Analysis of a new genetic cross between two East African Trypanosoma brucei clones

Mating compatibility in the parasitic protist Trypanosoma brucei

Background

Genetic exchange has been described in several kinetoplastid parasites, but the most well-studied mating system is that of Trypanosoma brucei, the causative organism of African sleeping sickness. Sexual reproduction takes place in the salivary glands (SG) of the tsetse vector and involves meiosis and production of haploid gametes. Few genetic crosses have been carried out to date and consequently there is little information about the mating compatibility of different trypanosomes. In other single-celled eukaryotes, mating compatibility is typically determined by a system of two or more mating types (MT). Here we investigated the MT system in T. brucei.

Methods

We analysed a large series of F1, F2 and back crosses by pairwise co-transmission of red and green fluorescent cloned cell lines through experimental tsetse flies. To analyse each cross, trypanosomes were cloned from fly SG containing a mixture of both parents, and genotyped by microsatellites and molecular karyotype. To investigate mating compatibility at the level of individual cells, we directly observed the behaviour of SG-derived gametes in intra- or interclonal mixtures of red and green fluorescent trypanosomes ex vivo.

Results

Hybrid progeny were found in all F1 and F2 crosses and most of the back crosses. The success of individual crosses was highly variable as judged by the number of hybrid clones produced, suggesting a range of mating compatibilities among F1 progeny. As well as hybrids, large numbers of recombinant genotypes resulting from intraclonal mating (selfers) were found in some crosses. In ex vivo mixtures, red and green fluorescent trypanosome gametes were observed to pair up and interact via their flagella in both inter- and intraclonal combinations. While yellow hybrid trypanosomes were frequently observed in interclonal mixtures, such evidence of cytoplasmic exchange was rare in the intraclonal mixtures.

Conclusions

The outcomes of individual crosses, particularly back crosses, were variable in numbers of both hybrid and selfer clones produced, and do not readily fit a simple two MT model. From comparison of the behaviour of trypanosome gametes in inter- and intraclonal mixtures, we infer that mating compatibility is controlled at the level of gamete fusion.

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

The genetic system on Trypanosoma brucei has been analysed by generating large numbers of independent progeny clones from two crosses, one between two cloned isolates of Trypanosoma brucei brucei and one between cloned isolates of T. b. brucei and Trypanosoma brucei gambiense, Type 2. Micro and minisatellite markers (located on each of the 11 megabase housekeeping chromosomes) were identified, that are heterozygous in one or more of the parental strains and the segregation of alleles at each locus was then determined in each of the progeny clones. The results unequivocally show that alleles segregate in the predicted ratios and that alleles at loci on different chromosomes segregate independently. These data provide statistically robust proof that the genetic system is Mendelian and that meiosis occurs. Segregation distortion is observed with the minisatellite locus located on chromosome I of T. b. gambiense Type 2 and neighboring markers, but analysis of markers further along this chromosome did not show distortion leading to the conclusion that this is due to selection acting on one part of this chromosome. The results obtained are discussed in relation to previously proposed models of mating and support the occurrence of meiosis to form haploid gametes that then fuse to form the diploid progeny in a single round of mating.

Sex and evolution in trypanosomes

Trypanosoma brucei is still the only kinetoplastid known to undergo genetic exchange, but it seems unreasonable to suppose that it evolved this process all by itself. The position of T. brucei on a molecular phylogenetic tree constructed from 18S ribosomal RNA gene sequences offers no clues to the likely existence of genetic exchange in trypanosome species other than the Salivaria, because this group of trypanosomes appears to have diverged from the rest a very long time ago. Antigenic variation is one characteristic shared by the Salivaria, which has been particularly well-studied in T. brucei. The large proportion of the genome devoted to variant antigen genes and related sequences in T. brucei, suggests a possible role for genetic exchange in enhancing the diversity of the repertoire. Alternatively, genetic exchange may counter potential excessive double-strand DNA damage brought about by the DNA rearrangements associated with antigenic variation. The remarkable biparental inheritance of organelle DNA (=kinetoplast DNA) in T. brucei is without precedent in other eukaryotes. The result of genetic exchange is to enhance the heterogeneity of the kinetoplast DNA minicircles.

Fish trypanosomes: their position in kinetoplastid phylogeny and variability as determined from 12S rRNA kinetoplast sequences

Fish trypanosomes have traditionally been classified according to the host species from which they were isolated, each isolate being regarded as a distinct species. To test the soundness of this practice, the genetic variabilities of the kinetoplast 12S rRNA-encoding genes of different fish trypanosomes isolates were compared. The DNAs were extracted from trypanosomes cloned from blood samples of 15 donors representing ten different fish species in four orders from waters of three major river systems of Central and Northern Europe. Comparison with other trypanosomatid sequences revealed that the fish trypanosomes form a monophyletic group with Trypanosoma brucei as a sister group. Pairwise comparisons of genetic distances yielded a wide range of continuous variation with no indication of any discontinuities attributable to barriers to gene flow. The genetic distances did not correlate with either the identity of the host species or geography. The host specificity of fish trypanosomes appears to be limited.

Genetic exchange in the trypanosomatidae

The only trypanosomatid so far proved to undergo genetic exchange is Trypanosoma brucei, for which hybrid production after co-transmission of different parental strains through the tsetse fly vector has been demonstrated experimentally. Analogous mating experiments have been attempted with other Trypanosoma and Leishmania species, so far without success. However, natural Leishmania hybrids, with a combination of the molecular characters of two sympatric species, have been described amongst both New and Old World isolates. Typical homozygotic and heterozygotic banding patterns for isoenzyme and deoxyribonucleic acid markers have also been demonstrated amongst naturally-occurring T. cruzi isolates. The mechanism of genetic exchange in T. brucei remains unclear, although it appears to be a true sexual process involving meiosis. However, no haploid stage has been observed, and intermediates in the process are still a matter for conjecture. The frequency of sex in trypanosomes in nature is also a matter for speculation and controversy, with conflicting results arising from population genetics analysis. Experimental findings for T. brucei are discussed in the first section of this review, together with laboratory evidence of genetic exchange in other species. The second section covers population genetics analysis of the large body of data from field isolates of Leishmania and Trypanosoma species. The final discussion attempts to put the evidence from experimental and population genetics into its biological context.

Intraclonal mating in Trypanosoma brucei is associated with out-crossing

To examine whether mating can occur within as well as between clones of Trypanosoma brucei, we transformed three T. brucei subspecies stocks with heterologous genes conferring resistance to either hygromycin or Geneticin and carried out a series of inter- and intraclone matings in all possible double drug combinations. Double drug-resistant hybrids were recovered from three of the six out-crosses, but not from any of the three intraclone matings. However, further analysis of cloned progeny trypanosomes from one of the out-crosses using RFLP markers, molecular karyotyping and RAPD (random amplification of polymorphic DNA) produced unequivocal evidence that intra- as well as interclone mating had occurred. The progeny of interclone mating were double drug-resistant and heterozygous at 9 of 13 loci examined. In contrast, the progeny of intraclone mating had no demonstrable input of genetic material from the hygromycin-resistant parent and were similar to the Geneticin-resistant parent for most markers, except for five loci which were heterozygous in the Geneticin-resistant parent but homozygous in these clones (aldolase THT1 glucose transporter, procyclin, tubulin and cDNA 23). In addition, PFGE showed considerable karyotypic rearrangements in these clones and loss of genetic material was evident from RAPD and VSG (variant surface glycoprotein) gene fingerprint analysis. We conclude that intraclone mating can occur in trypanosomes, but only during out-crossing, suggesting that meiosis and/or fusion are triggered by a diffusible factor.

A family of trypanosome cdc2-related protein kinases

The cyclin-dependent kinases, most notable of which is cdc2, are key regulators of the cell cycle, and are highly conserved in evolution. We have cloned and analysed three cdc2-related kinase-encoding genes (tbcrk1-3) from the protozoan parasite Trypanosoma brucei. tbcrk1 encodes a 34-kDa protein with 54% amino acid (aa) identity to the human cdc2, tbcrk2 a 39-kDa protein with 49% identity and tbcrk3 a 35-kDa protein with 54% identity. tbcrk1-3 have substitutions in the 16-aa sequence, the 'PSTAIRE' domain, that characterises the cdc2-related kinase family, to give PCTAIRE, PSTAVRE and PQTALRE motifs, respectively. The three kinases have conserved Tyr and Thr residues that are sites of phosphorylation in cdc2 and are important for regulating kinase activity. Southern blot analysis revealed that each tbcrk is a single copy gene. Pulse-field electrophoresis located the tbcrk genes to some of the largest of the trypanosome chromosomes at greater than 3 Mb. Western blots with anti-PSTAIRE polyclonal antibody detected proteins of 32, 43 and 65 kDa in all life-cycle stages and a 90-kDa protein in bloodstreams forms, implying the presence of a family of cdc2-related kinases. Trypanosomes have a remarkably large gene family of cdc2-related kinases for such a primitive organism. The crk genes may be involved in controlling aspects of the cell cycle which are linked to the differentiation of the parasite during its complex life cycle.

Genetic diversity and population structure of Trypanosoma brucei: clonality versus sexuality

Genomic fingerprinting by arbitrarily primed PCR was used to analyze the genetic variability among 59 Trypanosoma brucei stocks representing the three T. brucei subspecies isolated from various hosts and different countries in Africa. 14 oligonucleotide primers revealed 355 polymorphic binary characters which were used for phenetic and phylogenetic analysis and to perform recombination tests exploring the linkage disequilibrium in the sample. There was good concordance between arbitrarily primed PCR polymorphisms and isoenzyme data previously collected for many of the same strains [1]. However, the arbitrarily primed PCR typing was more discerning than multilocus enzyme electrophoresis typing. Phenetic and phylogenetic analysis using arbitrarily primed PCR markers did not confirm T. brucei brucei and T. brucei rhodesiense as separate subspecies, but T. brucei gambiense group I was monophyletic, confirming this group as suitable for the subspecies status. With this exception, there were no clear lineages among the sample, other than clustering of East African stocks and clustering of West African stocks. Some features of the phylogenetic analysis suggested that the population structure was not strictly clonal though recombination tests showed linkage disequilibrium, even in the absence of repeated genotypes. While genotypes appear stable enough for tracking in applied studies, sexuality will impact at the evolutionary time scale, and may be more frequent under some ecological conditions. The arbitrarily primed PCR approach should be an effective and simple approach to follow epidemics and to quantify the role of sexuality in T. brucei populations.

A successful backcross in Trypanosoma brucei

Genetic exchange can take place between different strains of Trypanosoma brucei ssp. when they are cotransmitted via the tsetse fly vector, but the mechanism and limits of compatibility between strains are ill-defined as yet. Following the recovery of several hybrid genotypes with single drug resistance from a cross of drug resistant parental strains, we attempted a series of backcrosses and F1 crosses, selecting hybrids by double drug resistance. Of 4 backcrosses, one produced hybrid progeny, the analysis of which is presented here, but none of the 4 F1 crosses produced hybrid progeny. However, among experimental flies from the 8 crosses, although there were large numbers of salivary gland infections, very few consisted of a mixture of parental clones, a prerequisite for mating. In the successful backcross both parents were diploid, but none of the crosses involving triploid clones produced hybrid progeny. The hybrid-secreting fly from the backcross contained a mixture of hybrid and parental clones. The hybrid clones had approximately 3n DNA contents relative to the 2n parental clones and fell into 2 groups with respect to restriction site polymorphisms in kinetoplast DNA maxi-circles. Fingerprinting by random PCR amplification using 8 different arbitrary primers showed minor variation between the hybrid clones.

Induction of resistance to melarsenoxide cysteamine (Mel Cy) in Trypanosoma brucei brucei

A population of Trypanosoma brucei brucei with reduced sensitivity to melarsenoxide cysteamine (Mel Cy) was produced in immunosuppresed mice using subcurative drug treatment. Melarsenoxide cysteamine resistance was stable after cyclical transmission through Glossina morsitans centralis. In vitro, the blood-stream forms showed 15-fold higher values for the minimal inhibitory concentration as compared with the parental clone. Cross-resistance could be determined with another arsenical drug, melarsoprol (14-fold) and to two different diamidines (diminazene aceturate: 47-fold; pentamidine methanesulphonate: 34-fold), but not to suramin. When cells were transformed to procyclic forms and tested in vitro, the sensitivity of the resistant population to melarsenoxide cysteamine was only 6-fold lower than that of the parent, but comparatively high cross-resistance could be shown to other drugs (melarsoprol; 85-fold; pentamidine methanesulphonate: 17-fold; quinapyramine sulphate: 40-fold). Selection of the resistant trypanosomes from non-resistant ones was possible under pentamidine methanesulphonate pressure in cell culture.