Mitochondrial genomes of human helminths and their use as markers in population genetics and phylogeny.

Molecular Parasitology Unit, Australian Centre for International and Tropical Health and Nutrition, The Queensland Institute of Medical Research and The University of Queensland, 300 Herston Road, Qld 4029, Brisbane, Australia.
Acta Tropica (Impact Factor: 2.52). 01/2001; 77(3):243-56. DOI: 10.1016/S0001-706X(00)00157-1
Source: PubMed

ABSTRACT To date, over 100 complete metazoan mitochondrial (mt) genomes of different phyla have been reported. Here, we briefly summarise mt gene organisation in the Metazoa and review what is known of the mt genomes of nematodes and flatworms parasitic in humans. The availability of complete or almost complete mtDNA sequences for several parasitic helminths provides a rich source of genetic markers for phylogenetic analysis and study of genetic variability in helminth groups. Examples of the application of mtDNA in studies on Ascaris, Onchocerca, Schistosoma, Fasciola, Paragonimus, Echinostoma, Echinococcus and Taenia are described.

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    ABSTRACT: Stafford in 1905 established the genus Glypthelmins to include Distomum quietum, species collected and described by himself in 1900 as a parasite of anurans from Canada. Since this time, about 36 species parasitizing apodes and anurans have been described around the world, most of them from American continent. Nowadays, there is not a general agreement about the number of species included in Glypthelmins, because some of them have been included in genera such as: Margeana, Haplometrana, Choledocystus, Microderma, Repandum, Rauschiella, Reynoldstrema e Hylotrema. Different taxonomic studies have considered some of these genera as synonyms of Glypthelmins or as valid genera. Nevertheless, the taxonomic classification has never been based on a phylogenetic hypothesis and in consequence the monophyly of the group has not been demonstrated. In order to determinate the species richness of Glypthelmins in Mexico, from December, 1996 to September, 2002, 33 species of anurans from 50 localities distributed in 13 states of Mexican Republic were collected. A total of 1060 host were studied, and nine species of Glypthelmins and two morphotypes were obtained. Of the 33 species of anurans studied only 13 were parasitized by one or more species of Glypthelmins. The different species of Glypthelmins are distributed in 20 of the 50 studied localities. This study increases the host range and geographic distribution of the genus, because 13 new host records and 25 new locality records were additioned. Three new species are described: Glypthelmins poncedeleoni, parasitizing Leptodactylus melanonotus from Tabasco and Guerrero; G. brownorumae, parasitizing Rana brownorum from Tabasco, and G. tuxtlasensis, parasite of R. vaillanti from Veracruz, Mexico. The last two species were described by using a combination of molecular data from mtDNA (gen cox1), rDNA genes 5.8S y 28S, and ITS2), and morphological characters. To probe if the genus Glypthelmins is a monophyletic group and to define the species composition, a phylogenetic analysis based on the principles of phylogenetic systematics was conducted. A database including 45 morphological characters with 112 characters states was assembled. Database included 35 taxa, 30 of them belonging to the ingroup, and the remaining five were used as outgroups. The phylogenetic analysis using a heuristic search under the maximum parsimony criteria produced one single most parsimonious tree (L= 1587, CI=0.40, RI= 0.76). The results demonstrated that Glypthelmins not conform a monophyletic assemblage, because the tree is constituted by three main groups, two of them conformed by Choledocystus and Rauschiella. Glypthelmins can be defined as a monophyletic group only if it include: G. quieta, G. californiensis, G. parva, G. intestinalis, G. shastai, G. facioi, G. pennsylvaniensis, G. hyloreus, G. brownorumae y G. tuxtlasensis. This species conform a group supported by a metraterm running dorsally to the cirrus sac. In this study, it is proposed the re-establishment the genera Choledocystus and Rauschiella, and it is considered that these are represented by eight and 12 species, respectively. The genus Choledocystus is supported by having a ratio between oral sucker-acetabulum equal or higher to 0.9 times. The species included in Rauschiella share an inequivocal synapomorphy, vitelline follicles are grouped in lateral fields along the body. With the aim of clarifying the systematic position of Glypthelmins within the order Plagiorchiida, partial sequences of the 28S ribosomal gene of 11 species of Glypthelmins were obtained. The fragment of the ribosomal gene 28S for 11 species of Glypthelmins was of 1270 bp. Analyzing the obtained sequences of some species of Glypthelmins along with other sequences of 54 species of digeneans included in Plagiorchiida, we obtained four most parsimonious trees generated through 1000 random taxa addition heuristic searches with Tree Bisection-Reconnection (TBR) branch swapping options, using a heuristic search. All the threes showed that the 11 species of Glypthelmins do not constitute a monophyletic assemblage, since Glypthelmins hepatica, G. tineri, and G. poncedeleoni appear closely related to other genera of digeneans. The results of the molecular analysis agree with those of the morphological analysis. Both analyses demonstrated that G. hepatica belongs to Choledocystus, while G. tineri and G. poncedeleoni are members of Rauschiella. With the information we obtained in this study, we propose that the genera Glypthelmins, Choledocystus and Rauschiella should be considered valid taxonomically. Genera Margeana, Haplometrana and Hylotrema are considered synonyms of Glypthelmins. Additionally, the genus Repandum is synonymy of Rauschiella, and Reynoldstrema is not considered valid. Following the results of the morphological and molecular studies we define that only five species of Glypthelmins are distributed in anurans from Mexico. These species are parasites of members of the “R. pipiens” and “R. palmipes” complex of both the nearctic and neotropical biogeographical regions. The remaining species, anteriorly considered members of Glypthelmins, should be placed in Choledocystus (C. hepatica), and in Rauschiella (R. tineri, R. linguatula and R. poncedeleoni).
    06/2004, Degree: Ph. D., Supervisor: Gerardo Pérez