Abstract
Diseases caused by trypanosomatids (Sleeping sickness, Chagas disease, and leishmaniasis) are a serious public health concern in low-income endemic countries. These diseases are produced by single-celled parasites with a diploid genome (although aneuploidy is frequent) organized in pairs of non-condensable chromosomes. To explain the way they reproduce through the analysis of natural populations, the theory of strict clonal propagation of these microorganisms was taken as a rule at the beginning of the studies, since it partially justified their genomic stability. However, numerous experimental works provide evidence of sexual reproduction, thus explaining certain naturally occurring events that link the number of meiosis per mitosis and the frequency of mating. Recent techniques have demonstrated genetic exchange between individuals of the same species under laboratory conditions, as well as the expression of meiosis specific genes. The current debate focuses on the frequency of genomic recombination events and its impact on the natural parasite population structure. This paper reviews the results and techniques used to demonstrate the existence of sex in trypanosomatids, the inheritance of kinetoplast DNA (maxi- and minicircles), the impact of genetic exchange in these parasites, and how it can contribute to the phenotypic diversity of natural populations.
Highlights
A number of observations based on population genetics supported the strict clonal theory: (i) all populations studied showed a strong imbalance in allele frequencies, regardless the stage of life cycle considered; (ii) all populations showed almost total absence of recombinant genotypes; (iii) an over-representation of dominant genotypes distributed over wide geographical areas was observed, and (iv) a clear association between unrelated genetic markers was detected [10]
When Lachaud and co-workers analyzed in 2014 the ploidy of three Old World Leishmania spp. (L. infantum, L. donovani, and L. tropica), and one from the New World (L. amazonensis) by fluorescent in situ hybridization (FISH), they found an aneuploid mosaic for six different chromosomes, which pointed out this feature as a general characteristic of the genus Leishmania [71]
The population would have reproduced by binary fission, occasional recombination events would have taken place, albeit with a low frequency. These results show that behind the epidemic episodes of leishmaniasis in a specific geographical focus, there can be genetic exchange phenomena between populations or even between different Leishmania species [77]
Summary
First evidence provided by restriction site polymorphisms, gene silencing, gene recombination, and karyotype alterations, suggested that the predominant state of Leishmania was the diploidy [66]. (L. infantum, L. donovani, and L. tropica), and one from the New World (L. amazonensis) by fluorescent in situ hybridization (FISH), they found an aneuploid mosaic for six different chromosomes, which pointed out this feature as a general characteristic of the genus Leishmania [71] This does not seem to be just restricted to this trypanosomatid, since the existence of widespread aneuploidy has been reported in the T. cruzi genome [71]. The first empirical demonstration that Leishmania was able to undergo sexual reproduction resulting from interspecific and intraspecific crossings was presented by Akopyants and co-workers in 2009 These authors crossed two parental lines of L. major: LV39c5 (HYG), heterozygous for an allelic replacement of the LPG5A on chromosome 24 by a hygromycin B resistance cassette, and the modified strain of L. major Friedlin V1, FV1 (SAT), the latter containing a heterozygous nourseothricin resistance (SAT) marker integrated along with a linked firefly luciferase (LUC) reporter gene into one allele of the 24S ribosomal RNA locus of chromosome 27. These vacuoles can harbor several amastigotes of these species, unlike what it is observed with Old World species, where each vacuole harbors one amastigote, preventing physical contact between parasites [86]
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