Parthenogenesis and asexual multiplication among parasitic platyhelminths.

Among flatworms with parasitic and commensal modes of existence, parthenogenesis and asexual multiplication appear to be largely confined to the Digenea and Cestoda, the only parasitic platyhelminths that routinely utilize indirect life-cycles. Parthenogenesis is apparently restricted to a minority of adult digeneans and cestodes inhabiting their final hosts, and a survey is made of the particular modes of parthenogenesis (i.e. apomictic, automictic and generative) which are employed by such adults. Asexual (amictic) multiplication, in the form of fissioning, is demonstrated by young adults of the cyclophyllidean cestode, Mesocestoides corti, but is otherwise not exhibited by adult cestodes or digeneans, other than in the perplexing phenomenon of proglottid formation in polyzoic tapeworms. Secondary multiplication is of ubiquitous occurrence in digenean life-cycles in the form of the proliferation which takes place within sporocysts and rediae (germinal sacs) located in the first intermediate host. The controversy concerning the nature of this multiplication is reconsidered in the context of recent findings which have centred on cellular aspects. On the basis of present evidence germinal sac multiplication should be regarded as an asexual rather than a parthenogenetic process. The cestode asexual multiplication which occurs in intermediate hosts is a function of the metacestode stage of development. Metacestode proliferation is only known from about 20 species and 6 families of polyzoic cestodes with approximately half the described instances occurring in the family Taeniidae. The organization of these proliferative metacestodes, findings concerning their totipotent stem cells and the ontogeny of buds and new scolices are all reviewed. Finally, the capacity for population expansion of multiplicative larval digeneans and metacestodes are compared, while the ecological roles and the genetical consequences of both parthenogenesis and amictic multiplication in the two taxa are also examined.