Histopathological evaluation of the infection elicited by three species of Myxobolus (Cnidaria: Myxosporea), with identification of Myxobolusxiaoganensis n. sp. isolated from the silver carp Hypophthalmichthys molitrix.

Several parasites infecting the Asian carp species have been broadly spread along with the global fish trade. However, the diversity of specific parasite groups and their pattern of parasitism remain insufficiently elucidated even in their native regions. Here, we conducted a holistic identification and histological analysis of three Myxobolus species. The oblate myxospores of the first isolates were found infecting the spleen of silver carp Hypophthalmichthys molitrix, measuring 6.4 ± 0.4 (5.4-7.1) µm in length, 8.2 ± 0.4 (7.5-9.0) in width, and 5.9 ± 0.3 (5.2-6.2) in thickness. They were morphologically distinct from other congeners and regarded as a novel species, Myxobolus xiaoganensis n. sp. For the second isolates, we associated the formation of round plasmodia on the gill raker of silver carp with Myxobolus allotypica Chen, 1998. The third isolates, encapsulated in the intestinal serosa membrane of the grass carp Ctenopharyngodon idella, was proved to be conspecific to Myxobolus huasaensis Chen, 1998. While M. xiaoganensis n. sp. and M. huasaensis exhibited distinct origins, with genetic differences exceeding 4% from other congeners, M. allotypica displayed complete genetic congruence with an item available in Genbank. Histologically, myxospores of M. xiaoganensis n. sp. were scattered in the spleen, and the branchial and intestinal infections of M. allotypica and M. huasaensis were determined, respectively. Phylogenetic analysis demonstrated a non-monophyletic origin of both the Thelohanellus and Myxobolus genera, with a remarkable association of host affinity with myxosporean clustering.