MOLECULAR CHARACTERIZATION OF INTERMEDIATE SNAIL HOSTS AND THE SEARCHFOR RESISTANCE GENES

The relationship between schistosomes and their intermediate hosts is an extremely intricate one with strains and species of the parasite de pending on particular species of snail, which in turn may vary in thei r susceptibility to the parasites. In order to gain a better understan ding of the epidemiology of the disease we have been investigating the use of molecular markers for snail identification and for studying ho st-parasite relationships. In this paper we will draw on examples conc erning schistosomiasis in West and East Africa to illustrate how a mol ecular analysis can be used as part of a ''total evidence'' approach t o characterisation of Bulinus species and provide insights into parasi te transmission. Particular emphasis is given to ribosomal RNA genes ( rRNA), random amplified polymorphic DNA (RAPDs) and the mitochondrial gene cytochrome oxidase I (COI). Snails resistant to infection occur n aturally and there is a genetic basis for this resistance. III Biompha laria glabrata resistance to Schistosoma mansoni is known? to be a pol ygenic trait and we have initiated a preliminary search for snail geno mic regions linked to, or involved in, resistance by using a RAPD base d approach in conjunction with progeny pooling methods. We ar-e curren tly characterising a variety of STSs (sequence tagged sites) associate d with resistance. These can be used for local linkage and interval ma pping to define genomic regions associated with the resistance trait. The development of such markers into simple dot-blot or specific PCR-b ased assays may have a direct and practical application for the identi fication of resistant snails in natural populations.