Cystodiscus nigromaculatus sp. n. (Myxozoa: Myxidiidae) infecting the gallbladder of the frog Pelophylax nigromaculatus in China.

Myxosporean infections in frogs have been reported in countries worldwide. This study conducted a survey of myxosporean infection in cultured frogs in China, and identified a new species of Cystodiscus in Pelophylax nigromaculatus. This new species, named Cystodiscus nigromaculatus sp. n., was exclusively located in the gallbladder of P. nigromaculatus. The observed plasmodia appeared as oval formations within the bile, reaching sizes of up to 1.2 mm in diameter. Mature myxospores ellipsoid to ovoid in shape, measuring 13.6 ± 0.4 (13.0-14.6) μm in length and 7.8 ± 0.5 (6.8-8.9) μm in width, with a slightly curved ridge along the myxospore's medial axis. The polar capsules pyriform and uniform in size, measuring 4.3 ± 0.3 (3.7-4.9) μm long and 3.5 ± 0.2 (3.0-3.9) μm wide, with polar tubules coiled into 3-5 turns. The pairwise genetic distances and varying numbers of base differences among C. nigromaculatus sp. n. and other Cystodiscus spp. with available 18S rRNA gene sequences in GenBank ranged from 0.092/75 (C. axonis, MZ412745) to 0.111/89 (C. australis, HQ822149). Phylogenetic analysis placed C. nigromaculatus sp. n. as an early-diverging species within the Cystodiscus clade, with strong support values.