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Genetic analysis of the 8D3;8D8-9 segment of the Drosophila melanogaster X chromosome has assigned seven complementation groups to this region, three of which are new. A Polycomb group (Pc-G) gene, multi sex combs (mxc), is characterized and mutant alleles are described. Besides common homeotic transformations characteristic of Pc-G mutants that mimic the ectopic gain of function of BX-C and ANT-C genes, mxc mutants show other phenotypes: they zygotically mimic, in males and females, the characteristic lack of germ line seen in progeny of some maternal effect mutants of the so-called posterior group (the grandchildless phenotype). Loss of normal mxc function can promote uncontrolled malignant growth which indicates a possible relationship between Pc-G genes and tumour suppressor genes. We propose that gain-of-function of genes normally repressed by the wild-type mxc product could, in mxc mutants, give rise to an incoherent signal which would be devoid of meaning in normal development. Such a signal could divert somatic and germ line development pathways, provoke the loss of cell affinities, but allow or promote growth.

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Polycomb group (PcG) genes maintain cell identities during development in insects and mammals and their products are required in many developmental pathways. These include limb morphogenesis in Drosophila melanogaster, since PcG genes interact with identity and pattern specifying genes in imaginal discs and clones of polyhomeotic (ph) null cells induce abnormal limb patterning. Such clones are associated with ectopic expression of engrailed, hedgehog, patched, cubitus interruptus and decapentaplegic, in a compartment specific manner. Our results also reveal negative engrailed regulation by ph in both disc compartments: ph silences engrailed in anterior cells and maintains the level of engrailed expression in posterior ones. We suggest that PcG targets are not exclusively regulated by an on/off mechanism, but that the PcG also exerts negative transcriptional control on active genes.

Chromodomains are found in many regulators of chromatin structure, and most of them recognize methylated lysines on histones. Here, we investigate the role of the Drosophila melanogaster protein Corto's chromodomain. The Enhancer of Trithorax and Polycomb Corto is involved in both silencing and activation of gene expression. Over-expression of the Corto chromodomain (CortoCD) in transgenic flies shows that it is a chromatin-targeting module, critical for Corto function. Unexpectedly, mass spectrometry analysis reveals that polypeptides pulled down by CortoCD from nuclear extracts correspond to ribosomal proteins. Furthermore, real-time interaction analyses demonstrate that CortoCD binds with high affinity RPL12 tri-methylated on lysine 3. Corto and RPL12 co-localize with active epigenetic marks on polytene chromosomes, suggesting that both are involved in fine-tuning transcription of genes in open chromatin. RNA–seq based transcriptomes of wing imaginal discs over-expressing either CortoCD or RPL12 reveal that both factors deregulate large sets of common genes, which are enriched in heat-response and ribosomal protein genes, suggesting that they could be implicated in dynamic coordination of ribosome biogenesis. Chromatin immunoprecipitation experiments show that Corto and RPL12 bind hsp70 and are similarly recruited on gene body after heat shock. Hence, Corto and RPL12 could be involved together in regulation of gene transcription. We discuss whether pseudo-ribosomal complexes composed of various ribosomal proteins might participate in regulation of gene expression in connection with chromatin regulators.

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Identification of the events responsible for rapid morphological variation during evolution can help understand how developmental processes are changed by genetic modifications and thus produce diverse body features and shapes. Sex combs, a sexually dimorphic structure, show considerable variation in morphology and numbers among males from related species of Sophophora, a subgenus of Drosophila. To address which evolutionary changes in developmental processes underlie this diversity, we first analyzed the genetic network that controls morphogenesis of a single sex comb in the model D. melanogaster. We show that it depends on positive and negative regulatory inputs from proximo-distal identity specifying genes, including dachshund, bric à brac, and sex combs distal. All contribute to spatial regulation of the Hox gene Sex combs reduced (Scr), which is crucial for comb formation. We next analyzed the expression of these genes in sexually dimorphic species with different comb numbers. Only Scr shows considerable expression plasticity, which is correlated with comb number variation in these species. We suggest that differences in comb numbers reflect changes of Scr expression in tarsus primordia, and discuss how initial comb formation could have occurred in an ancestral Sophophora fly following regulatory modifications of developmental programs both parallel to and downstream of Scr.

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