Trypanosome hybrids generated in tsetse flies by nuclear fusion.

Abstract

Genetic exchange may occur between two particular Trypanosoma brucei clones simultaneously transmitted by the same tsetse fly. We report here that this exchange takes place in the fly, through nuclear fusion. The resulting hybrids appear to be sub‐tetraploid, some particular DNA sequences from one of the parental stocks being lost before enough cloned hybrid trypanosomes could be harvested for DNA analysis. A further reduction of the DNA content of these hybrids occurs gradually upon growth and yields near diploid value in a major part of the population. This mode of hybrid generation is different from the fusion of haploid gametes, which is thought to occur normally upon inoculation of metacyclic trypanosomes in their mammalian host. In this respect, the sub‐tetraploid hybrids appear to undergo meiosis in the fly, generating sub‐diploid metacyclic forms, then fusion in the mammalian blood.

Allelic segregation and independent assortment in T. brucei crosses: Proof that the genetic system is Mendelian and involves meiosis

“…The data presented here clearly do not support the parental fusion and chromosome loss model originally proposed to explain the increased DNA content of some progeny [14,15]. Essentially two other models have been proposed: (1) a conventional system where parental cells undergo meiosis to produce haploid gametes that then fuse to generate diploid recombinant progeny and (2) a 'ciliate/flagellate' model in which fusion of diploid cells occurs, followed by meiosis in a heterokaryon.…”

“…The DNA contents of the progeny from the first cross [14,15] were shown to be elevated relative to the parental lines and this has also been observed in a high proportion (average 59%, n=24) of progeny from crosses between T.b.brucei and T.b.rhodesiense where marker analysis suggests that these products of mating are trisomic or triploid [13]. In contrast, crosses between either T.b.brucei stocks or…”

Intraclonal mating in Trypanosoma brucei is associated with out-crossing

“…Genetic exchange was first demonstrated by Jenni et al (1986) after cotransmission of two different trypanosome clones through tsetse flies. In this first successful laboratory cross and several subsequent ones (Gibson, 1989;Turner et al, 1990; Abbreviations: hph, hygromycin phosphotransferase gene; neo, neomycin phosphotransferase gene; H, hygromycin-B-resistant; N, Geneticin (G418)-resistant; ALD, aldolase; GAPDH, glyceraldehyde phosphate dehydrogenase; GPI, glucose phosphate isomerase; PARP,, procyclic acidic repetitive protein; PGK, phosphoglycerate kinase; PLC, phospholipase C; PYK, pyruvate kinase; RAPD, random amplification of polymorphic DNA; RNAP, RNA polymerase; SSC, standard saline citrate; TIM, triose phosphate isomerase; VSG, variant surface glycoprotein.…”

“…Analysis of isoenzymes and RFLPs indicates that a meiotic division probably occurs during genetic exchange : segregation and reassortment of genetic markers is observed (Jenni et al, 1986;Paindavoine et al, 1986;Wells et al, 1987;Gibson, 1989;Sternberg et al, 1989;Turner et al, 1990;Gibson & Garside, 1991;Gibson et al, 1992Gibson et al, ,1995Gibson & Whittington, 1993;Schweizer et al, 1994;Gibson & Bailey, 1994;Degen et al, 1995) and also a high frequency of chromosomal recombination in hybrids (Gibson et al, 1992;Gibson & Bailey, 1994). However, hybrids with a 3n DNA content have been found in four of five crosses for which DNA contents were measured (Paindavoine et al, 1986;Wells et al, 1987;Gibson et al, 1992Gibson et al, , 1995Gibson & Bailey, 1994), including two early crosses which did not employ selectable markers. These 3n hybrids have been interpreted as triploid, since DNA contents of independent clones clustered at 3n, rather than a range of intermediate values, and in addition, two clones analysed in depth showed trisomy for several chromosomes (Gibson et al, 1992;Gibson & Bailey, 1994).…”

“…brucei, except Type I T . 6. gambiense, have been successfully mated (Jenni et al, 1986;Gibson, 1989;Turner et al, 1990;Gibson & Garside, 1991;Schweizer et al, 1994;Degen et al, 1995). Other lower eukaryotes have multiple mating types, determined by loci which encode or control production of specific pheromones ; such systems ensure that mating occurs only between organisms of different mating types and hence favour out-crossing.…”