MOLECULAR SYSTEMATICS OF TOXOPLASMA GONDII AND RELATED COCCIDIA

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).

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