Myxobolus xiushanensis n. sp. (Myxozoa: Myxobolidae) infecting the gill arches of Yellowhead Catfish Tachysurus fulvidraco from China.

Abstract

Myxosporidiosis of bagrid fishes has been a focus of aquaculture research in recent years. The purpose of this study is to characterize a novel myxobolid, named Myxobolus xiushanensis n. sp., infecting Yellowhead Catfish Tachysurus fulvidraco in China. We used molecular biology, morphology, phylogeny, and histopathology in the present study. Mature myxospores were circular to ellipsoidal in valve view, measuring 12.2 ± 0.4 μm (mean ± SD; range = 11.2-13.2 μm) in length and 10.6 ± 0.4 μm (9.5-11.1 μm) in width. Two oval polar capsules were equal in width (3.4 ± 0.2 μm; 3.0-3.8 μm) but slightly unequal in length: 5.6 ± 0.3 μm (5.3-6.1 μm) and 4.7 ± 0.2 μm (4.4-5.5 μm). The polar capsule was packed with five to seven spirals of polar tubules. Histopathological investigation demonstrated that the plasmodium under the cuticular layer of the gill arch only induced a local inflammatory response and did not cause serious damage to the gill arch's internal structure. The two small subunit (SSU) ribosomal DNA sequences of M. xiushanensis n. sp. showed 100% similarity and uniqueness, and the highest similarity with other myxosporean sequences in GenBank was 90.27% (query coverage = 94%). The secondary structures of the SSU ribosomal RNA revealed that the present species was distinctly different from related species in regions V4 and V7. Phylogenetic analysis showed that M. xiushanensis n. sp. clustered independently within a branch. These results enrich our understanding of the biodiversity of myxobolids infecting bagrid fishes and provide fundamental data for the diagnosis of myxosporidiosis.