Some current problems in the systematics of Trypanosomatids

The evolution of trypanosomatid taxonomy

Trypanosomatids are protozoan parasites of the class Kinetoplastida predominately restricted to invertebrate hosts (i.e. possess a monoxenous life-cycle). However, several genera are pathogenic to humans, animals and plants, and have an invertebrate vector that facilitates their transmission (i.e. possess a dixenous life-cycle). Phytomonas is one dixenous genus that includes several plant pathogens transmitted by phytophagous insects. Trypanosoma and Leishmania are dixenous genera that infect vertebrates, including humans, and are transmitted by hematophagous invertebrates. Traditionally, monoxenous trypanosomatids such as Leptomonas were distinguished from morphologically similar dixenous species based on their restriction to an invertebrate host. Nonetheless, this criterion is somewhat flawed as exemplified by Leptomonas seymouri which reportedly infects vertebrates opportunistically. Similarly, Novymonas and Zelonia are presumably monoxenous genera yet sit comfortably in the dixenous clade occupied by Leishmania. The isolation of Leishmania macropodum from a biting midge (Forcipomyia spp.) rather than a phlebotomine sand fly calls into question the exclusivity of the Leishmania-sand fly relationship, and its suitability for defining the Leishmania genus. It is now accepted that classic genus-defining characteristics based on parasite morphology and host range are insufficient to form the sole basis of trypanosomatid taxonomy as this has led to several instances of paraphyly. While improvements have been made, resolution of evolutionary relationships within the Trypanosomatidae is confounded by our incomplete knowledge of its true diversity. The known trypanosomatids probably represent a fraction of those that exist and isolation of new species will help resolve relationships in this group with greater accuracy. This review incites a dialogue on how our understanding of the relationships between certain trypanosomatids has shifted, and discusses new knowledge that informs the present taxonomy of these important parasites.

Morphology and growth characteristics of cultured Leptomonas ctenocephali from Ctenocephalides felis felis (Siphonaptera: Pulicidae) of dogs in Brazil

To confirm the taxonomic identification of a trypanosomatid found in the hindgut, rectum and Malpighian tubules of dog fleas captured in Belo Horizonte, Minas Gerais, Brazil, between April and November of 2005, 910 specimens of Ctenocephalides felis felis were removed from street dogs and dissected, and isolates from their digestive tracts were cultivated in NNN-alpha-MEM medium. Four different morphological forms were observed in culture: long, slender, twisted promastigotes with a long flagellum; short, stubby, non-twisted promastigotes; rounded amastigotes; and cyst-like bodies. Twisted and non-twisted promastigotes were frequently seen forming rosettes, and these two forms presented significant differences (P<0.01) in terms of their morphological characteristics. Unlike the promastigote forms observed throughout the culture period, rounded amastigotes were seen only in the lag phase, and the cyst-like bodies were only seen in the decline phase. The trypanosomatid DNA obtained from the culture was analyzed by the polymerase chain reaction (PCR) method and found to be negative for Leishmania infantum chagasi. Based on the growth pattern, morphological parameters and molecular analysis, the flagellates were confirmed to be Leptomonas ctenocephali. The significance of this infection for animals is also commented.

Herpetomonas trimorpha sp. nov. (Trypanosomatidae, Kinetoplastida), a parasite of the biting midge Culicoides truncorum (Ceratopogonidae, Diptera)

Monoxenous trypanosomatid Herpetomonas trimorpha sp. nov. was isolated from the digestive tract of the biting midge Culicoides truncorum (Ceratopogonidae, Diptera). This species forms three distinct morphotypes in culture: the microflagellate promastigote, the small promastigote and the long promastigote. The last form is unique for the newly described species. Phylogenetic analyses of SSU rRNA and glycosomal glyceraldehyde phosphate dehydrogenase genes showed that H. trimorpha sp. nov. is the closest relative of Herpetomonas ztiplika, another monoxenous trypanosomatid isolated from biting midges. However, morphological and randomly amplified polymorphic DNA analyses confirmed that H. trimorpha sp. nov. is distinct from H. ztiplika.

Discovery and barcoding by analysis of spliced leader RNA gene sequences of new isolates of Trypanosomatidae from Heteroptera in Costa Rica and Ecuador

Trypanosomatid diversity in Heteroptera was sampled using a culture-independent approach based on amplification and sequencing of Spliced Leader RNA gene repeats from environmental samples. By combining the data collected herein with that of previous work, the prevalence of parasites was found to be 22%-23%. Out of approximately 170 host species investigated nearly 60 were found to harbor trypanosomatids. The parasites found were grouped by cluster analysis into 48 typing units. Most of these were well separated from the known groups and, therefore, likely represent new trypanosomatid species. The sequences for each typing unit serve as barcodes to facilitate their recognition in the future. As the sampled host species represent a minor fraction of potential hosts, the entire trypanosomatid diversity is far greater than described thus far. Investigations of trypanosomatid diversity, host-specificity, and biogeography have become feasible using the approach described herein.

Sergeia podlipaevi gen. nov., sp. nov. (Trypanosomatidae, Kinetoplastida), a parasite of biting midges (Ceratopogonidae, Diptera)

Three strains of a trypanosomatid protozoan were isolated from the midguts of two naturally infected species of biting midges [Culicoides (Oecacta) festivipennis and Culicoides (Oecacta) truncorum] and characterized by light and electron microscopy and by molecular techniques. Morphological characteristics and sequences of the 18S rRNA, 5S rRNA, spliced leader RNA and glycosomal glyceraldehyde-3-phosphate dehydrogenase genes indicate that the studied flagellates represent a novel phylogenetic lineage within the Trypanosomatidae. Based on phylogenetic analyses, the novel endosymbiont-free, monoxenous trypanosomatid was classified as Sergeia podlipaevi gen. nov., sp. nov. Interestingly, it is closely related to another trypanosomatid species that parasitizes the sand fly Lutzomyia evansi, a blood-sucking dipteran from South America. The type strain of S. podlipaevi sp. nov., ICUL/CZ/2000/CER3, was obtained from Malpighian tubes. Of 2518 females of seven species of biting midges trapped in the Czech Republic, more than 1.5 % were infected by trypanosomatid parasites. An unrelated insect species, Culicoides (Monoculicoides) nubeculosus, was experimentally infected with S. podlipaevi, demonstrating that its host range extends to different subgenera of biting midges.

Patterns of co-evolution between trypanosomes and their hosts deduced from ribosomal RNA and protein-coding gene phylogenies

Trypanosomes (genus Trypanosoma) are widespread blood parasites of vertebrates, usually transmitted by arthropod or leech vectors. Most trypanosomes have lifecycles that alternate between a vertebrate host, where they exist in the bloodstream, and an invertebrate host, where they develop in the alimentary tract. This raises the question of whether one type of host has had greater influence on the evolution of the genus. Working from the generally accepted view that trypanosomes are monophyletic, here we examine relationships between trypanosomes using phylogenies based on the genes for the small subunit ribosomal RNA (SSU rRNA) and the glycosomal glyceraldehyde phosphate dehydrogenase (gGAPDH). New analysis of a combined dataset of both these genes provides strong support for many known clades of trypanosomes. It also resolves the deepest split within the genus between the Aquatic clade, which mainly contains trypanosomes of aquatic and amphibious vertebrates, and a clade of trypanosomes from terrestrial vertebrates. There is also strengthened support for two deep clades, one comprising a wide selection of mammalian trypanosomes and a tsetse fly-transmitted reptilian trypanosome, and the other combining two bird trypanosome subclades. Considering the vertebrate and invertebrate hosts of each clade, it is apparent that co-speciation played little role in trypanosome evolution. However most clades are associated with a type of vertebrate or invertebrate host, or both, indicating that 'host fitting' has been the principal mechanism for evolution of trypanosomes.

The ubiquitous gp63-like metalloprotease from lower trypanosomatids: in the search for a function

Plant and insect trypanosomatids constitute the "lower trypanosomatids", which have been used routinely as laboratory models for biochemical and molecular studies because they are easily cultured under axenic conditions, and they contain homologues of virulence factors from the classic human trypanosomatid pathogens. Among the molecular factors that contribute to Leishmania spp. virulence and pathogenesis, the major surface protease, alternatively called MSP, PSP, leishmanolysin, EC 3.4.24.36 and gp63, is the most abundant surface protein of Leishmania promastigotes. A myriad of functions have been described for the gp63 from Leishmania spp. when the metacyclic promastigote is inside the mammalian host. However, less is known about the functions performed by this molecule in the invertebrate vector. Intriguingly, gp63 is predominantly expressed in the insect stage of Leishmania, and in all insect and plant trypanosomatids examined so far. The gp63 homologues found in lower trypanosomatids seem to play essential roles in the nutrition as well as in the interaction with the insect epithelial cells. Since excellent reviews were produced in the last decade regarding the roles played by proteases in the vertebrate hosts, we focused in the recent developments in our understanding of the biochemistry and cell biology of gp63-like proteins in lower trypanosomatids.

Use of proteolytic enzymes as an additional tool for trypanosomatid identification

The expression of proteolytic activities in the Trypanosomatidae family was explored as a potential marker to discriminate between the morphologically indistinguishable flagellates isolated from insects and plants. We have comparatively analysed the proteolytic profiles of 19 monoxenous trypanosomatids (Herpetomonas anglusteri, H. samuelpessoai, H. mariadeanei, H. roitmani, H. muscarum ingenoplastis, H. muscarum muscarum, H. megaseliae, H. dendoderi, Herpetomoas sp., Crithidia oncopelti, C. deanei, C. acanthocephali, C. harmosa, C. fasciculata, C. guilhermei, C. luciliae, Blastocrithidia culicis, Leptomonas samueli and Lept. seymouri) and 4 heteroxenous flagellates (Phytomonas serpens, P. mcgheei, Trypanosoma cruzi and Leishmania amazonensis) by in situ detection of enzyme activities on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE ) containing co-polymerized gelatine as substrate, in association with specific proteinase inhibitors. All 23 trypanosomatids expressed at least 1 acidic proteolytic enzyme. In addition, a characteristic and specific pattern of cell-associated metallo and/or cysteine proteinases was observed, except for the similar profiles detected in 2 Herpetomonas (H. anglusteri and H. samuelpessoai) and 3 Crithidia (C. fasciculata, C. guilhermei and C. luciliae) species. However, these flagellates released distinct secretory proteinase profiles into the extracellular medium. These findings strongly suggest that the association of cellular and secretory proteinase pattern could represent a useful marker to help trypanosomatid identification.

Differential lectin recognition of glycoproteins in choanomastigote-shaped trypanosomatids: taxonomic implications

The glycoprotein profiles of seven choanomastigote-shaped trypanosomatids (six Crithidia spp. and one Herpetomonas sp.), which have been suggested to form three distinct taxonomic groups (Crithidia, Angomonas and Strigomonas), were analyzed by Western blotting using the lectins Limax flavus (LFA), Sambucus nigra (SNA) and Maackia amurensis (MAA), which specifically recognize sialic acid residues, and concanavalin A (Con A) that recognizes mannose-like residues in glycoconjugates. All lectins showed a sugar-inhibited recognition with the parasite extracts, with the exception of LFA, which did not show any reactivity with the studied species. The SNA agglutinin presented a characteristic and specific pattern for each taxonomic group. The MAA lectin showed an identical profile for all species analyzed, while Con A grouped the choanomastigote-shaped species in two different patterns, one specific for the Angomonas group, and the other comprehending both Strigomonas and Crithidia groups. These results illustrate the heterogeneity of the genus Crithidia. The possible taxonomic redistribution of the choanomastigote-shaped trypanosomatids is also discussed.