The Epidemiology And Pathogensis Of The Parasitic Dinoflagellate Hematodinium Sp.infecting Marine Crustaceans Along The Coast Of China

Parasitic dinoflagellates in the genus Hematodinium are serious infectious pathogens causing epidemic disease in a broad range of marine crustaceans.In recent years,frequent outbreaks of the disease had seriously impacted the wild stocks of many wild crustaceans around the world,as well as the sustainable development of marine crustacean cultures in China.The studies on its epidemology,the pathogenesis of Hematodinium sp.and the host immunologic responses against Hematodinium infection are the fundamental basis to find solution on how to prevent and control this infectious disease.Thus,this dissertation conducted three aspects of researches,including the epidemiology of Hematodinium sp.in wild and cultured stocks of major marine crustaceans along the coast of China,the developmental cycle of Hematodinium sp.and the immune response of P.trituberculatus responding to the Hematodinium parasite.The major findings are listed as following:1.Hematodinium sp.infections in wild and cultured crustaceans along the eastern coast of China were diagnosed by hemolymoph smear assay and molecular biological method.Hematodinium infections were widely distributed in China.Hematodinium infection was detected in wild Chinese swimming crabs Portunus trituberculatus in Liaoning,Shandong and Zhejiang provinces and mud crabs Scylla paramamosain in Guangdong province.In addition,Hematodinium infections were persistent in the polyculture ponds in Shandong during the culture period,with a peak prevalence(90 %)in late July.Furthermore,Hematodinium sp.infection was identified for the first time in the giant tiger prawn P.monodon co-cultured with P.trituberculatus in the polyculture system during outbreak of the disease.2.The biological characteristic of Hematodinium sp.infecting cultured P.trituberculatus and P.monodon was studied by molecular biological method and in vitro culture.Most typical life stages were observed in in vitro cultures, including filamentous trophont,ameboid trophont,arachnoid trophont,arachnoid sporont,schizonts,clump colony,sporoblast,prespores and dinospores.Phylogenetic analysis indicated the Hematodinium isolate infecting P.monodon was identical to the isolate infecting the co-cultured P.trituberculatus,and the isolates were grouped into the cluster(Hematodinium perezi II)together with the Hematodinium sequences obtained from other places in China.3.The pathogenic mechanism of Hematodinium sp.was explored with the hemolymoph smear assay and H&E assay.Histopathology indicated that Hematodinium sp.lives and proliferate in the hemolymph and hemocoel of major organs of infected hosts,caused systemic infections to its hosts and resulted in overt pathological alterations in hepatopancreas,heart,gills,and muscles.The systemic infections of Hematodinium sp.in P.trituberculatus led to malfunction or dysfunction of major organs,and eventually mortality of infected hosts.4.The immune response of host against Hematodinium was studied.A novel selenium-dependent glutathione peroxidase gene(Pt GPx)was isolated and characterized from Portunus trituberculatus.The length of se-GPx gene c DNA was 696 bp,including a 507 bp open reading frame(ORF)encoding 168 amino acids.Amino acid comparison and phylogenetic analyses indicated that the Pt GPx was closely related to and clustered with the crustacean Se GPxs.The Pt GPx transcripts could be extensively and significantly induced during the immune response of P.trituberculatus responding to the Hematodinium parasite,implying a critical role of GPx in the crustacean innate immune defense against the parasitic infection.