Molecular approaches for inferring evolutionary relationships among protistan parasites
Introduction
With the advent of molecular methods and, in particular, sequence data, the suite of characters available for inferring relationships among protistan parasites of vertebrates has expanded greatly (Sogin and Silberman, 1998). These molecular characters have been used to augment classification schemes that were originally erected based on morphological and life history criteria. In some cases, the phylogenetic hypotheses have been in serious conflict with taxonomic systems erected using other means. Examples include the recognition of Blastocystis species as stramenopiles (instead of flagellates, amoebae, yeasts or sporozoa—see Silberman et al., 1996), the determination that Pneumocystis carinii was actually related to higher fungi (Edman et al., 1988); and, recognition of myxozoan parasites as cnidarian metazoa rather than protists (Siddall et al., 1995). Although conflicts such as these disturb the stability of well-accepted taxonomic schemes, the recognition that the taxonomic scheme (or the molecular analyses) may be flawed is often central to understanding the biology of a group of parasites. The purpose of this paper is not to review the methods used in molecular systematics; there is a rich and rapidly expanding primary literature in this field and many excellent texts (e.g. Hillis et al., 1996, Graur and Li, 2000) that describe methodologies and debate the relative predictive power of these methods. Instead, the intent is to highlight the use of molecular approaches for inferring the phylogenetic relationship among protistan taxa with special reference to parasites of veterinary relevance.
For the purposes of illustration, I will outline a number of examples using apicomplexan and myxozoan parasites to highlight the utility of molecular methods in erecting evolutionary hypotheses for protistan parasites. These examples will demonstrate the central role that molecular systematics has in inferring relationships among these protistan parasites and, importantly, in supporting biological investigations into the life history, cell and molecular biology, diagnosis, and treatment of these agents.
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Background and rationale
The goal of a robust taxonomic framework in which all protistan parasites can be housed remains an elusive goal but one which is becoming more attainable and practical with the advent of molecular investigations (Sogin and Silberman, 1998). The last quarter century has seen a dramatic movement from sole reliance on morphological characters (light and electron microscopic observations principally) as well as life history characteristics and host range for inferring relationships among protistan
The apicomplexan parasites
Ultrastructural features fairly readily identified an important group of parasites as distinct within the “Sporozoa”, the spore-forming protozoa. The phylum Apicomplexa (Levine, 1970) was erected to accommodate parasites with prototypical apical structures. However, the relationships between members of the phylum Apicomplexa and other protistan taxa were difficult to establish based on morphological characters. Sequence data from the 18S rRNA genes suggested that apicomplexan parasites were
Myxozoan parasites—life cycles and metazoan affinities
The demonstration that the myxosporean parasites infecting fish were alternate life cycle stages for actinosporean parasites (Wolf and Markiw, 1984) was greeted with considerable debate in the literature. These important parasites of fish and shellfish have been extremely difficult to study because they could not be propagated in the laboratory setting. Even more startling was the suggestion that the myxosporean parasites may actually not be protists at all.
In both cases, the use of molecular
Summary
Molecular characters have become an essential adjunct to existing morphological and biological characters for use in the inference of relationships among protistan taxa. The use of appropriate (and preferably multiple) genes for phylogenetic study has permitted the reliable inference of the evolutionary relationships that exist among extant protistan parasites. The generation of a robust taxonomic framework for the protists, including obligately parasitic, facultatively parasitic and
Future directions
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There is an understandable over-representation of parasitic protists of medical and veterinary importance for which sequence data are currently available. Molecular systematics studies benefit procedurally from both depth and breadth of taxonomic richness. Many questions, particularly regarding closely related parasitic protists, may not be resolved without additional intermediary taxa. A much broader range of taxa, especially free-living relatives of parasitic protists and parasites of
Acknowledgements
This research program has benefited from ongoing support of the Natural Sciences and Engineering Research Council of Canada.
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