Minisatellite marker analysis of Trypanosoma brucei : Reconciliation of clonal, panmictic, and epidemic population genetic structures

Abstract

The African trypanosome, Trypanosoma brucei, has been shown to undergo genetic exchange in the laboratory, but controversy exists as to the role of genetic exchange in natural populations. Much of the analysis to date has been derived from isoenzyme or randomly amplified polymorphic DNA data with parasite material from a range of hosts and geographical locations. These markers fail to distinguish between the human infective (T. b. rhodesiense) and nonhuman infective (T. b. brucei) ''subspecies'' so that parasites derived from hosts other than humans potentially contain both subspecies. To overcome some of the inherent problems with the use of such markers and diverse populations, we have analyzed a well-defined population from a discrete geographical location (Busoga, Uganda) using three recently described minisatellite markers. The parasites were primarily isolated from humans and cattle with the latter isolates further characterized by their ability to resist lysis by human serum (equivalent to human infectivity). The minisatellite markers show high levels of polymorphism, and from the data obtained we conclude that T. b. rhodesiense is genetically isolated from T. b. brucei and can be unambiguously identified by its multilocus genotype. Analysis of the genotype frequencies in the separated T. b. brucei and T. b. rhodesiense populations shows the former has an epidemic population structure whereas the latter is clonal. This finding suggests that the strong linkage disequilibrium observed in previous analyses, where human and nonhuman infective trypanosomes were not distinguished, results from the treatment of two genetically isolated populations as a single population.

Susceptibility of <i>Simulium damnosum</i> complex larvae to temephos in the Tukuyu onchocerciasis focus, southwest Tanzania

“…This division tallies with previous clinical and molecular characterization studies indicating the existence of northern and southern strains of T. b. rhodesiense (Hide & Tilley, 2001;Ormerod, 1967;Gibson et al, 1980;Macleod et al, 2000). In this study a prominent band of a 3.9kb was amplified from Tanzanian isolates of T. b. rhodesiense genomic DNA.…”

The genetic structure of Borrelia afzelii varies with geographic but not ecological sampling scale

“…Neisseria gonorrhoeae), whereas others are largely clonal (e.g. Salmonella and Trypanosoma) (Maynard Smith et al, 1993;MacLeod et al, 2000;Feil et al, 2001;Dykhuizen & Kalia, 2008). The rate of recombination influences the probability that beneficial mutations will spread in a population (Hill & Robertson, 1966;Otto & Barton, 1997).…”

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

“…Analysis of T.b.gambiense isolates using a range of different markers has lead to the definition of two discrete groups, termed Type 1 and 2 [2]. While there is considerable controversy about the existence of genetic exchange between different strains within each subspecies in the field [3][4][5][6][7], there is unequivocal evidence for genetic exchange when two stocks of the parasite are used to infect the tsetse fly vector in the laboratory [8][9][10][11] As no chromosome condensation has been observed in any life cycle stage and no gamete-like stages identified [12], the main approach to determining whether this parasite has a sexual cycle and undergoes meiosis has been to undertake classical genetic analysis.…”