| Myxosporea belongs to the phylum Cnidaria,subphylum Myxozoa,and is a type of hazardous metazoan parasite that mainly infects fishes and are widely distributed across the world.This animal is microscopic in terms of their body size,and simple in structure with a complicated life cycle;it has,therefore,been generally recognized as a difficult group to study internationally.Although the previous researches have revealed that myxosporean is origin from multicellular animal and has close relationship with cnidarian,there are still many taxonomy problems for certain groups of this animal that need to be resolved,e.g.,the systematic relationship among the myxidiid myxosporean(mainly refers to the Myxidiidae Thélohan,1892 and Sphaeromyxidae Lom & Noble,1984)has always been a concern of scholars as well as a difficult point in taxonomy,viz.the independence of the genera Myxidium Butschli,1882、Zschokkella Auerbach,1910 in the family Myxidiidae,and the taxonomy and origin of the morphologically similar genus of Myxidium,namely Sphaeromyxa(the only genus of the family Sphaeromyxidae Lom & Noble,1984)and so on.In the present study,the myxidiid myxosporean infecting marine and freshwater fishes in China were investigated.By combining morphology,molecular biology and ecology,and so on,we studied the systematic classification of myxidiid myxosporean.On the basis of adding new resources of myxidiid myxosporean,we summarized the species list of myxidiid myxosporean in China.The main research results and conclusions are as follows.1.New resources of myxidiid myxosporeanTwelve new species and new record for three species were obtained from the Yangtze River,Jialing River,Huai River,and Moshui River systems,the Yellow Sea,the East China Sea and the South China Sea,viz.Myxidium spinibarba sp.n.,Myxidium asiaticum sp.n.,Myxidium callionyma sp.n.,Myxidium karalla sp.n.,Zschokkella chongqingensis sp.n.,Sphaeromyxa photopectoralis sp.n.,Sphaeromyxa sebastisca sp.n.,Sphaeromyxa pseudorhomba sp.n.,Parasphaeromyxa xiamenensis sp.n.,Parasphaeromyxa evynnises sp.n.,Parasphaeromyxa chaeturichthyses sp.n.,Parasphaeromyxa chirolophises sp.n..New geographical records of Zschokkella parasiluri Fujita,1927,Myxidium cuneiforme Fujita,1924,and Zschokkella nova Klokacéwa,1914.Zschokkella.nova Klokacéwa,1914 was transformed to Myxidium as Myxidium nova comb.n.A new genus Parasphaeromyxa gen.n.was established according to the difference of spore morphology between ―balbianii‖ and ―incurvata‖sphaeromyxids.P.xiamenensis sp.n.,P.evynnises sp.n.,P.chaeturichthyses sp.n.,P.chirolophises sp.n.,and 26 known species previously classified in the Sphaeromyxa were transferred to Parasphaeromyxa gen.n.Parasphaeromyxa incurvata(Doflein,1898)comb.n.was considered as the type-species of this genus.2.Species list and revision on the myxidiid myxosporean in ChinaRevisions on the synonyms of myxidiid myxosporeans recorded in early China has been completed according to the International Code of Zoological Nomenclature.The most up-to-date and complete list of myxidiid myxosporean in China was summarized including 122 nominal species.Among them,five previously classified in Myxidium species were assigned to Zschokkella,and three previously identified as Zschokkella species were assigned to Myxidium.3.Molecular systematics of the myxidiid myxosporeanThe relation between the habitat of the myxidiid myxosporean and their molecular lineage: The phylogenetic analysis based on 18 S rDNA showed that myxidiid myxosporean mainly separated into two clades abiding by the host‘s habitat(freshwater/marine),and the structures of H17 and H37 of the 18 S rRNA secondary structures for the two clades were significantly different.In particular,the species of Myxidium and Zschokkella were also split into two different clades mainly depending on the host‘s habitat rather than the genus,and the structures of H17 and H37 between the two clades showed obviously difference.These results indicated that marine and freshwater myxidiid myxosporean have undergone different evolutionary pathways,and these results supported that a new genus should be established to accommodate the marine Myxidium-Zschokkella species,which are significantly different from freshwater Myxidium-Zschokkella species in their secondary structure of H17 and H37.The sphaeromyxids(Sphaeromyxidae),whose hosts are mostly marine fishes,were closely related to freshwater Myxidium-Zschokkella species,but far related to marine Myxidium-Zschokkella species and other species of genera in Myxidiidae.In addition,the structures of H17 and H37 of the 18 S rRNA secondary structures in sphaeromyxids were consistent with those of freshwater Myxidium-Zschokkella species,and inconsistent with those of other species mentioned above,indicating that the sphaeromyxids have the same origin with freshwater Myxidium-Zschokkella species which further supported the hypothesis that the sphaeromyxids originate from a freshwater ancestral specie,and the results does not supported the the delimitation of the family Sphaeromyxidae.The relation between the morphology of the myxidiid myxosporean and their molecular lineage: In the internal branches of the phylogenetic tree,most species still clustered preferentially on the basis of the spore morphology,indicating that there is a certain genetic similarity among the myxidiid myxosporean with similar morphology.The Parasphaeromyxa gen.n.and Sphaeromyxa were separated into two clades,respectively,which is consistent with their morphological differentiation,and the two genera also possessed obvious differences in the structures of H17,E23＿15,and H24 of the 18 S rRNA,showing that there are significant differences both in the morphology and genes between the two genera.These results further confirm the validity of the new genus Parasphaeromyxa gen.n.Prasphaeromyxa limocapitis(Barto?vá-Sojková et al.,2015)comb.n.,with ―limocapitis‖ type spore,has similar structures of H17,E23＿15,and H24 of the 18 S rRNA with the species of Prasphaeromyxa gen.n.,these results do not support the division of ―limocapitis‖ sphaeromyxids.The phylogenetic analysis also showed that the two clades formed by the marine and freshwater habitats of myxidiid myxosporean were closely related to the presence or absence of valvular striations.And the H17 and H37 structures of myxidiid myxosporean with valvular striations were significantly different from those of these species without the striations.The present results suggested that the species with the valvular striations and without the valvular striations evolved independently,respectively.The feature mapping analysis of the morphology and ecology showed that the ellipsoidal spores,oval polar capsules,and short tubular polar filaments are probably the ancestral morphology of these groups,and the host gallbladder might be the original infected site of this group.The myxidiid myxosporean in the marine and freshwater might have evolved from different ancestral spore types.To sum up,a total of 15 myxidiid myxosporeans reported from the present study provide new data to research the taxonomy,phylogeny and evolutionary relationship of this group.The species list of this group has enriched the database data of China.The phylogenic study of Myxidium,Zschokkella,Sphaeromyxa and Parasphaeromyxa gen.n.support the validity of the genus Parasphaeromyxa gen.n.and the hypothesis of freshwater origin of sphaeromyxids.The evolutionary history of myxidiid myxosporean has been studied,and the hypothesis of different evolutionary paths for marine and freshwater species of this group is put forward,which provides an important scientific evidence for further taxonomic study of this group. |
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