The resolution of the taxonomic dilemma of Myxobolus colossomatis and description of two novel myxosporeans species of Colossoma macropomum from Amazon basin.

This study presents morphologic, molecular and phylogenetic data about two new species of the genus Myxobolus and of the previously described Myxobolus colossomatis, all which are found infecting the Colossoma macropomum, a fish whose natural habitat is the Amazon Basin of Brazil, from where the specimens for this study were caught. A total of 51 C. macropomum specimens were examined between October of 2014 and January of 2016. Plasmodia of the myxosporeans were found infecting several organs: Myxobolus matosi n. sp. and Myxobolus longissimus n. sp. were respectively found in the inner face of the operculum and in the wall external surface of the stomach and gill arch. M. matosi n. sp. were 9.6 ± 0.4 μm in length, 7.0 ± 0.3 μm in width and 5.0 ± 0.3 μm in thickness of the myxospore. M. longissimus n. sp. measured 19.1 ± 0.4 μm in length, 9.4 ± 0.3 μm in width and 8.3 ± 0.4 μm in thickness. The polar capsules, which were elongated, showed 4.3 ± 0.4 μm in length and 1.9 ± 0.1 μm in width for M. matosi n. sp. and 10.5 ± 0.2 μm in length and 2.5 ± 0.1 μm in width for M. longissimus n. sp. The Myxobolus colossomatis had two myxospore morphotypes: 1) Ellipsoidal myxospores measuring 11.6 ± 0.4 μm in length and 7.6 ± 0.2 μm in width. Their elongated polar capsules measured 5.6 ± 0.2 μm in length and 2.5 ± 0.2 μm in width; 2) Oval myxospores measuring 10.4 ± 0.5 μm in length and 7.7 ± 0.3 μm in width. Their polar capsules were 5.4 ± 0.2 μm in length and 2.4 ± 0.0 μm in width. The number of turns of the polar filament was 7-8 coils. The molecular comparison of the small subunit ribosomal DNA (ssrDNA) showed a genetic divergence of 10.3% between M. matosi n. sp. and M. colossomatis, 22.4% between M. matosi n. sp. and M. longissimus n. sp., and 23.2% between M. longissimus n. sp. and M. colossomatis. Myxobolus cf. colossomatis, a parasite of Piaractus mesopotamicus, showed 11.1% of genetic divergence to M. colossomatis, demonstrating them to be distinct species. Phylogenetic analysis, based on sequences of the ssrDNA, showed the M. matosi n. sp. to be a sister species of M. colossomatis, and it also showed M. longissimus n. sp. to be a sister branch in the lineage composed by Myxobolus cf. cuneus and Henneguya pellucida.