Cytotaxonomy of Sirba form populations of the Simulium damnosum complex in west Africa: amendments to sex chromosomes and sibling status.

The banding pattern of larval salivary polytene chromosomes was analysed to re-examine the distribution, by sex and geography, of the sex linked inversion (IS-3) in "Sirba" form which was previously described as consisting of Simulium sirbanum and S. sudanense (Vajime and Dunbar, 1975). West African larval samples from seven countries within the Onchocerciasis Control Programme area and some adjacent locations were used. It was concluded that "Sirba" consists of two indisputable cytoforms, one distinguished by X0Y0 males and X0X0 females, the other by X1Y0 males and X1X1 females; and that the cytoforms are cytologically variable populations of a single cytospecies, S. sirbanum. It is argued that the species is in the process of incipient speciation.