Abstract
Evolutionary theory predicts that the lack of recombination and chromosomal re-assortment in strictly asexual organisms results in homologous chromosomes irreversibly accumulating mutations and thus evolving independently of each other, a phenomenon termed the Meselson effect. We apply a population genomics approach to examine this effect in an important human pathogen, Trypanosoma brucei gambiense. We determine that T.b. gambiense is evolving strictly asexually and is derived from a single progenitor, which emerged within the last 10,000 years. We demonstrate the Meselson effect for the first time at the genome-wide level in any organism and show large regions of loss of heterozygosity, which we hypothesise to be a short-term compensatory mechanism for counteracting deleterious mutations. Our study sheds new light on the genomic and evolutionary consequences of strict asexuality, which this pathogen uses as it exploits a new biological niche, the human population.
Highlights
Obligate asexual reproduction has been argued to carry considerable negative evolutionary consequences (Maynard Smith, 1986) most clonal species undergo some degree of recombination, albeit infrequent, which generates novel genotypes (Heitman, 2006)
T.b. gambiense Group 1 showed a 5–10 fold lower number of single nucleotide polymorphisms (SNPs) (11,398) and SNP density compared to the other groups (Figure 1—source data 1), despite an over-representation in terms of the number of samples
Phylogenetic network analysis revealed that T.b. gambiense Group 1 genotypes showed an extremely low level of intra-group diversity and formed a monophyletic group (Figure 1A)
Summary
Obligate asexual reproduction has been argued to carry considerable negative evolutionary consequences (Maynard Smith, 1986) most clonal species undergo some degree of recombination, albeit infrequent, which generates novel genotypes (Heitman, 2006). We investigate the reproductive strategy of a putatively asexual yet successful human pathogen, Trypanosoma brucei gambiense Group 1, the main causative agent of human African trypanosomiasis, which contrasts with closely-related, sexually reproducing sub-species. T.b. gambiense has been divided into two groups
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