https://www.thelancet.com/pdfs/journals/lancet/PIIS0140-6736(09)60829-1.pdf

Human African trypanosomiasis

We propose a general theory of clonal reproduction for parasitic protozoa, which has important medical and biological consequences. Many parasitic protozoa have been assumed to reproduce sexually, because of diploidy and occasional sexuality in the laboratory. However, a population genetic analysis of extensive data on biochemical polymorphisms indicates that the two fundamental consequences of sexual reproduction (i.e., segregation and recombination) are apparently rare or absent in natural populations of the parasitic protozoa. Moreover, the clones recorded appear to be stable over large geographical areas and long periods of time. A clonal population structure demands that the medical attributes of clones be separately characterized; ubiquitous clones call for priority characterization. Uniparental reproduction renders unsatisfactory Linnean taxonomy; this needs to be supplemented by the "natural clone" as an additional taxonomic unit, which is best defined by means of genetic markers.

/pdf/a-clonal-theory-of-parasitic-protozoa-the-population-1wtyzmucnb.pdf

A clonal theory of parasitic protozoa: the population structures of Entamoeba, Giardia, Leishmania, Naegleria, Plasmodium, Trichomonas, and Trypanosoma and their medical and taxonomical consequences

We argue that the mode of reproduction of microorganisms in nature can only be decided by population genetic information. The evidence available indicates that many parasitic protozoa and unicellular fungi have clonal rather than sexual population structures, which has major consequences for medical research and practice. Plasmodium falciparum, the agent of malaria, is a special case: the scarce evidence available is contradictory, some suggesting that uniparental lineages may exist in nature. This is puzzling (because P. falciparum is known to have a sexual stage) and poses a challenge that can be readily settled by ascertaining the frequency distribution of genotypes in natural populations.

/pdf/are-eukaryotic-microorganisms-clonal-or-sexual-a-population-i9imowjx6i.pdf

Are eukaryotic microorganisms clonal or sexual? A population genetics vantage.

Molecular basis for trypanosome antigenic variation.

Trypanosomes of the species Trypanosoma brucei reproduce primarily by binary fission, but the frequency of enzyme electrophoretic variants in natural populations of T. brucei has provided indirect evidence for the existence of a sexual cycle. These studies, coupled with studies of restriction fragment length polymorphisms of genes encoding glycolytic enzymes, have also provided evidence for T. brucei being diploid. Here we report direct evidence of gene exchange between two different clones of trypanosomes after mixed infection and full cyclical development in the tsetse fly vector.

Hybrid formation between African trypanosomes during cyclical transmission.

Sensitive detection of trypanosomes in tsetse flies by DNA amplification.

Publisher Summary The subgenus Trypanozoon contains several kinds of trypanosome, which are morphologically indistinguishable but differ in their behavior. At various times specific or subspecific status has been accorded to each kind, but, in recent years, those cyclically transmitted by tsetse in Africa have been separated into subspecies of Trypanosoma brucei , while the others, transmitted non-cyclically outside the African tsetse belts, have retained separate species status. However, although such new terminology may be more correct taxonomically, the practical problems of distinguishing the trypanosomes remain. Enzyme electrophoresis is being used on an increasingly wide scale for the intrinsic characterization. However, as the number of enzyme systems being used for identification grows, comparison of stocks becomes increasingly difficult and intricate, particularly when an enzyme occurs in a multibanded form after electrophoresis. After using 12 enzyme systems to characterize 160 Trypanozoon stocks, the results have been analyzed to establish the degree of relatedness of each of the 59 zymodemest encountered. The detailed analysis is presented in this chapter.

Numerical analysis of enzyme polymorphism: a new approach to the epidemiology and taxonomy of trypanosomes of the subgenus Trypanozoon.

The prevalence of various species and subgroups of trypanosomes in the Sinfra area of C⊚te d'Ivoire was determined using the polymerase chain reaction (PCR). Using this technique to amplify specific satellite DNA targets, it was possible to identify developmental-stage trypanosomes in the midguts and the proboscides of tsetse without expansion of parasite populations. The predominant tsetse species in the area was Glossina palpalis, while G. pallicera and G. nigrofusca were also present. Microscopical examination of 811 non-teneral flies revealed an infection rate of 14% in midguts and/or proboscides. Three subgroups of Trypanosoma congolense (Savannah, Forest & Kilifi), T. simiae, T.godfreyi, West African T. vivax and T. brucei ssp. were identified using PCR. T. congolense Forest was the most abundant of the Nannomonas trypanosomes. Approximately 40% of all infections were mixed, and there was a significantly higher prevalence of apparently mature T. brucei ssp. trypanosomes than has previously been reported. The present study demonstrates that PCR facilitates the easy identification of mature trypanosome infections in tsetse, providing a reliable estimation of trypanosomiasis challenge.

A high prevalence of mixed trypanosome infections in tsetse flies in Sinfra, Cote d'Ivoire, detected by DNA amplification

Trypanosome identification in wild tsetse populations in Tanzania using generic primers to amplify the ribosomal RNA ITS-1 region.

220 Trypanosoma (Trypanozoon) brucei sp. stocks isolated between 1969 and 1983 from the Lambwe valley sleeping sickness focus in South Nyanza, Western Kenya, were characterized by isoenzyme electrophoresis using 12 enzymes. 12 different zymodemes of T. (T.) b. rhodesiense were isolated from patients during the 13-year period and identical stocks were also found in cattle, reedbuck (Redunca redunca) and tsetse (Glossina pallidipes). Cattle may have played an important role in maintaining and increasing peridomestic transmission of trypanosomes during the 1980 outbreak of sleeping sickness in the valley, even though they themselves suffered heavy mortality. Sleeping sickness in Lambwe valley is unlikely to have been introduced from elsewhere, since T. (T.) b. rhodesiense stocks isolated from the valley were different from those from neighbouring epidemic areas. Alternatively, the recent outbreak may have been caused by the increased transmission associated with an expanding tsetse population. The possibility that genetic exchange contributed to the biochemical diversity of the trypanosomes examined is discussed.

Wendy C. Gibson

Papers

Sensitive detection of trypanosomes in tsetse flies by DNA amplification.

Numerical analysis of enzyme polymorphism: a new approach to the epidemiology and taxonomy of trypanosomes of the subgenus Trypanozoon.

A high prevalence of mixed trypanosome infections in tsetse flies in Sinfra, Cote d'Ivoire, detected by DNA amplification

Trypanosome identification in wild tsetse populations in Tanzania using generic primers to amplify the ribosomal RNA ITS-1 region.

Characterization of Trypanozoon stocks from the South Nyanza sleeping sickness focus in Western Kenya.