Such a behavior was described for W1 selleck and could depend on the chromatin structure around the repeats and/or flanking regions. Several of these W specific repeats are transcribed in the miracidia and cercariae stages but never in the adults. Role of W specific repeats In most species that possess sex chromosomes of the Y or W type it was found that repetitive sequences accumulate on these chromosomes, large regions are heterochromatic and these chromosomes deteriorate or are completely absent in the extreme case. We show that the W chromosome in S. mansoni is no exception to this rule. What is unknown, however, is the suite of events in the evolution of sex chromosomes and the role of the different elements in sex determination. We believe our present study sheds some light on this mat ter.

Heterochromatization of the W chromosome in schistosomes has been known for a long time and has been even used as a marker for sex identification in morphologically indistinguishable cercariae. Based on cytogenetic analysis, some authors argued that heterochromatization of the W starts in miracidia. Since it is impossible to determine chromosome banding in miracidia and then reuse the larvae for infection and production of adults, these results are difficult to verify. Our results clearly show that the repeats that are located in the W heterochromatic region carry a euchromatic signature in miracidia and lose their euchromatic char acter progressively during the development into adults. This process is accompanied by a decrease of transcrip tion until complete silencing of the repeats in the sexu ally mature adult stage.

During the miracidia to cercaria transition that is, precisely when sexual dimorphism starts to develop the repeats heterochromatize. Sex specific repeats are found in many species. In some cases transcription has been described and it was suspected that these repeats play a role in the sex deter mination process. The transcription of repetitive elements of the satellite type in S. mansoni is particu larly interesting in the light of the recent discovery of stage dependent expression of the elements that consti tute the RNA interference pathway in schisto somes. In many organisms RNAi and chromatin structural changes are linked and it is tempt ing to speculate that transcription of W specific repeats is actually the origin of chromatin compaction on the W chromosome during the life cycle.

A hypothetical scheme is shown in Figure 5. In our model, reset of the repeat chromatin structure occurs during early embryo genesis. In the miracidia, repeats are euchromatic and several of them are tran scribed. Transcripts are processed through a pathway that has similarity to RNAi and a hypothetical repeat Anacetrapib induced silencing complex is formed that induces the formation of heterochromatin around the repeats.