Morphological and molecular characterization of Myxobolus sheyangensis n. sp. (Myxosporea: Myxobolidae) with intralamellar sporulation in allogynogenetic gibel carp, Carassius auratus gibelio (Bloch) in China.

Allogynogenetic gibel carp is one of the most important freshwater cultured species in China. However, myxosporidiosis represents a severe threat to prevent the sustainable development of aquaculture of this species. During the investigation of myxosporean diversity of reared allogynogenetic gibel carp in East China, a new myxosporean with typical characteristics of Myxobolus was found from 169 out of 210 (80.4 %) examined samples, designated as Myxobolus sheyangensis n. sp. by combing comparative analysis of morphological and molecular data. The diagnostic features of this species are reminiscent with Myxobolus pyramidis Chen, 1958 as follows: round or ellipsoidal, grayish white, cyst-like polysporous plasmodia, averaging 219.3 ± 11.9 (98.7-421.7) × 158.4 ± 9.7 μm (79.9-191.8) in size; spores flat-pear shaped in frontal view with tapering anterior and rounded posterior ends and lemon-shaped in sutural view, averaging 11.0 ± 0.31 (10.5-11.9) × 10.2 ± 0.25 (9.2-10.7) × 6.3 ± 0.23 μm (5.9-6.9) in size; and two equal pyriform polar capsules averaging 5.5 ± 0.3 (4.5-6.1) × 3.4 ± 0.26 μm (2.9-4.0) in size with convergent longitudinal axes and polar filaments wounded in seven to eight coils, perpendicularly to the longitudinal axis of the polar capsules. Spore surface was smooth and two spore valves were symmetrical, with a thin and straight sutural ridge. Occasionally, abnormal spores with typical Henneguya-like caudal appendage and atypical papillary caudal appendage could be observed. The sporogenesis is asynchronous. Histopathological investigation showed that the plasmodia developed inside the capillary network of gill lamellae, belonging to the intralamellar vascular type, and no significant inflammatory responses were provoked by the infection. Homology search by BLAST showed that the newly obtained sequence did not match any available sequences in GenBank. Phylogenetic analysis of the aligned sequences indicated that M. sheyangensis n. sp. positioned in a clade composed of Myxobolus species infecting the gill of several freshwater cyprinid fish.