Extreme inbreeding in Leishmania braziliensis

Abstract

Leishmania species of the subgenus Viannia and especially Leishmania braziliensis are responsible for a large proportion of New World leishmaniasis cases. The reproductive mode of Leishmania species has often been assumed to be predominantly clonal, but remains unsettled. We have investigated the genetic polymorphism at 12 microsatellite loci on 124 human strains of Leishmania braziliensis from 2 countries, Peru and Bolivia. There is substantial genetic diversity, with an average of 12.4 +/- 4.4 alleles per locus. There is linkage disequilibrium at a genome-wide scale, as well as a substantial heterozygote deficit (more than 50% the expected value from Hardy-Weinberg equilibrium), which indicates high levels of inbreeding. These observations are inconsistent with a strictly clonal model of reproduction, which implies excess heterozygosity. Moreover, there is large genetic heterogeneity between populations within countries (Wahlund effect), which evinces a strong population structure at a microgeographic scale. Our findings are compatible with the existence of population foci at a microgeographic scale, where clonality alternates with sexuality of an endogamic nature, with possible occasional recombination events between individuals of different genotypes. These findings provide key clues on the ecology and transmission patterns of Leishmania parasites.

Fig. 1.

F_IS and 95% confidence intervals obtained by bootstrap over loci, for the 124 human Leishmania braziliensis strains collected in 2 different countries in 2 different years. F_IS measures the local deficiency of heterozygous genotypes due to nonrandom mating. There is a large heterozygote deficiency in each population, shown because F_IS is significantly greater than zero.

Fig. 2.

F_IS for each of 12 microsatellite loci in the 4 populations (and their mean) of Leishmania braziliensis collected in Peru and Bolivia. There is a large heterozygote deficiency at each locus.

Fig. 3.

F_IS for Leishmania braziliensis strains in each population and within their subdivisions as identified by BAPS. The decrease of F_IS in the subdivision suggests a Wahlund effect. However, the residual F_IS values are still high, which suggests the persistence of nonrandom mating (due, for example, to selfing).