In the first model, transcription of non coding RNA from repetitive DNA thing elements was the initial event in sex chromosome evolution of schisto somes. Non coding RNA would have induced hetero chromatization and suppression of recombination. Both favored expansion of repeats and organization in large blocks. Satellite expansion would have rein forced the system and led finally to the beginning of genetic changes in the W chromosome. The very basal phylogenetic position of leuphotrochozoans such as S. mansoni permits a general model for the main stages of sex chromosome evolution to be proposed the estab lishment of a sex determining region, recruitment of repeats for production of non coding RNA, RNA direc ted heterochromatization and repeat expansion, local suppression of recombination, and shrinkage of the chromosome by deletion.

In the second model, a small mutation and/or local heterochromatization could have been the initial event, leading to recombination repression in the first place. Repetitive DNA accumulated subsequently. During germ cell formation or during early embryogenesis euchroma tization occurs. Cytogenetic evidence in other species in which the female is the heterogametic sex shows that the W chromosome is often condensed in somatic cells, and becomes euchromatic in early oocytes. This releases transcription repression and repeats are transcribed, leading subsequently to heterochromati zation. Our preliminary data suggest that chromatin structural changes do not occur in trans that is, not on the Z chromosome but on the adjacent regions of the W chromosome.

We cannot formally exclude that sex determination is based on a specific protein coding gene that is absent or present on the W chromosome. But we show that the most pronounced difference in transcription between ZZ and ZW individuals is at the level of non coding RNA. We therefore favor the hypothesis that sex differ entiation in S. mansoni is based on developmental stage dependent tagging of the W chromosome by non coding RNA and a chromatin marking system. Our model predicts that chromatin structural changes influ ence transcription of one or several genes in the close vicinity of the core heterochromatic region and that transcriptional activation or inactivation of these leads to morphological and/or physiological changes that are the bases for development of the male and female phe notypes in the adult stage.

Materials and methods Parasite culture and drug treatment Eggs were axenically recovered from 60 day infected hamster livers and miracidia were hatched from eggs in 5 ml of spring water over 2 to 3 hours under light. Mir acidia were concentrated by sedimentation on ice for 15 minutes. Cercariae were recovered from infected AV-951 snails and collected by pipetting. They were then concentrated by cold centrifugation at 1,200 rpm for 5 minutes and the supernatant was removed.