Hybrid formation in the life cycle of Trypanosoma (T.) brucei: detection of hybrid trypanosomes in a midgut-derived isolate.

Until recently, African trypanosomes of the species Trypanosoma (T . ) brucei which are the causative agents of sleeping sickness in man and Nagana disease in cattle, were believed to multiply solely by binary fission. In the early eighties, evidence for a system of genetic exchange was obtained indirectly from the analysis of enzyme electrophoretic variation between stocks isolated from natural populations (Gibson et al., 1980; Tait, 1980; Tait et al., 1985). Direct demonstration of gene exchange was only possible recently, with the detection of ~hybrid' trypanosomes obtained after the simultaneous transmission of two different T. brucei clones through the tsetse fly vector (Jenni et al., 1986). Ever since, investigation has focussed mainly on three topics: (1) frequency of hybrid formation (Schweizer et al., 1988; Sternberg et al., 1989), (2) mechanisms of gene exchange (Jenni et al., 1986; Paindavoine et al., 1986; Sternberg et al., 1987; Sternberg et al., 1988; Gibson, 1989; Sternberg et al., 1989; Tait et al., 1989) and finally (3) localization of hybrid formation in the parasite's life cycle. Up to now, no results have been published on this topic. The present study provides preliminary data. Progeny clones derived from single metacyclic forms out of mixed-infected vectors can express hybrid characteristics (Jenni et al., 1986; Sternberg et al., 1988). To investigate the localization of this hybrid formation in the tsetse fly, we isolated trypanosome populations from different parts of tsetse with mixed infections; salivary glands (SG), proventricle (PROV), anterior third (MG ant., including the mycetome) and posterior two thirds (MG post.) of the midgut. The different populations were characterized by isoenzyme analysis for isocitrate dehydrogenase (ICD; EC 1.1.1.42) which has proved to be quick and reliable for the detection of hybrid organisms (Jenni et al., 1986; Schweizer et al., 1988). A pair of parental clones which had