Intracellular promiscuity in Schistosoma mansoni: nuclear transcribed DNA sequences are part of a mitochondrial minisatellite region

It has been shown that the mtDNA of the parasitic trematode Schistosoma mansoni is hypervariable in size. We report here that this length variation is due to a large polymorphic minisatellite composed of two types of repeated sequences of 558 bp and 62 bp. Each minisatellite repeat is made up of a large 558-bp component and a variable tandem array of the small 62-bp unit. Of more fundamental interest was the finding that both the 558-bp and 62-bp components have significant homology with a gene, SM750, previously identified in the nuclear genome of S. mansoni. The small 62-bp unit is identical to the nuclear polymorphic repeat element, which is apparently spread throughout the nuclear genome and is abundant among transcripts, in addition to being present in five tandem copies in SM750. The presence, in the S. mansoni mtDNA, of fragments of genes that are present in and transcribed from the nuclear genome raises the question of the origin of these sequences. The arrangement and the variability that the mtDNA minisatellite embodies were explored as an identity test for S. mansoni based on the use of PCR for tallying the relative abundance of the several repeat numbers of the tandem arrays of the 62-bp unit within the minisatellite structure.