**1.1 General taxonomic aspects and life cycle of** *Trypanosoma cruzi*

Kinetoplastida is a class of unicellular eukaryotes characterized by the presence of the kinetoplast, a feature formed by circular concatenated DNA molecules (kDNA) inside a solitary mitochondrion [10]. Trypanosomatida is an order of kinetoplastids composed of obligatory parasites of invertebrates, plants, and all classes of vertebrates [11, 12]. This order includes genera that are exclusive arthropod parasites, such as *Crithidia* [13, 14] and *Herpetomonas* [11], and also genera with heteroxenous life cycles (involving more than one host), such as *Leishmania* and *Trypanosoma*. These two genera are of great medical importance as they contain species that are etiological agents for serious human diseases, such as sleeping sickness, Chagas' disease, and

*Exploring the Evolutionary Origin and Biological Role of the* Trypanosoma cruzi *Ecotin-Like… DOI: http://dx.doi.org/10.5772/intechopen.109929*

visceral and cutaneous leishmaniasis. Some Trypanosomatida genera have obligate endosymbiont bacteria, which have been used as model organisms in studies on unicellular symbiosis [15, 16].

Species in the *Trypanosoma* genus are heteroxenous and usually have complex life cycles [17]. In trypanosomatids belonging to the *Stercoraria* section, trypanosomes are transmitted to vertebrate hosts by hemipteran insects in the Reduviidae family [18], such as *Trypanosoma cruzi*, the etiological agent of Chagas' disease. Motile metacyclic trypomastigote infective form of *T. cruzi* penetrates the skin or skin lesion of the vertebrate host after being expelled with the feces of the insect. Once inside the vertebrate host, the infective metacyclic trypomastigotes transform into bloodstream trypomastigotes, which invade cells of various tissues and, differentiate into amastigotes, static forms that multiply inside the cells by binary fission. After a number of division cycles, *T. cruzi* amastigotes transform into bloodstream trypomastigotes and are released into the circulatory system, infecting other cells in the body. When a triatomine insect vector feeds on an infected vertebrate with the motile trypomastigotes present in the blood, the ingested parasite differentiates into epimastigote forms that multiply by binary fission and migrate to the posterior intestine of the insect where it transforms to vertebrate infective metacyclic trypomastigotes, completing the parasite cycle. In members of the *Salivaria* section of *Trypanosoma* genera, such as *Trypanosoma brucei*, the metacyclic trypomastigote is injected directly from the salivary gland of the insect host, flies from *Glossina* genus, into the bloodstream of the vertebrate host [19, 20]. *T brucei* trypomastigotes can directly multiply by binary fission in the vertebrate blood. When a hematophagous insect vector feeds on the blood of an infected vertebrate it consumes bloodstream trypomastigotes forms, which differentiate inside the insect into procyclic trypomastigotes and then into epimastigotes, which are capable of multiplying by binary fission. The cycle is closed with the transformation of epimastigote forms into metacyclic trypomastigote forms in the salivary gland of the insect fly that infects a new vertebrate host during blood feeding [18].

The heteroxenous life cycle in trypanosomatids may have an evolutionary history beginning before the start of the tertiary period, but the overall Kinetoplastida phylogeny is still filled with uncertainty, even more so among trypanosomatids [21]. The existence of bacterial endosymbionts in the group is of marked interest to researchers, possibly being related to the transition from free to parasitic life cycles or being involved in lateral gene transfers between bacteria and eukaryotes [22]. Trypanosomatids have polycistronic DNA transcription which tends to keep coding sequences conserved in contiguous groups, resulting in multiple gene loci being preserved between different species [23].
