Hsp90 prevents phenotypic variation by suppressing the mutagenic activity of transposons.

The canalization concept describes the resistance of a developmental process to phenotypic variation, regardless of genetic and environmental perturbations, owing to the existence of buffering mechanisms. Severe perturbations, which overcome such buffering mechanisms, produce altered phenotypes that can be heritable and can themselves be canalized by a genetic assimilation process. An important implication of this concept is that the buffering mechanism could be genetically controlled. Recent studies on Hsp90, a protein involved in several cellular processes and development pathways, indicate that it is a possible molecular mechanism for canalization and genetic assimilation. In both flies and plants, mutations in the Hsp90-encoding gene induce a wide range of phenotypic abnormalities, which have been interpreted as an increased sensitivity of different developmental pathways to hidden genetic variability. Thus, Hsp90 chaperone machinery may be an evolutionarily conserved buffering mechanism of phenotypic variance, which provides the genetic material for natural selection. Here we offer an additional, perhaps alternative, explanation for proposals of a concrete mechanism underlying canalization. We show that, in Drosophila, functional alterations of Hsp90 affect the Piwi-interacting RNA (piRNA; a class of germ-line-specific small RNAs) silencing mechanism leading to transposon activation and the induction of morphological mutants. This indicates that Hsp90 mutations can generate new variation by transposon-mediated 'canonical' mutagenesis.

The Drosophila simulans Genome Lacks the crystal-Stellate System.

The cry-Ste system is a genetic interaction system between heterochromatin and euchromatin in Drosophila melanogaster, regulated via the piRNA pathway. Deregulation of this system leads to meiotic defects and male sterility. Although the cry-Ste system is peculiar to D. melanogaster, ancestors of Ste and Su(Ste) elements are present in the three closely related species, D. simulans, D. sechellia, and D. mauritiana. The birth, evolution, and maintenance of this genetic system in Drosophila melanogaster are of interest. We investigate the presence of sequences homologous to cry and Ste elements in the simulans complex and describe their chromosomal distribution. The organization and expression of cry- and Ste-like sequences were further characterized in the D. simulans genome. Our results allow us to conclude that the cry-Ste genetic interaction system is absent in the D. simulans genome.

VHL frameshift mutation as target of nonsense-mediated mRNA decay in Drosophila melanogaster and human HEK293 cell line.

There are many well-studied examples of human phenotypes resulting from nonsense or frameshift mutations that are modulated by Nonsense-Mediated mRNA Decay (NMD), a process that typically degrades transcripts containing premature termination codons (PTCs) in order to prevent translation of unnecessary or aberrant transcripts. Different types of germline mutations in the VHL gene cause the von Hippel-Lindau disease, a dominantly inherited familial cancer syndrome with a marked phenotypic variability and age-dependent penetrance. By generating the Drosophila UAS:Upf1(D45B) line we showed the possible involvement of NMD mechanism in the modulation of the c.172delG frameshift mutation located in the exon 1 of Vhl gene. Further, by Quantitative Real-time PCR (QPCR) we demonstrated that the corresponding c.163delG human mutation is targeted by NMD in human HEK 293 cells. The UAS:Upf1(D45B) line represents a useful system to identify novel substrates of NMD pathway in Drosophila melanogaster. Finally, we suggest the possible role of NMD on the regulation of VHL mutations.

Does Stellate cause meiotic drive in Drosophila melanogaster?

Drosophila melanogaster males deficient for the crystal (cry) locus of the Y chromosome that carry between 15 and 60 copies of the X-linked Stellate (Ste) gene are semisterile, have elevated levels of nondisjunction, produce distorted sperm genotype ratios (meiotic drive), and evince hyperactive transcription of Ste in the testes. Ste seems to be the active element in this system, and it has been proposed that the ancestral Ste gene was "selfish" and increased in frequency because it caused meiotic drive. This hypothetical evolutionary history is based on the idea that Ste overexpression, and not the lack of cry, causes the meiotic drive of cry(-) males. To test whether this is true, we have constructed a Ste-deleted X chromosome and examined the phenotype of Ste(-)/cry(-) males. If hyperactivity of Ste were necessary for the transmission defects seen in cry(-) males, cry(-) males completely deficient for Ste would be normal. Although it is impossible to construct a completely Ste(-) genotype, we find that Ste(-)/cry(-) males have exactly the same phenotype as Ste(+)/cry(-) males. The deletion of all X chromosome Ste copies not only does not eliminate meiotic drive and nondisjunction, but it also does not even reduce them below the levels produced when the X carries 15 copies of Ste.

Loss of Pol32 in Drosophila melanogaster causes chromosome instability and suppresses variegation.

Pol32 is an accessory subunit of the replicative DNA Polymerase δ and of the translesion Polymerase ζ. Pol32 is involved in DNA replication, recombination and repair. Pol32's participation in high- and low-fidelity processes, together with the phenotypes arising from its disruption, imply multiple roles for this subunit within eukaryotic cells, not all of which have been fully elucidated. Using pol32 null mutants and two partial loss-of-function alleles pol32rd1 and pol32rds in Drosophila melanogaster, we show that Pol32 plays an essential role in promoting genome stability. Pol32 is essential to ensure DNA replication in early embryogenesis and it participates in the repair of mitotic chromosome breakage. In addition we found that pol32 mutants suppress position effect variegation, suggesting a role for Pol32 in chromatin architecture.

The "Special" crystal-Stellate System in Drosophila melanogaster Reveals Mechanisms Underlying piRNA Pathway-Mediated Canalization.

The Stellate-made crystals formation in spermatocytes is the phenotypic manifestation of a disrupted crystal-Stellate interaction in testes of Drosophila melanogaster. Stellate silencing is achieved by the piRNA pathway, but many features still remain unknown. Here we outline the important role of the crystal-Stellate modifiers. These have shed light on the piRNA pathways that defend genome integrity against transposons and other repetitive elements in the gonads. In particular, we illustrate the finding that HSP90 participates in the molecular pathways of piRNA production. This observation has relevance for the mechanisms underlying the evolutionary canalization process.

Structure, regulation and evolution of the crystal-Stellate system of Drosophila.

The crystal-Stellate system is one of the most known example of interaction between heterochromatin and euchromatin: a heterochromatic locus on the Y chromosome (crystal) 'represses' a euchromatic locus (Stellate) on the X chromosome in Drosophila melanogaster. The molecular mechanism regulating this interaction is not completely understood. It is becoming clear that an RNA interference (RNAi) mechanism could be responsible for the silencing carried out by crystal on the Stellate sequences. Here, a detailed structural analysis of all the sequences involved in the system is reported, demonstrating a their 'puzzling' structure. In addition three autosomal mutations: sting, scratch and sirio are described that interfere with the system. All of them are male sterile mutations and exhibit crystals made by the STELLATE protein in their primary spermatocytes. They are requested during oogenesis and early in embryogenesis as well. Hypothesis on the involvement of these genes in activating the Stellate sequences are discussed.