Nevertheless, Criscione et al. discovered a small selleck JQ1 region of roughly 18 Mb on the sex chromosome that shows recombination repression. Several open questions remain to be answered. First, it is not clear what are the genetic differences between W and Z chro mosomes of S. mansoni, or in other words, what are the W and what are the Z specific sequences. Second, the mechanism of recombination repression between S. mansoni sex chromosomes is not clear. As outlined above, either inversion events or heterochromatization have been proposed for other species. The spe cific objectives of the present study were to determine what the sex specific DNA sequences of S. mansoni are, and how heterochromatization of the W chromosome might be initiated. We present here evidence that S.
mansoni sex chromosomes contain large pseudoautoso mal regions. Outside these regions, Z specific sequences are composed of unique sequences and interspersed repeats. W specific sequences are almost entirely com posed of satellite type repeats located in the heterochro matic region of the W chromosome. While no female specific gene could be identified, many of the female repeats are transcribed in the larval stages of the para site but never in the adults. This loss of transcriptional activity and the development into adults is accompanied by chromatin structural changes around the W specific repeats. We develop a model in which female specific repeats are expressed to induce a change in chromatin structure of the W chromosome specifically in the sex ual part of the life cycle, leading to functional heterogametism.
Results The S. mansoni sex chromosomes Z and W share large pseudoautosomal regions We had previously sequenced genomic DNA of female and male S. mansoni individuals of the DFO strain using Illumina sequencing submis sion number SRA012151. We aligned the 8,600,198 sequences from the male samples and the 9,355,380 sequences from the female samples to the 19,022 known scaffolds of the S. mansoni genome assembly using SOAP. We then calculated for each scaffold the ratio between sequences that match with the scaffold in question for the male and the female DNA. The rationale behind this approach was that, in males, Z specific scaffolds should show two times higher hit counts than in females. We searched for scaffolds with at least 10 hits per 1 kb in the female and the male genome, at least 10 kb in length, and a male/female hit count ratio 1.
68. Using these parameters we identified 15 scaffolds spanning 6,436,718 bp. We consider these scaffolds to be specific for the Z chromosome. We confirmed these in silico results for representative regions in a subset of 13 arbitrarily chosen scaffolds by quantitative PCR. With the exception of one scaffold, GSK-3 qPCR con firmed next generation sequencing hit count ratios.