Molecular approaches for inferring evolutionary relationships among protistan parasites

Abstract

Within the diverse group of parasites broadly recognized as protists, there are limited morphological characters that can be used to distinguish species and even fewer characters that can infer evolutionary relationships among species. For this reason, molecular data are commonly used to infer relationships among species and strains. These studies most commonly rely on sequences associated with the ribosomal RNA genes but increasingly other nuclear, mitochondrial or plastid genes are contributing data. Molecular systematics has been invaluable in expanding the range of characters that are available for inferring relationships among protistan taxa. As an adjunct to morphological characters, sequence data allow us to better understand the evolutionary history of this group of parasites and thereby erect a well-supported taxonomic framework that reflects these historical relationships. Importantly, the predictive nature of such a framework can aid the search for therapeutic compounds (e.g. via shared biochemical pathways) and highlight organisms that should be tested for cross-reactivity in immunological or molecular diagnostic methods (e.g. use of the closest relatives to assess test specificity). For emergent and poorly known parasites, molecular characterization and placement within the broader phylogenetic framework can help predict likely life history traits, including possible or likely definitive hosts.

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.