The trematode genera Paragonimus and Schistosoma in East Asia: molecular evolution, phylogeny and biogeography

van Herwerden, Lynne (1998) The trematode genera Paragonimus and Schistosoma in East Asia: molecular evolution, phylogeny and biogeography. PhD thesis, James Cook University.

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Trematode infections affect one fifth of the worlds' population, with more than 200 million Chinese alone being at risk (World Health Organisation 1997, Hotez et at 1997). Of particular interest to this study are the Asian species of the trematode genera Paragonimus and Schistosoma, which infect many mammals, including humans. It is becoming clear that within both genera species complexes exist that require further characterisation. Molecular markers have given some insight into the nature of these complexes. In this thesis additional molecular markers are investigated as sources of information about the nature and distribution of the S. japonicum, P. westermani and P. ohirai species complexes. These two genera are of additional interest because they are thought to have shared a similar biogeographic history in Asia, which should be reflected in their molecular evolution.

I have obtained DNA sequences from one nuclear (ITS1) and one mitochondrial (ND1) marker of five Paragonimus species (P. westermani, P. miyazakii, P. macrorchis, P. ohirai and P. iloktsuenensis) present in East Asia, including numerous geographic isolates of the P. westermani and P. ohirai species complexes. In addition, I have obtained ITS1 sequences of three schistosome species (S. japonicum, S malayensis and S. mekongi) present in East Asia, which comprise the S. japonicum species complex and an African schistosome species (S. mansoni) for comparative purposes. I could differentiate all species using ITS1 and ND1 sequence data.

ITS1 is contained within the ribosomal DNA gene cluster, which is repeated in tandem many times. ITS1 does not experience the constraints of coding regions and theoretically undergoes sequence homogenisation within individuals and species. Despite this, intra-individual sequence variation was sometimes greater than inter-species variation, particularly among northeast Asian P. westermani isolates. Such variation confounded phylogenetic inferences for this group. There are some interesting aspects relating to processes of concerted evolution generally, when considering the differences within and between the two trematode genera, Paragonimus and Schistosoma, in this study. Additionally, a transcriptional enhancer motif (TATAAT) is embedded within the repeats of the ITS1 of the Asian schistosomes. The repeats are the cause of size variation and given the improbability of such motifs occurring by chance, I propose that the abundant, large variants containing multiple copies of repeats may have a role in the stage- or tissue-specific regulation of transcription of the ribosomal genes.

In contrast, the ND1 gene codes for a mitochondrial protein and is therefore functionally constrained. Phylogenetic inferences could be made from ND1 sequence data obtained from Paragonimus. However, many clones had to be sequenced per individual to achieve this, as multiple ND1 lineages occurred within individuals of all Paragonimus species and strains investigated. Presumed pseudogenes were identified, which may be present in either the nuclear genome or in different types of mitochondria, as Paragonimus species have two types of mitochondria which differ structurally. It seems likely that both nuclear pseudogenes and heteroplasmic mitochondrial genes occur, though this hypothesis remains to be tested.

The Paragonimus westermani species complex has been further studied, to determine the origins of parthenogenetic triploid forms. Ribosomal DNA-ITS restriction fragments, ND1 sequences and simple sequence repeat (SSRs) "fingerprints" of triploids from China, Korea and Japan all indicate that the triploids are genetically different from one another. I conclude from this that triploids may have arisen more than once independently, possibly by relatively rare coatings between diploid and tetraploid P. westermani individuals, which occur in sympatry with the triploids in NE China. Alternatively, triploid lineages may have arisen once and diverged subsequently by mutation alone.

Despite their suggested shared biogeographic history, members of the two genera exhibit rather different properties with respect to their ND1 and ITS1 genes. This is likely to be a reflection of their different phylogenetic histories. Molecular techniques have been used successfully to infer phylogenies which could be used to evaluate a previously published biogeographic hypothesis, but only to a limited extent, using the markers. It is clear that the situation for P. westermani in particular is complex and requires further investigation. Molecular evolution of these markers has proved to be an interesting component of the study that has brought to light some novel ideas and applications of these markers, not as phylogenetic tools, but as tools to study processes of concerted evolution in the nuclear genome and mitochondrial evolution.

Item ID: 24131
Item Type: Thesis (PhD)
Keywords: East Asia; molecular genetics; Paragonimus; phylogeny; Schistosoma
Date Deposited: 12 Dec 2012 02:17
FoR Codes: 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060309 Phylogeny and Comparative Analysis @ 100%
SEO Codes: 96 ENVIRONMENT > 9604 Control of Pests, Diseases and Exotic Species > 960405 Control of Pests, Diseases and Exotic Species at Regional or Larger Scales @ 100%
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