Taxonomic Studies On Four Myxosporean Species Infecting Yellow Catfish

Myxozoans are metazoan,microscopic,spore-forming endoparasites of aquatic vertebrates and invertebrates.About more than 2500 species have been described,most of them are not highly pathogenic for their hosts,some cause severe disease and economic losses in aquaculture around the world.In Chinese aquaculture,yellow catfish Tachysurus fulvidraco Richardson,1846 is considered an important species because of high commercial value/demand.The farming of yellow catfish often suffers from outbreaks of myxozoans infection that cause morbidity and low market value/demand.Many aquatic parasites systematicians defined several myxozoans species from this commercially significant host by the traditionally acceptable morphology-based taxonomic system.Recently,this traditional taxonomic system is in a state of flux given new insights provided by the expanding dataset of biological traits(host/organ/tissue specificity)and molecular sequences(small subunit ribosomal DNA,SSUr DNA/internal transcribed spacer,ITS/large subunit,LSU).Thus,the present study initiated a project in 2015,focusing on the myxozoans in yellow catfish from 4 provinces of China.The present study,reported the prevalence of myxozoans from yellow catfish(10.2%)65/632,yielded 4 myxosporean taxa belong to genus Myxobolus.To defined novel taxa and available morphological units,data are accompanied by biological traits(host preference/organ affected/tissue specificity)and molecular data(mainly SSU rDNA).At beginning of this project,a species of Myxobolus was collected from the fins of yellow catfish,which was consistent in spore morphology and host/organ specificity with Chinese records of Myxobolus physophilus Reuss,1906.However,these earlier records and our own findings are inconsistent with the original description of M.physophilus from Russia.Specifically,there are differences in spore morphology(shape,intracapsular appendix,and polar capsule size),the infection site(air bladder vs.fins),and the host affinity(common rudd vs.yellow catfish).The inconsistencies allow us to conclude that both the present Myxobolus species and Chinese records of M.physophilus are distinct from the original description of M.physophilus and represent a new Myxobolus species,which we named Myxobolus xiantaoensis n.sp.Subsequently,the M.xiantaoensis was collected from four different localities,exhibit some morphological variations under the light microscope.The ultrastructural examination based on scanning and transmission electron microscopy revealed morphological features of the parasite external surface,asynchronous spore development,and host-parasite interface.The phylogenetic analyses based on ITS(ITS-1,5.8S.ITS-2)and LSU data were performed by means of Bayesian inference and maximum likelihood methods.The ITS-based phylogeographic results,though differing somewhat among ITS-1,5.8S,and ITS-2,provide inconsistent support values for most clades.The pathogenesis and histochemical cell response due to M.xiantaoensis in the affected fins was assessed by cartilage-specific histopathological examination.In the early stages of infection,biopsies are characterized by damage of surface collagen of fin cartilage resultant to weakness,breaks,and missing fin cartilage.The closure period of this project reported three Myxobolus spp.from the skin,gill arch,and kidney of the yellow catfish.To favors a more comprehensive characterization strategy these three species were also experimented by modem-taxonomic-system(combining spore morphology,biological traits,and sequence data).These three species were identified as Myxobolus pseudowuli encysted in the skin,Myxobolus voremkhai encysted in the gill arch,and a novel Myxobolus species encysted in the kidney.The spores of M.voremkhai were found consistent in spore morphology and biological traits with previous Chinese records of M.voremkhai.However,these earlier records and our own findings are inconsistent with the original description of Myxobolus voremkhai Akhmerov,1960.Specifically,there are differences in spore morphology and biological traits(host and organ).These inconsistencies allow us to conclude that both the present Myxobolus species and Chinese record of M.voremkhai are distinct from the original description of and represent a new Myxobolus species.Collectively,this work highlights the expediency of morphological features in conjunction with biological traits and sequence analysis in myxozoans definition.Generally,this scheme must be applied in a systematic way,primarily to distinguish between morphologically defined sister species or cryptic species of myxozoans.In the present study,it allowed a comprehensive integrative taxonomic-revision of M.physophilus and Myxobolus voremkhai recorded from the cartilages(fin and gill)of yellow catfish in Chinese records.In previous morphology-based taxonomic reports,besides yellow catfish,M.physophilus and Myxobolus voremkhai were also reported to affect the multi-infection-sites of the widely distributed multi-fish-hosts.Here,we provide the molecular sequence data of these species,which will facilitate future work designed to evaluate the validity of previous records multi-infection sites of the multi-fish host.