The Chironomus tentans genome sequence and the organization of the Balbiani ring genes

Abstract

Background: The polytene nuclei of the dipteran Chironomus tentans (Ch. tentans) with their Balbiani ring (BR) genes constitute an exceptional model system for studies of the expression of endogenous eukaryotic genes. Here, we report the first draft genome of Ch. tentans and characterize its gene expression machineries and genomic architecture of the BR genes. Results: The genome of Ch. tentans is approximately 200 Mb in size, and has a low GC content (31%) and a low repeat fraction (15%) compared to other Dipteran species. Phylogenetic inference revealed that Ch. tentans is a sister clade to mosquitoes, with a split 150-250 million years ago. To characterize the Ch. tentans gene expression machineries, we identified potential orthologus sequences to more than 600 Drosophila melanogaster (D. melanogaster) proteins involved in the expression of protein-coding genes. We report novel data on the organization of the BR gene loci, including a novel putative BR gene, and we present a model for the organization of chromatin bundles in the BR2 puff based on genic and intergenic in situ hybridizations. Conclusions: We show that the molecular machineries operating in gene expression are largely conserved between Ch. tentans and D. melanogaster, and we provide enhanced insight into the organization and expression of the BR genes. Our data strengthen the generality of the BR genes as a unique model system and provide essential background for in-depth studies of the biogenesis of messenger ribonucleoprotein complexes.