Redescription and phylogenetic analysis of Myxobolus tricostatus (Myxozoa: Myxobolidae)

Myxobolus tricostatus was found initially to infect the gills of grass carp in the late 1970s with simple and insufficient taxonomic data, so the taxonomic validity was problematic. In order to supplement description and verify the validity of M. tricostatus, we redescribed M. tricostatus collected from Lake Tangxun in Wuhan city, Hubei Province, using the integrative taxonomic approach of combining myxospore morphology, molecular data and ecological information of host and infection site, which has been widely accepted for the taxonomy of Myxozoa. Here, the plasmodia, tissue tropism and molecular data of M. tricostatus are first provided. The morphological features of the concerned species are as follows: whitish plasmodia infecting the gills measured (2.4±0.3) (2.1-2.7) mm in length and (0.8±0.1) (0.6-0.9) mm in width; mature spores encapsulated by a transparent mucous envelope were ovoid in frontal view and fusiform in sutural view, measuring (10.3±0.4) (9.4-11.0) μm long, (9.5±0.5) (8.7-10.9) μm wide, and (7.4±0.5) (6.4-8.0) μm thick; no "V"-shaped folds were observed on the posterior end of the spore; three parallel ridges could be observed in sutural view; two equal polar capsules were pyriform and coiled with 7-8 turns of polar filament. Histological observation showed that the plasmodia dwelled between the gill filaments, and no serious inflammation was found in the surrounding tissues. BLAST search indicated that M. tricostatus was most similar to M. elegans with the similarity of 89.58%, which was definitely below the intraspecies similarity range. Phylogenetic analysis revealed that M. tricostatus branched with M. elegans from the gills of Leuciscus idus and Rutilus rutilus with high support values within the clade of Cyprinidae gill-infecting Myxobolus spp. including M. enoblei, M. minutus and M. bibullatus from the gills of Catostomidae fish. The present phylogenetic results further confirmed that host affinity and tissue tropism provide strong evolutionary signal for tissue-infecting Myxobolidae. In conclusion, we provided here supplemeemtary morphological, histological and molecular data of M. tricostatus, and verified the validity of its classification.