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Cytotaxonomy of Simulium siamense Takaoka and Suzuki (Diptera: Simuliidae) in Thailand

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Larval polytene chromosomes of 1824 individuals of Simulium siamense Takaoka and Suzuki from 32 locations in northern, northeastern, eastern, and central Thailand were examined. Differences in sex chromosomes, fixed inversions, and the kind and frequency of floating inversions revealed five cytoforms (A, B, C, D, and E). In contrast to most Simulium species, females of S. siamense are the heterogametic sex (ZW) and males are the homogametic sex (ZZ). Cytoform A differs from the others by having 23 floating inversions and undifferentiated sex chromosomes (W0 was undifferentiated from Z0). Cytoforms B, C, and D differ from each other by having different W chromosomes (W1, W2, and W3) in females. Cytoform E is characterized by having two fixed inversions, IS-1 and IIIS-2. Cytoform A is widely distributed throughout the geographic ranges of the species in northern, northeastern, eastern, and central Thailand, whereas cytoforms B, C, and D are restricted to a few locations in central Thailand. Although the larvae of the five cytoforms are morphologically similar, they seem to occupy different ecological habitats, which should be further investigated. At present, no definite conclusion can be drawn as to the taxonomic status of the cytoforms of S. siamense. However, this study indicates that S. siamense is represented by at least five cytoforms, of which cytoform B is the most distinct by having complete sex linkage and no floating inversions.

Les chromosomes polytènes de 1 824 larves du Simulium siamense Takaoka et Suzuki provenant de 32 sites du nord, du nord-est, de l’est et du centre de la Thaïlande ont été examinés. Des différences touchant les chromosomes sexuels (inversions fixées de même que le nombre et la fréquence d’inversions flottantes) ont permis de révéler cinq cytoformes (A, B, C, D et E). Contrairement à la plupart des espèces du genre Simulium, les femelles du S. siamense sont le sexe hétérogamétique (ZW) et les mâles sont le sexe homogamétique (ZZ). La cytoforme A diffère de autres en ayant 23 inversions flottantes et des chromosomes sexuels non différenciés (le chromosome W0 n’étant pas différent du chromosome Z0). Les cytoformes B, C et D diffèrent les unes des autres en ayant des chromosomes W différents (W1, W2 ou W3) chez les femelles. La cytoforme E est caractérisée par la présence deux inversions fixées : IS-1 et IIIS-2. La cytoforme A est largement distribuée au sein de l’aire géographique de l’espèce dans le nord, le nord-est, l’est et le centre de la Thaïlande. Bien que les larves des cinq cytoformes soient de morphologie semblable, elles semblent occuper des niches écologiques différentes et ce phénomène mériterait de plus amples études. En ce moment, il est impossible de tirer des conclusions définitives quant au statut taxonomique des cytoformes du S. siamense. Par contre, cette étude montre que le S. siamense comprend au moins cinq cytoformes dont la cytoforme B est la plus distincte en présentant une liaison sexuelle complète et aucune inversion flottante.

Document Type: Research Article

Publication date: December 1, 2008

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  • From its inception in 1957, this international cytogenetics journal has catered to the research areas of the members of the Genetics Society of Canada; traditionally, these have included agriculture, entomology, genetics/cytogenetics, and evolutionary mechanisms. The contents of the journal have evolved as contributors developed new technologies and interests. A 20-member Editorial Board is composed of scientists from around the world. Reviews and commentary from respected experts are often featured.
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