Genetic exchange in Trypanosoma brucei brucei: variable chromosomal location of housekeeping genes in different trypanosome stocks

A Trypanosoma brucei brucei clone from West Africa was crossed with another T. b. brucei clone from the East African kiboko group. This group is defined by characteristic isoenzyme patterns and kinetoplast DNA maxicircle polymorphisms, and is associated with a wild animal-tsetse transmission cycle. Three types of clone were isolated from the cross, 2 of which were hybrid. The hybrids were heterozygotic at 7 loci where the parents were homozygotic and the hybrids also had molecular karyotypes different from those of both parents. Both molecular karyotypes had an extra non-parental band, which was shown to have a different origin in the 2 sets of clones by Southern analysis with various housekeeping gene probes. This analysis also revealed that although the GPI and PGK genes reside on the same chromosome in parent J10, they are on different chromosomes in parent 196. Hybridisation of PFG blots carrying a variety of other trypanosome stocks confirmed that the GPI gene is not always in the same linkage group as the PGK gene cluster. Given that genetic exchange in trypanosomes involves meiosis, such differences in gene linkage will give rise to progeny with incorrect gene dosage, i.e., certain crosses will be partially infertile. This incipient speciation may explain why natural populations of T. brucei spp. are observed not to be in a randomly mating equilibrium.