INTRA-SPECIFIC AND INTER-SPECIFIC VARIATION IN NUCLEAR RIBOSOMAL INTERNAL TRANSCRIBED SPACER-1 OF THE SCHISTOSOMA-JAPONICUM SPECIES COMPLEX

The first internal transcribed spacer (ITS1) of the nuclear ribosomal DNA repeat was sequenced for members of the Schistosoma japonicum spec ies complex (S. malayensis, S. mekongi and 2 geographical isolates of S. japonicum). The ITS1 is composed of 3 distinct regions: the 5' end (23 nucleotides); a tract of approximately 90-140 nucleotides, which o ccurs up to 7 times in tandem, the number varying even within an indiv idual in all species investigated in this study; the 3' region (378 nu cleotides), which lacks repeats. There is size and sequence variation among copies of the ITS1 repeat within a single individual. The relati ve abundances of size variants of ITS1 in S. japonicum have been ascer tained by hybridizing genomic digests with an ITS1 probe. Multiple rep eats and intra-individual variation in numbers and abundance of these is a feature of the Asian schistosomes, but not generally of African s chistosomes. Possible reasons for this difference in ITS1 between Afri can and Asian schistosomes are discussed. The ITS1 repeat sequences de scribed for African schistosomes are different to, and cannot be align ed with, those from the Asian species described here, whereas the rema inder of the ITS1 can be aligned quite easily.