs. 9–10 Chromosome IS of S. squamosum showing IS-23 and altered centromere region.
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Légende Figs. 11–14 Chromosome II of S.squamosum/S.yahense (11). Standard sequence of Chromosome II for West.African S. damnosum s.l. as found in S. squamosum. (12) Chromosome II of S. yahense; IIL-18/18, IIS/6. (13) Long arm of S. yahense, IIL/18. (14) Long arm of S. squamosum; IIL/42. B = Ring of Balbiani, db = double bulb. PB = para-Balbiani. C = centromere.
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Légende Fig. 15 Long arm of chromosome II of S. yahense heterozygous for IIL-61 (IIL-18/18.61). C = centromere.
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Légende Figs. 16–20 Chromosome III of S. sqamosum/S. yahense. (16) Standard sequençe of Chromosome III for West African S. damnosum s.l. as found in S. squamosum/S. yahense. (17) band dimorphism of contromere of chromosome III; SCI. (18) Chromosome III of S. squamosum showing band dimorphism of the centromere and a heterozygous inversion-IIIDL/41. (19) Chromosome III of S. squamosum; IIIL/82. (20) Chromosome III of S. squamosum; IIIL/42. b = blister. C = centromere.
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Légende Figs. 21–24 Chromosome I of S. sanctipauli subcomplex. (21) Chromosome I with IS-A and IL-B homozygous; IS-A/A, IL-B/B. (22) Short arm showing IS-A heterozygous. (23) Short arm with IS-21 heterouzygous. (24) Short arm with IS-24. C = centromere.
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Légende Figs. 25–26 (25) Full karyotype of S. leonense (S. soubrense B); IS-A/A, IL-B.A/B.A, IIS-7/7. IIL-4.6.D.7/4.6.D.7. IIIL-2/2.17.4. (26) Chromosome I of S. sanctipauli s.l.; IL-B/B. B = Ring of Balbiani. db = double bulb. PB = para-Balbiani. b = blister. C = centromere. NO = nucleolar organiser.
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Légende Figs. 27–29 Chromosome II of S. sanctipauli s.l. (27) Chromosome II of S. sanctipauli: IIL-4.6A/4.6.A. (28) Chromosome II of S. sanctipauli; IIL-4.6.A.7/4.6.A.7. (29) Chromosome II of S. sanctipauli; IIL-4.6.A/4.6.A.7. B = Ring of Balbiani, db = double bulb. PB = para-Balbiani. C = centromere.
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Légende Fig. 30 Chromosome II of S. sanctipauli IIS/6b, IIL-4.6.A/4.6.A. B = Ring of Balbiani. db = double bulb. PB = para-Balbiani. C = centromere
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Légende Figs. 31–34 Chromosome II of S. sanctipauli s.l. (31) Chromosome II of S. soubrense’, IIL-4.6/4.6. (32) Chromosome II typical for S. leonense. S. konkourense Menenkaya form and S. soubrense St. Paul form; IIS-7/7, IIL-4.6.D.7/4.6.D.7. (33) Chromosome II of S. konkourense Konkouré form; IIL-4.X/4.X. (34) Chromosome II of S. konkourense Konkouré form; IIL-4.X/4.6.D.7. B = Ring of Balbiani. db = double bulb. Pb = para-Balbiani. C = centromere.
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Légende Figs. 35–37 Chromosome III of S. sanctipauli s.l. (35) Chromosome III homozygous for IIIL-5 (IIIL-2.17.4.5/2.17.4.5). (36) Chromosome III with IIIL-17 and IIIL-4heterozygous; IIIS-st/st, IIIL-2/2.17.4. (37) Chromosome III found commonly in all members of the S. sanctipauli subcomplex; IIIL-2.17.4/2.17.4. C = centromere.
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Légende Figs. 38–41 Chromosome III of S. sanctipauli s.l. (38) Long arm of chromosome III homozygous for IIIL-2.17.B.4. (39) Long arm of Chromosome III; IIIL-2.17.B.4.24/2.17.4.24. (40) Chromosome III; IIIS-st/st, IIIL-2.17.B.4/2.17.4. (41) Asynapsis of homologous arms of Chromosome MIL due to an inclusion (arrow). C = centromere.
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Légende Figs. 42–45 (42) Chromosome I recorded for S. damnosum s.s.l. S. sirbanum: IS-2.3/2.3, IL-1/1.2, IL-1 not shown). (43) Chromosome I of S. damnosum s.s/S. sirbanum with IS-3 heterozygous; IS-2/2.3. (44) Chromosome I of S. dieguerense: IS-2/2, IL-35/35. (45) Altered centromere region of Chromosome I (arrow) found in-S. dieguerense males. C = centromere. NO = nucleolar organiser.
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Légende Figs.46–48 Chromosome I of S. damnosum s.s./S. sirbanum. (46) Long arm of Chromosome I typical of S. damnosum s.s./S. sirba-num; IL-3.1/3.1. (47) Long arm heterozygous for IL-2. (48) Chromosome I homozygous for IL-2 and heterozygous for IS-2 and IS-3 (IS/2.3, IL-2/2). C = centromere.
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Légende Figs. 49–52 (49) Chromosome II with standard sequence for IIS and heterozygous for inversions on the long arm typical of both S. damnosum s.s. and S. sirbanum; IIS-st/st, III-C/C.8.3. (50) Chromosome II with the inversions IIL-C/C.8 common to S damnosum s.s. males. (51) short arm of chromosome II of S. sirbanum heterozygous for IIS-45. (52) Short arm of Chromosome II of S. sirbanum heterozygous for IIS-8. B = Balbiani ring, db = double bulb. C = centromere, PB = Para-Balbiani.
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Légende Figs. 53–56 (53) Chromosome II of S. damnosum s.s.; IIS-st/st, III-C/C. (54) Chromosome II of S. sirbanum/S. dieguerense: IlS-st/st, IIL-C.8/C.8. (55) Chromosome II of S. sirbanum; IlS-st/st, IIL-C.8.3/C.8.3. (56) Chromosome II of S. sirbanum: IlS-st/st. IIL-C.8./C.8.64. B = Balbiani ring, db = double bulb. C = centromere. PB = Para-Balbiani.
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Légende Fig. 57–59 (57) Chromosome III of S. damnosum s.s./S. sirbanum/S. dieguerense, IIIS-st/st, IIIL-2/2. (58) Chromosome III of S. damnosum s.s./S. sirbanum; IHS-st/st, IIIL-2.6.7/2.6.7. (59) Chromosome III of S. damnosum s.s./S. sirbanum; IHS-st/st, IIIL-2/2.6.7. C = centromere.
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Légende Figs. 60–61 Chromosome III of S.-damnosum s.s./S. sirbanum; (60) IIIS-st/st, IIIL-2.7/2.7. (61) IIIL-2/2.7. C = centromere.
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Auteur

World Health Organisation. Onchocerciasis Control Programme, Ouagadougou. Burkina Faso
Department of Population Biology
University of Leiden Schelpenkade 15 a, 2313 ZT Leiden
The Netherlands

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