Trypanosome trees and homologies

A METHOD FOR ISOLATING AND SEQUENCING TRYPANOSOME CELLS TO INVESTIGATE SPECIES ASSOCIATIONS IN MULTIPLE MORPHOTYPE INFECTIONS

Trypanosome infections containing multiple morphologies have been described from all classes of vertebrates, including mammals, birds, non-avian reptiles, amphibians, and fish. These mixed infections make it challenging to evaluate trypanosome diversity, as it is not immediately clear whether the forms present in the bloodstream represent different species or a single pleomorphic species. Amphibians are common hosts for trypanosomes and are often infected by multiple trypanosome morphologies in the bloodstream. Based on morphological observations and life cycle studies, many authors have considered multiple trypanosome morphotypes found infecting the same frogs to be a single pleomorphic species. However, molecular evidence supporting pleomorphic trypanosome species in amphibians is lacking, primarily because linking sequence data to bloodstream trypanosome morphology in mixed infections is extremely challenging. Here we present methods to isolate individual trypanosome cells of 6 morphotypes from frog blood for nested PCR of the 18S rRNA and gGAPDH genes. Single trypanosome cells were isolated by dilution and 3 DNA extraction methods, and 5 nested PCR primer regimes were utilized to optimize amplification from very low starting concentrations. The success rates of extraction methods ranged from 29 to 50% with the use of a Direct PCR kit having the highest success rate. Although the success rate varied in the different combinations of extraction methods and primer regimes, multiple individuals of all 6 trypanosome morphotypes were sequenced for both genes in a novel way that links sequence data to cell morphology by observing isolated cells with a microscope before PCR amplification. All 6 morphologically distinguishable morphotypes coinfecting a frog were genetically distinct. The only other recent molecular study on amphibian trypanosomes also found genetic differences between morphotypes in multiple infections. Together these studies suggest that the occurrence of pleomorphism may be overestimated in amphibian trypanosomes. The methods presented here offer a promising solution to characterize trypanosome diversity within multiple morphotype infections.

Molecular taxonomy of trypanosomatids: some problems and pitfalls

Trypanosomids appear to have attracted the particular attention of taxonomists, and a wealth of data from studies using a variety of techniques are available. There are, however, some potential pitfalls in such studies. A general problem in the taxonomy of Trypanosomatids resides in that only a small amount of its true diversity is reflected in the reduced number of isolates identified and studied from this family. An associated problem is that of confusion over the identity of the organisms. Other concerns include the problems of long branch attractions and mutational saturation, the loss of phylogenetic signal from the accumulation of overlapping mutations, and the fact that gene phylogeny cannot be equated with organism phylogeny and that organisms are more than just the sum of their genes. Additional complications can occur due to numerous cases of horizontal gene transfer between organisms. The use of a large sample of recent isolates from the field is also important so that the true diversity of these organisms is reflected in these studies, and bias due to selection, contamination, and misidentification when isolates have been maintained for long periods in culture is eliminated.

Evolutionary rate differences in trypanosomes

Ribosomal RNA-based studies of trypanosome phylogenies have highlighted considerable differences in genetic diversity within clades in the genus Trypanosoma and several-fold substitution rate differences between clades have been identified. While early 18S rRNA-based studies were hampered by highly variable substitution rates and long-branch attraction, it is apparent that genuine differences in evolution rates within localized clades do exist and questions remain regarding what rate or rates such clades are evolving at and why is the application of a single clock to trypanosome evolution so inappropriate? In this study, we explore rate heterogeneity in the commonly used 18S rRNA gene across genus Trypanosoma, using a maximum likelihood (ML) approach to explore local rate variations in clades of biological interest.

Future trypanosomatid phylogenies: refined homologies, supertrees and networks

There has been good progress in inferring the evolutionary relationships within trypanosomes from DNA data as until relatively recently, many relationships have remained rather speculative. Ongoing molecular studies have provided data that have adequately shown Trypanosoma to be monophyletic and, rather surprisingly, that there are sharply contrasting levels of genetic variation within and between the major trypanosomatid groups. There are still, however, areas of research that could benefit from further development and resolution that broadly fall upon three questions. Are the current statements of evolutionary homology within ribosomal small sub-unit genes in need of refinement? Can the published phylograms be expanded upon to form 'supertrees' depicting further relationships? Does a bifurcating tree structure impose an untenable dogma upon trypanosomatid phylogeny where hybridisation or reticulate evolutionary steps have played a part? This article briefly addresses these three questions and, in so doing, hopes to stimulate further interest in the molecular evolution of the group.

Speculations on the origin and evolution of the genus Leishmania

Recently two hypotheses have been proposed for the evolution of Leishmania involving respectively a Neotropical or Paleartic origin for the species. Here an alternative proposal on the phylogeny of Leishmania based on the major divisions within the genus is presented. In this hypothesis a Neotropic origin is retained for L. (Viannia) and Paraleishmania, a recently described section within the genus Leishmania, while an African origin is proposed for L. (Leishmania) and possibly Sauroleishmania. The current distribution of Leishmania in the Neotropics is explained as the product of multiple introductions of Leishmania parasites into the New World. Problems with organismal identity in Sauroleishmania and the use of molecular sequence data in inferring phylogenies are also discussed.

Trypanosoma cruzi - the vector-parasite paradox

Trypanosoma cruzi and the majority of its insect vectors (Hemiptera, Reduviidae, Triatominae) are confined to the Americas. But while recent molecular studies indicate a relatively ancient origin for the parasite ( approximately 65 million years ago) there is increasing evidence that the blood-sucking triatomine vectors have evolved comparatively recently (<5 mya). This review examines the evidence for these ideas, and attempts to reconcile the apparent paradox by suggesting that marsupial opossums (Didelphidae) may have played a role, not just as original reservoir hosts, but also as original vectors of the parasite.

Analysis of genetic diversity of Trypanosoma cruzi: an application of riboprinting and gradient gel electrophoresis methods

Analysis of restriction fragment length polymorphism (RFLP) profiles derived from digestion of polymerase chain reaction (PCR) products of the ribosomal 18S from Trypanosoma cruzi yields a typical 'riboprint' profile that can vary intraspecifically. A selection of 21 stocks of T. cruzi and three outgroup taxa: T. rangeli, T. conorhini and Leishmania braziliensis were analysed by riboprinting to assess divergence within and between taxa. T. rangeli, T. conorhini and L. braziliensis could be easily differentiated from each other and from T. cruzi. Phenetic analysis of PCR-RFLP profiles indicated that, with one or two exceptions, stocks of T. cruzi could be broadly partitioned into two groups that formally corresponded to T. cruzi I and T. cruzi II respectively. To test if ribosomal 18S sequences were homogeneous within each taxon, gradient gel electrophoresis methods were employed utilising either chemical or temperature gradients. Upon interpretation of the melting profiles of riboprints and a section of the 18S independently amplified by PCR, there would appear to be at least two divergent 18S types present within T. cruzi. Heterogeneity within copies of the ribosomal 18S within a single genome has therefore been demonstrated and interestingly, this dimorphic arrangement was also present in the outgroup taxa. Presumably the ancestral duplicative event that led to the divergent 18S types preceded that of speciation within this group. These divergent 18S paralogues may have, or had, different functional pressures or rates of molecular evolution. Whether or not these divergent types are equally transcriptionally active throughout the life cycle, remain to be assessed.