Chromosomal differentiation of the Schistosoma japonicum complex

The C-banding pattern, location of telomere sequence and chiasma frequency of four species of the Schistosoma japonicum complex were compared with tho se of two African species, Schistosoma mansoni and Schistosoma haematobium. In the six species, C-banding patterns of seven autosomes and the two sex chromosomes (Z and W) showed relatively species-specific and geographical ( Asian and African) differences. Particularly, a plausible pathway of altera tion of chromosome 2 revealed a direction from the A-chromosome to the M- c hromosome in terms of rearrangements of pericentric inversion and eliminati on of constitutive heterochromatin (AM inversion). This chromosome change s uggested hypothetically that the S. japonicum complex is the original type, and the African species represents the derived type. Moreover, the mosaic construct of the Asian and African types in Schistosoma sinensium chromosom es prompted us to propose that the species might have been formed by hybrid speciation of the genomes of Asian and African species. Localisation of te lomeric repeats enabled Asian and African schistosomes to be distinguished clearly by simple terminal location and by terminal and interstitial locati ons, respectively. Change of chiasma frequency in the S. japonicum complex might be caused by the reduction of interstitial chiasmate (Xi) in the larg er chromosomes, I and Z (ol W), and the change seems to have progressed to Japan from South East Asia. These data enabled us to predict a tentative ev olutionary pathway of schistosomes at the cytogenetic level. (C) 2000 Austr alian Society for parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.