Symbiotic Specialization,Stability And Their Pathogenicity Of Symbiotic Bacteria Associated With Entomopathogenic Nematode Of Heterorhabditidoides

Entomopathogenic nematodes (EPNs) are a kind of parasitic nematodes carrying symbiotic bacteria. They are widely lethal to insect pests but don’t induce resistance. In addition, they are safe for human and livestock, also can actively search for host. They have become promising biological control factors since the early 20th century. These nematodes act as a spreading medium of symbiotic bacteria for communication in the biological control, and the metabolic product of symbiotic bacteria plays a main role on pathogenicity to insects. To recover the symbiosis specialization and stability between entomopathogenic nematodes and symbiotic bacteria is basis for using them as biological control agent. Two strains of Serratia nematodphila, DZ0503SBS1 and DR186, were isolated from intestine of Heterorhabditidoides chongmingensis DZ0503CMFT and H. rugaoensis RG081015, respectively. This kind of symbiotic-complex is the major entomopathogenic nematode groups widespread in Yangtze river delta and their relevant symbiosis has never been reported. This paper demonstrated the symbiosis specialization, stability and their pathogenicity among the new symbiosis-complex. The methods and results are as following:Sterile eggs of DZ0503CMFT and RG081015 developed to sterile infective juveniles (Us) on the sterile culture medium in vitro. And then exposed theirselves to strains DZ0503SBS1 and DR186, respectively, to make them carrying one bacterial strain. Effect of bacterial strain on the Us recovery, Us yield, sex ratio, Us length and width of those monoxenie nematodes were evaluated. The symbiosis specialization between Serratia nematodphila and RG081015 was weaker than those between DZ0503CMFT and bacterial strains. Both DZ0503SBS1 and DR186 could induce recovery of RG081015 Us, while only DZ0503SBS1 can induce that of DZ0503CMFT. The sex ratio of DZ0503CMFT Us cultured with DZ0503SBS1 was 2.61, while the sex ratio of DZ0503CMFT Us cultured with DR186 was 0.63. But the sex ratio of RG081015 Us (2.05) cultured with DZ0503SBS1 was a little more than that with DR186 (1.79). Us length and width of those nematodes had no significant difference. Rates of carring bacteria of the first generation of the four kinds of monoxenic combination were about 75% and the pathogenicity to Galleria mellonella larvae was so strong that they all dead in 72h. Specialization of nematodes RG081015 associated with two S. nematodiphila strains was not strong.The further study on the symbiotic stability and their pathogenicity for the 4 combinations showed that after the transfer of culture from generation to generation with Galleria mellonella larvae in vivo, the pathogen ratios all appeared to downtrend, in which the rate of RG081015 cultived with DR186 and DZ0503SBS1 reduced to 50% after five infection cycles, and that of DZ0503CMFT cultived with DR186 was significantly so low that almost no symbiotic bacterial cells was isolated after third infection cycle, but had no effct on it breeding. The stability of nematodes DZ0503CMFT and its symbiotic bacterial strain DZ0503SBS1 was strong. RG081015 could carry two kinds of Serratia nematodphila, while DZ0503CMFT almost could not carry non-symbiotic bacteria DR186. The bacteria carriage rate and pathogenicity of DZ0503CMFT associated with two Serratia nematodphila both were significantly different. In contrast, the difference of the bacteria carriage rate of RG081015 with two Serratia nematodphila was 20%, but the pathogenicity to the Galleria mellonella larvae was not significantly different..