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Molecular Phylogenetics and Evolution
68, No. 1, 2013; Pages: 93 - 105

Sphaerospora sensu stricto: Taxonomy, diversity and evolution of a unique lineage of myxosporeans (Myxozoa)

Pavla Bartošová, Ivan Fiala, Miloslav Jirku, Martina Cinková, Monica Caffara, Maria Letizia Fioravanti, Stephen Douglas Atkinson, Jerri Lee Bartholomew, Astrid Sibylle Holzer

Institute of Parasitology, Biology Centre of the Academy of Sciences of the Czech Republic, Ceské Budejovice, Czech Republic.


Myxosporeans (Myxozoa) are eukaryotic parasites, primarily of fish, whose classification is in a state of flux as taxonomists attempt to synthesize the traditional morphology-based system with emerging DNA sequence-based phylogenies. The genus Sphaerospora Thélohan, 1892, which includes pathogenic species that cause significant impacts on fisheries and aquaculture, is one of the most polyphyletic taxa and exemplifies the current challenges facing myxozoan taxonomists. The type species, Selegans, clusters within the Sphaerospora sensu stricto clade, members of which share similar tissue tropism and long insertions in their variable rRNA gene regions. However, other morphologically similar sphaerosporids lie in different branches of myxozoan phylogenetic trees. Herein, we significantly extend taxonomic sampling of sphaerosporids with SSU + LSU rDNA and EF-2 sequence data for 12 taxa including three representatives of the morphologically similar genus Polysporoplasma Sitjà-Bobadilla et Álvarez-Pellitero, 1995. These taxa were sampled from different vertebrate host groups, biogeographic realms and environments. Our phylogenetic analyses and statistical tests of single and concatenated datasets revealed Sphaerospora ss. as a strongly supported monophyletic lineage, that clustered sister to the whole myxosporean clade (freshwater + marine lineages). Generally, Sphaerospora ss. rDNA sequences (up to 3.7 kb) are the longest of all myxozoans and indeed metazoans. The sphaerosporid clade has two lineages, which have specific morphological, biological and sequence traits. Lineage A taxa (marine Sphaerospora spp.) have a single binucleate sporoplasm and shorter AT-rich rDNA inserts. Lineage B taxa (freshwater/brackish Sphaerosporaspp. + marine/brackish Polysporoplasma spp.) have 2–12 uninucleate sporoplasms and longer GC-rich rDNA inserts. Lineage B has four subclades that correlate with host group and habitat; all Polysporoplasmaspecies, including the type species, cluster together in one of these subclades. We thus suppress the genusPolysporoplasma and the family Polysporoplasmidae and emend the generic diagnosis of the genusSphaerospora. The combination of morphological, biological and DNA sequence data applied in this study helped to elucidate an important part of the taxonomic puzzle within the phylum Myxozoa.


Keywords: Genus emendment; Suppression of Polysporoplasma; Phylogeny; Sporoplasmic organization; rDNA inserts; GC content



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