SCHISTOSOMA MANSONI: CHARACTERIZATION OF CLONES MAINTAINED BY THE MICROSURGICAL TRANSPLANTATION OF SPOROCYSTS

ABSTRACr: Five male and 5 female clones of Schistosoma mansoni were established and maintained for 3 yr by the serial microsurgical transplantation of sporocysts from infected to uninfected Biomphalaria glabrata snails. The clones were initially derived from 10 randomly selected snails with monomiracidial infections. Clones were characterized by several criteria, including their infectivities for mice and snails, their cercarial outputs, and their ability to produce immunity in mice. The mean infectivities of individual clones in mice ranged from 26 to 44%, and were highly consistent within each clone. The infectivities of cloned sporocysts in snails ranged from 44 to 100% and were also highly consistent within clones. Mean cercarial outputs from individual clones ranged from 450 to 4,300 per snail. In mice, clones differed significantly from each other in their ability to immunize and in their susceptibility to immunity. Each clone was unique and did not appear to differ with time or subpassaging through snails, suggesting that the differences had a genetic basis. The schistosomes have a complex life cycle in which sexually reproducing stages in the vertebrate definitive host alternate with asexually reproducing forms in the molluscan intermediate host. Under natural conditions intramolluscan parasites derived from a single miracidium are genetically unique, and all adult schistosomes derived from them are members of a single clone. However, the life span of any clone is limited to the longevity of its snail host. An infected snail releases varying numbers of cercariae on different days, so its contribution to the cercarial pool varies from day to day. Also, a snail is often infected by more than 1 miracidium, and each