MOLECULAR SYSTEMATICS OF TOXOPLASMA GONDII AND RELATED COCCIDIA:
EXPANDING THE RANGE OF DISTINGUISING CHARACTERS FOR TAXONOMY, SYSTEMATICS AND
DIAGNOSTICS
Within the group of apicomplexan parasites broadly recognized as the
coccidian, there is a limited range of morphological characters that can be used
to distinguish species and even fewer characters that one can use to infer
evolutionary relationships. For this reason, molecular data, most notably
sequences associated with the ribosomal RNA genes, have been used increasingly
to infer relationships among species and strains. The use of small subunit
ribosomal DNA sequences clearly demonstrated some well-supported clades within
the coccidian: 1) the eimeriid coccidian including Eimeria and Cyclospora
species as well as some avian Isopora species (those possessing Stieda bodies
and refractile bodies); 2) the isosporoid coccidian including species in the
genera Toxoplasma, Neospora, Frenkelia, Hammondia and Saracocystis as well as
some Isospora species (those lacking Stieda bodies and refractile bodies).
Within the latter clade, there appear to be two sub-clades that are comprised of
1) Sarcocystis and Frenkelia species; and 2)Toxoplasma, Neospora and Isospora
species. Recent observations suggest that Cryptosporidium species, long
considered to be “unusual” coccidian, are only distantly related to
the coccidian and may be most closely related to the gregarines that infect
invertebrates. Molecular systematics have been instrumental in expanding the
range of characters available for inferring relationships among apicomplexan
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 immuological or molecular diagnostic methods (e.g. use of
the closest relatives to assess test specificity).