Genome and Phylogenetic Analyses of Trypanosoma evansi Reveal Extensive Similarity to T. brucei and Multiple Independent Origins for Dyskinetoplasty

Abstract

Two key biological features distinguish Trypanosoma evansi from the T. brucei group: independence from the tsetse fly as obligatory vector, and independence from the need for functional mitochondrial DNA (kinetoplast or kDNA). In an effort to better understand the molecular causes and consequences of these differences, we sequenced the genome of an akinetoplastic T. evansi strain from China and compared it to the T. b. brucei reference strain. The annotated T. evansi genome shows extensive similarity to the reference, with 94.9% of the predicted T. b. brucei coding sequences (CDS) having an ortholog in T. evansi, and 94.6% of the non-repetitive orthologs having a nucleotide identity of 95% or greater. Interestingly, several procyclin-associated genes (PAGs) were disrupted or not found in this T. evansi strain, suggesting a selective loss of function in the absence of the insect life-cycle stage. Surprisingly, orthologous sequences were found in T. evansi for all 978 nuclear CDS predicted to represent the mitochondrial proteome in T. brucei, although a small number of these may have lost functionality. Consistent with previous results, the F1FO-ATP synthase γ subunit was found to have an A281 deletion, which is involved in generation of a mitochondrial membrane potential in the absence of kDNA. Candidates for CDS that are absent from the reference genome were identified in supplementary de novo assemblies of T. evansi reads. Phylogenetic analyses show that the sequenced strain belongs to a dominant group of clonal T. evansi strains with worldwide distribution that also includes isolates classified as T. equiperdum. At least three other types of T. evansi or T. equiperdum have emerged independently. Overall, the elucidation of the T. evansi genome sequence reveals extensive similarity of T. brucei and supports the contention that T. evansi should be classified as a subspecies of T. brucei.

Sex is a ubiquitous, ancient, and inherent attribute of eukaryotic life

“…The well-documented meiotic event occurs only in the insect (11), making the subspecies asexual. Extensive analysis of a set of T. b. evansi strains showed their multiple emergences from T. brucei, in time frames not even sufficiently long to precipitate the loss of meiotic genes (65). In the same vein, many truly asexual metazoans and plants have been described, but most of them seem to be recently derived lineages, closely related to sexual species, as well.…”

“…This is exemplified by T. b. evansi surviving loss of mitochondrial (kinetoplast) DNA by skipping the tsetse fly vector, in which it is needed, altogether (compare its loss of sex; see above). Based on almost zero mutation rates of hundreds of unused genes encoding mitochondrial proteins (65) with proven lack of essentiality (81), it has been proposed that after relatively short periods of time, these lineages vanish, being replaced by new strains that, via disruption of kinetoplast DNA (and hence of sex), entered the path to their slow, yet inevitable demise (65,82).…”

Cancer in the parasitic protozoans Trypanosoma brucei and Toxoplasma gondii

“…Adapted from the National Cancer Institute (www.cancer.gov). Procyclin-associated genes (7 genes) Absence of PAG3 and mutations in others/absence of PAG3 and mutations in other genes in some isolates (38) Proteins associated with differentiation (PADs, 8 genes)…”

Spliced Leader (SL) RNA: análises de genes e regiões intergênicas com aportes na filogenia, taxonomia e genotipagem de <i>Trypanosoma</i> spp. de todas as classes de vertebrados.

“…Os tripanossomas africanos são reconhecidos por serem os agentes causais de doenças graves em humanos e animais de importância pecuária (Lai et al, 2008;Carnes et al, 2015).…”

“…Estas três espécies são geneticamente idênticas, diferindo principalmente pelo modo de transmissão e o fato de que T. evansi e T. equiperdum não possuem o DNA do cinetoplasto ou possuem apenas parte dele. Por essas razões, alguns autores sugerem que T. evansi e T. equiperdum derivam de T. brucei e que deveriam ser consideradas subespécies (Lai et al, 2008;Carnes et al, 2015).…”