Approaches for characterising myxozoan species

Abstract

Myxozoan species and genera have been defined traditionally using morphological characteristics of spores and developmental stages, host preference and tissue specificity. Use of morphology is, however, limited in some taxa by a lack of reliable characters or ambiguities in their origin. For instance, morphological variation can characterise spores of closelyrelated species while similar spore morphologies in distantly-related species reflect convergence. Therefore, morphological traits are accompanied routinely by DNA sequences in the identification of myxozoan species. DNA sequencing methods have inherent limitations, too, which include co-amplification of host, inability to distinguish mixed infections and PCR and primer biases for some taxa over others. By combining several approaches, myxozoan researchers are revealing novel diversity and demonstrating that strains and morphologically cryptic species characterise many taxa. Extensive geographic sampling of economically significant myxozoans, including Myxobolus cerebralis, Kudoa thyrsites, Ceratonova shasta and Tetracapsuloides bryosalmonae, has demonstrated intraspecific genetic variation, strains linked with geographic localities, and species complexes. Application of new approaches to myxozoan classification is expected to facilitate reassessment of existing taxa, discovery of new taxa and further resolution of cryptic species.