Subject: FB Help Mailer: 618 Ultimate modification!
Rox 'n' Rab 
This is an additional comment on the repeated sequences present near the six Rab GTPases genes which are clustered in cytological region 9 of D. melanogaster, and that I reported last year in Advances in Genetics (Vol 58, 2007). These so called 6paRab9 genes (RabX2, Rab9D, Rab9Db, Rab9E, Rab9Fa and Rab9Fb) share more than 50 partial copies (fragments) near their 5' and 3' UTRs, with a 880 bp sequence which lies about 400 bp from the 3' end of the roX1 gene (CR32777), as judged by its roX1-RB transcript. Five such fragments are present in the one kb interval of genomic DNA which has consistently separated Rab9D from the hybrid rescuing gene Hmr for at least 40 million years. In addition to the above mentioned sites, fragments of the repeated sequence are present at more than 300 sites throughout the X chromosome, pointing to their possible role in attracting the dosage compensation complex (DCC), or propagating its effect from entry points. Indeed, it has been postulated that proper functioning of the DCC may become impaired in hybrids between D. melanogaster and its sibling species. The above roX1 linked 880 bp sequence has several partial copies (fragments) in the 1A region (between SP71 and CG3038 genes), in addition to more than 50 fragments at 9E, notably 30 bp from the start codon of CG9806. Remarkably, the same pattern of distribution of 10 fragments through 1913 bp of genomic DNA is present both near the 5' UTR of CG9806 and in a region encompassing both UTRs of the Rab9Db gene. With regard to this, the above fragments are prima facie good candidates as causes of mis-targetting by the DCC or its impaired signal propagation, in hybrids. This would imply that regions enriched in the above repeats may be prone to play an instrumental role in postzygotic reproductive isolation between incipient species. Remarkably, the high density of the above repeats within the 500 kb interval comprising the 6paRab9 genes, co-localizes with at least one gene, Hmr, whose mutations can override hybrid inviability.