River-specific macrogenomic diversity in Simulium guianense s. l. (Diptera: Simuliidae), a complex of tropical American vectors associated with human onchocerciasis

Abstract

Simulium guianense Wise is a Latin American vector complex of black flies associated with transmission of the causal agent of human onchocerciasis (river blindness). An analysis of the chromosomal banding patterns of 607 larvae of S. guianense s. l. revealed a high level of variation involving 83 macrogenomic rearrangements across 25 populations in Brazil, French Guiana, and Venezuela. The 25 populations were assigned to 13 cytoforms (A1, A2, B1-B4, C, D, E1-E4, and F), some of which are probably valid species. Based on geographical proximity, a member of the B group of cytoforms probably represents the name-bearing type specimen of S. guianense and the primary vector in the last-remaining onchocerciasis foci in the Western Hemisphere. Cytoform B3 in Amapá State is implicated as an anthropophilic simuliid in an area currently and historically free of onchocerciasis. Distributions of cytoforms are associated with geography, elevation, and drainage basin, and are largely congruent with ecoregions. Despite extraordinarily large larval populations of S. guianense s. l. in big rivers and consequent production of female flies for dispersal, the cytoforms maintain their chromosomal distinction within individual rivers, suggesting a high degree of fidelity to the specialized breeding habitats-rocky shoals-of the natal rivers.

Fig 1. Major rivers of Brazil, French Guiana, and Venezuela, showing sampling sites for Simulium guianense s. l.

Sampling sites are named by river, corresponding to names used in the text and Table 1. Type localities of S. guianense, S. ortizi, and S. pintoi, and current onchocerciasis foci are indicated.

Fig 2. Sampling sites for Simulium guianense cytoforms A and B.

A. Pandeiros River, Minas Gerais state. B. Teles Pires River, Mato Grosso state. C. Pitinga River, Amazonas state. D. Branco River, Roraima state. E. Oiapoque River, Amapá state. F. Yuruani River, Bolivar state.

Fig 3. Sampling sites for Simulium guianense cytoforms D, E, and F.

A. Xingu River, Pará state. B. São José River, Espírito Santo state. C. Araguaia River, Mato Grosso state. D. Pardo River, São Paulo state. E. Mortes River, Mato Grosso state. F. Longá River, Piauí state.

Fig 4. IS standard chromosomal sequence of cytoform F.

Breakpoints of 15 inversions of various cytoforms are indicated by brackets, and those of X-linked IS-14, which occurs only on top of IS-13 in the Pitinga River population of cytoform B2, by arrows. Male larva (Longá River). C = centromere, g = glazed marker, hb18 = location of heteroband, 2°NO = location of secondary nucleolar organizer, ‘3’ = 3 heavy marker.

Fig 5. IS distal half of cytoform B3, showing a partial IS-3 chromosomal sequence.

Breakpoints of floating inversions IS-15, IS-18, and IS-22 are indicated by brackets. Male larva (Oiapoque River).

Fig 6. IL base (above) and sections 28–41 of cytoform F, showing IL-5 chromosomal sequence.

Breakpoints of IL-7 and IL-11 are indicated by brackets. Male larva (Longá River). C = centromere, N.O. = nucleolar organizer.

Fig 7. IL distal to nucleolar organizer (N.O.) of cytoform B3, showing IL-6, 8 chromosomal sequence.

Breakpoints of 9 additional inversions are indicated by brackets. Male larva (Oiapoque River).

Fig 8. IIS standard chromosomal sequence of cytoform F.

Breakpoints of IIS-1, IIS-2, and IIS-3 are indicated by brackets. Composite female [sections 42–45] and male larva [sections 46–54] (Longá River). Bu = bulge, C = centromere, RoB = ring of Balbiani, ss = shoestring marker, T = trapezoidal marker.

Fig 9. IIL standard chromosomal sequence of cytoform F.

Breakpoints of 12 inversions of various cytoforms are indicated by brackets. Letters a—h, when assembled alphabetically, produce the IIL-2, 3, 13 sequence of cytoform B1 from the standard sequence. Male larva (Longá River). C = centromere, gB = gray band, J = jagged, P = puffing band, Pb = parabalbiani.

Fig 10. IIL of cytoform E4, showing the IIL-11, 12 chromosomal sequence.

Breakpoints of IIL-4, IIL-6, and IIL-8 are indicated by brackets. Female larva (Verdão River). C = centromere, gB = gray band, J = jagged, Pb = parabalbiani.

Fig 11. IIL distal half of cytoform B3, showing the IIL-2, 3 chromosomal sequence.

Breakpoints of 9 additional inversions of various cytoforms are indicated by brackets. Male larva (Oiapoque River). DNA = DNA puff, gB = gray band, J = jagged, Pb = parabalbiani.

Fig 12. IIIS standard chromosomal sequence of cytoform B3.

Breakpoints of IIIS-1 are indicated by a bracket. Male larva (Oiapoque River). Bl = blister, C = centromere, ca = capsule.

Fig 13. IIIL distal to section 88 of cytoform F, showing standard chromosomal sequence.

Breakpoints of 13 inversions of various cytoforms are indicated by brackets. Male larva (Longá River).

Fig 14. IIIL of cytoform B3, showing IIIL-7, 9 chromosomal sequence.

Breakpoints of 4 additional inversions of various cytoforms are indicated by brackets. Female larva (Oiapoque River). C = centromere.

Fig 15. Strict consensus tree of three most parsimonious trees for cytoform populations of Simulium guianense s. l.

Cytoforms are in quotation marks followed by their rivers of occurrence. Numbers below branches correspond to relative Bremer support.

Fig 16. Representative tree showing hypothesis of relationships among cytoform populations of Simulium guianense s. l.

Cytoforms are in quotation marks followed by their rivers of occurrence. Black circles indicate unique apomorphies (i.e. derived traits of a clade); open circles represent non-unique changes on the tree, either forward or reverse.

Fig 17. Freshwater ecoregions of South America [25] where cytoforms of Simulium guianense s. l. were collected.