title: Kinetoplast DNA dynamics in trypanosoma species: the impact of life cycle variations and reproduction strategies
Creators: name: Chen, Zihao
Publisher: name: The University of Edinburgh
Year: 2025
Subjects: subject: trypanosoma
subject: kinetoplast
subject: minicircles
subject: maxicircles
subject: RNA editing
subject: guide RNA

description: Trypanosomatids are unicellular, flagellated obligatory protozoa parasites. Many dixenous trypanosomatids, such as trypanosome parasites in the genus Trypanosoma, cause diseases in humans and livestock. Human diseases due to trypanosome parasites mainly occur in developing or undeveloped countries, including Chagas in South and Central America (Trypanosoma cruzi), chronic Human African Trypanosomiasis (HAT) in Central and West Africa (Trypanosoma. brucei. gambiense type 1), and acute HAT in East Africa (Trypanosoma brucei rhodesiense). Meanwhile, Trypanosoma brucei brucei, Trypanosoma congolense, Trypanosoma brucei equiperdum, and Trypanosoma evansi afflict animals and cause Animal African Trypanosomiasis (AAT), nagana, dourine, and surra respectively. The single mitochondrion of trypanosomatids contains a massive genome, the kinetoplast. Within an individual parasite, the kinetoplast DNA (kDNA) forms a chainmail-like network with two types of catenated DNA molecules: 20 to 50 copies of essentially identical maxicircles and thousands of highly heterogeneous minicircles. Maxicircle encodes ribosomal and electron transport chain subunits. The pre-mRNAs of 12 genes require post-transcriptional editing directed by short “guide RNAs” (gRNAs) encoded on minicircles. To produce translatable mRNAs, trypanosomatids must cover all editing sites with at least one gRNA. In species with extensive editing such as T. brucei, the kDNA network contains a highly diverse population of minicircles and encodes hundreds of distinct gRNAs. The lifecycle of dixenous trypanosomatids involves insect vectors and mammalian hosts. During clonal reproduction, imperfect replication and segregation of kDNA may cause some minicircles encoding essential genes to drift towards a dangerously low abundance. In trypanosome parasites, sexual reproduction occurs exclusively in the insect vectors and results in mixing of the mitochondrial genome in the progeny. Circulating minicircles among tsetse-transmissible isolates, sexual reproduction potentially rescues low-abundance gRNA genes in the progeny by replenishing it with copies from the other parent. In addition, the different metabolisms at the mammalian and insect stages entail a different set of essential maxicircle genes and a lower demand for editing capacity in the mammalian stage. Consequently, deviations from the typical lifecycle can present a unique challenge in maintaining the kDNA integrity. We propose that sexual reproduction is key in combating genetic diversity loss in kDNA. Conversely, the absence of sexual recombination reduces kDNA complexity. Using next-generation sequencing data, we have assembled and examined the kDNA from trypanosome species and subspecies with different life histories. Also transmitted by tsetse, the human pathogen T. b. gambiense type 1 reproduces strictly clonally. The clonal T. b. equiperdum and T. b. evansi no longer rely on tsetse but are transmitted directly between mammals. The dixenous T. congolense is colocalized with T. brucei and reproduces sexually in the proboscis of tsetse flies. We report a highly conserved minicircle population characteristic of T. b. gambiense type 1 in 117 isolates. Comparing T. b. gambiense type 1 to the sexual T. brucei subspecies, we observed substantial kDNA streamlining in the asexual isolates with evidence of approaching tsetse transmissibility loss. We confirm that in three groups of asexual kDNA-independent T. b. evansi and T. b. equiperdum, the minicircle genomes consist of thousands of a single minicircle class specific to each group. A putatively kDNA-independent ecotype, T. b. equiperdum group OVI retains moderate kDNA complexity and can probably produce fully edited mRNAs of A6 and RPS12, the only edited maxicircle genes required in mammalian bloodstream. Comparison between T. brucei and T. congolense gRNA annotation revealed highly conserved editing blocks that cover the edited mRNAs with minimal overlaps. The results shed light on the evolution of the editing cascade.