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

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.