Longevity determination genes in Drosophila melanogaster

Identification of longevity mutants is crucial for genetic approach to dissect the molecular mechanism of aging and longevity determination. In Drosophila melanogaster, several mutations have been shown to extend the longevity: methuselah encoding a putative G-protein coupled receptor, Indy encoding a sodium dicarboxylate cotransporter, chico encoding insulin receptor substrate, and InR encoding the insulin-like receptor. Extended longevity phenotypes were also observed in transgenic flies overexpressing antioxidant enzymes, Cu/Zn superoxide dismutase and Catalase, Cu/Zn SOD only, or a molecular chaperone, hsp70. Pleiotropism of mutations is a limitation associated with conventional mutagenesis for efficient detection of longevity determination genes. Using a conditional misexpression system, we identified Drosophila POSH (DPOSH), a scaffold protein containing RING finger and four SH3 domains, whose ubiquitous overexpression in adult stage extends the longevity. Neural-specific overexpression of DPOSH is sufficient to extend the longevity, whereas overexpression in non-neural tissues during development induces apoptosis through activation of JNK/SAPK pathway.     

Role of X chromosomal song genes in the evolution of species-specific courtship songs in Drosophila virilis group species

In the Drosophila virilis group the males of the virilis phylad species produce courtship song consisting of pulse trains with no pauses between successive sound pulses, whereas the males of the montana phylad species produce songs with clear pauses between the sound pulses. We obtained song data for F₁ hybrids between D. virilis (representing the virilis phylad) or D. flavomontana (representing the montana phylad) females and the males of several species of the D. virilis group to study the interaction of X chromosomal and autosomal song genes affecting species differences in song. In crosses with D. virilis females, X chromosomal (or maternal) factors masked variation in pulse length despite variation in heterospecific autosomal song genes. To the contrary, in crosses with D. flavomontana females, X chromosomal genes largely determined the pause length and interacted with autosomal genes to determine the pulse length. In the montana phylad species, pulse length showed dominance toward shorter pulses and pause length toward longer pauses. The first-mentioned trait also indicated the epistatic effects of X chromosomal and autosomal components.     

Age-dependent chromosomal distribution of male-biased genes in Drosophila

We investigated the correlation between the chromosomal location and age distribution of new male-biased genes formed by duplications via DNA intermediates (DNA-level) or by de novo origination in Drosophila. Our genome-wide analysis revealed an excess of young X-linked male-biased genes. The proportion of X-linked male-biased genes then diminishes through time, leading to an autosomal excess of male-biased genes. The switch between X-linked and autosomal enrichment of male-biased genes was also present in the distribution of both protein-coding genes on the D. pseudoobscura neo-X chromosome and microRNA genes of D. melanogaster. These observations revealed that the evolution of male-biased genes is more complicated than the previously detected one-step X→A gene traffic and the enrichment of the male-biased genes on autosomes. The pattern we detected suggests that the interaction of various evolutionary forces such as the meiotic sex chromosome inactivation (MSCI), faster-X effect, and sexual antagonism in the male germline might have shaped the chromosomal distribution of male-biased genes on different evolutionary time scales.It has been observed that male-biased genes in Drosophila are overrepresented on autosomes (Parisi et al. 2003; Ranz et al. 2003). Consistent with this result, a dynamic process that can explain the nonrandom autosomal distribution has also been observed, in which autosomal new genes with X-linked parental genes are often male-biased. Specifically, a significant excess of autosomal testis-expressed retrogenes were identified as RNA-duplicates of X-linked parental genes (Betran et al. 2002). Recently, similar X→A gene traffic was observed in the DNA-level duplication and relocation data set of the Drosophila genus (Vibranovski et al. 2009b), and was further confirmed for DNA-level duplications in the D. pseudoobscura neo-X chromosome (Meisel et al. 2009). In addition, selective extinction of neo-X linked male-biased genes also occurred in D. pseudoobscura (Sturgill et al. 2007). These three lines of genome-wide investigation support a common pattern of out-of-X traffic for male-biased genes, resulting in an enrichment of these genes on autosomes in the long term.Various hypotheses have been proposed to explain the chromosomal distribution of sex-biased genes. The sexual antagonism hypothesis (Rice 1984) predicts an increase of X-linked sexually antagonistic genes in a population over a wide range of parameters (e.g., recessive mutations advantageous for males and disadvantageous for females). With the assumption of a modifier gene that may restrict expression to the advantageous sex, X-linked antagonistic genes would be more likely to be involved in the evolution of sexually dimorphic traits (Rice 1984; Vicoso and Charlesworth 2006). In autosomal inheritance, when the advantageous effect on one sex is greater than the disadvantageous effect on the other sex, sexually antagonistic genes can spread in the population (Rice 1984). Although sexual antagonism has often been used to explain the evolution of sex-biased genes (Betran et al. 2002; Parisi et al. 2003; Ranz et al. 2003; Meisel et al. 2009), recent reports questioned the association between sex-biased expression and sexual antagonism (Vicoso and Charlesworth 2009; Innocenti and Morrow 2010). On the other hand, meiotic sex chromosome inactivation (MSCI) in spermatogenesis predicts X demasculinization via selection favoring autosomal male-biased genes, if the X chromosome is inactive during meiosis (Lifschytz and Lindsley 1972; Betran et al. 2002; Hense et al. 2007; Vibranovski et al. 2009a, 2010). Thus, if sex-biased expression were a proxy for sexual antagonism, sexually antagonistic selection is compatible with a paucity of X-linked somatic male-biased genes (Parisi et al. 2003). Additionally, the existence of MSCI-based selection is supported by the recent finding that autosomal genes retroposed from the X chromosome frequently demonstrate complementary expression in meiosis compared to their X-linked parental genes (Vibranovski et al. 2009a).However, a number of X-linked evolutionarily young genes have been identified recently (Arguello et al. 2006; Levine et al. 2006; Chen et al. 2007), all of which are male-biased. In order to understand whether these cases represent a general pattern, we examined the chromosomal distribution of male-biased genes of different evolutionary ages. Unexpectedly, we observed that the X chromosome has undergone an initial enrichment of young male-biased genes through intrachromosomal origination. However, as gene age increases, this excess gradually diminishes and is finally reversed, resulting in an overrepresentation of male-biased genes on the autosomes. This dynamic suggests a significant impact of the evolutionary time scale on the different mechanisms responsible for the evolution of male-biased genes. Here, we discuss how different evolutionary forces may impact the chromosomal distribution of male-biased genes with different ages.     

Retroposed new genes out of the X in Drosophila

New genes that originated by various molecular mechanisms are an essential component in understanding the evolution of genetic systems. We investigated the pattern of origin of the genes created by retroposition in Drosophila. We surveyed the whole Drosophila melanogaster genome for such new retrogenes and experimentally analyzed their functionality and evolutionary process. These retrogenes, functional as revealed by the analysis of expression, substitution, and population genetics, show a surprisingly asymmetric pattern in their origin. There is a significant excess of retrogenes that originate from the X chromosome and retropose to autosomes; new genes retroposed from autosomes are scarce. Further, we found that most of these X-derived autosomal retrogenes had evolved a testis expression pattern. These observations may be explained by natural selection favoring those new retrogenes that moved to autosomes and avoided the spermatogenesis X inactivation, and suggest the important role of genome position for the origin of new genes.     

An efficient method to generate chromosomal rearrangements by targeted DNA double-strand breaks in Drosophila melanogaster

Homologous recombination (HR) is an indispensable tool to modify the genome of yeast and mammals. More recently HR is also being used for gene targeting in Drosophila. Here we show that HR can be used efficiently to engineer chromosomal rearrangements such as pericentric and paracentric inversions and translocations in Drosophila. Two chromosomal double-strand breaks (DSBs), introduced by the rare-cutting I-SceI endonuclease on two different mobile elements sharing homologous sequences, are sufficient to promote rearrangements at a frequency of 1% to 4%. Such rearrangements, once generated by HR, can be reverted by Cre recombinase. However, Cre-mediated recombination efficiency drops with increasing distance between recombination sites, unlike HR. We therefore speculate that physical constraints on chromosomal movement are modulated during DSB repair, to facilitate the homology search throughout the genome.     

Behavior of life-shortening genes in genetic mosaics of Drosophila melanogaster

We have developed a novel method to examine the translational fidelity of mammalian ribosomes in vitro, where protamine mRNA was used as a template. This method enabled us to determine frequency of misrecognition of purine bases at the second position of arginine codons (AGR/AAR) in the mRNA. Using this method the fidelity of translation of ribosomes derived from mouse livers was found to remain unchanged from 2 to 29 months, the maximum life span of the animal. This conclusion is not consistent with the "error catastrophe" theory of aging. This is the first report in which translational fidelity of ribosomes of animals of various ages has been compared by an in vitro translation of a natural mRNA.     

Myotropic peptides in Drosophila melanogaster and the genes that encode them

Myotropic peptides are structurally dissimilar; thus, they comprise different families. The cellular expressions of myotropins suggest they act as hormones, transmitters, and modulators of numerous biological processes. Drosophila melanogaster allatostatin (AST), FMRFamide-containing, dromyosuppressin (DMS), and drosulfakinin (DSK) peptides represent four different myotropin families. A different gene encodes each of these four myotropin families. D. melanogaster AST, FMRFamide-containing, DMS, and DSK peptides are present in neural and gut tissue, but are not all expressed in the same cells. These four families of myotropins affect spontaneous contractions of gut, heart, and/or reproductive tissue, but their effects are dissimilar in magnitude and time course. Based on their structures, genes, distributions, and activities, the synthesis and release of these D. melanogaster myotropins are likely governed by different sensory inputs and regulatory mechanisms. The differences in structures, precursors, cellular expressions, and activities are consistent with the conclusion that they do not play redundant roles in their effects on the frequency of muscle contractions. Orthologs of these D. melanogaster myotropins exist in other animal species; thus, research on the mechanisms involved in their production and processing, functions, and signaling may be widely applicable. Here, we review research on D. melanogaster AST, FMRFamide-containing, myosuppressin, and sulfakinin peptides.     

X chromosomal regulation in flies: when less is more

In Drosophila, dosage compensation of the single male X chromosome involves upregulation of expression of X linked genes. Dosage compensation complex or the male specific lethal (MSL) complex is intimately involved in this regulation. The MSL complex members decorate the male X chromosome by binding on hundreds of sites along the X chromosome. Recent genome wide analysis has brought new light into X chromosomal regulation. It is becoming increasingly clear that although the X chromosome achieves male specific regulation via the MSL complex members, a number of general factors also impinge on this regulation. Future studies integrating these aspects promise to shed more light into this epigenetic phenomenon.     

Effects of mutagenized X chromosomes of Drosophila melanogaster on viability and fitness in males

Drosophila melanogaster males were treated with different doses of X-rays or ethyl methanesulfonate (EMS) and mated so that mutagenized X chromosomes could be recovered and tested for lethal mutations and for less drastic mutations affecting viability and other aspects of fitness. The lethals were detected in standard X-linked lethal tests. The less drastic mutations were detected in one generation tests for effects on viability and in multigeneration tests for effects on overall fitness. The Poisson-corrected frequencies of the lethal mutations increased linearly with dose for both X-rays and EMS. Based on the data, 1 Krad X-rays given acutely induces the same number of lethals as 0.55 mM EMS administered by feeding. For some of the X-ray and EMS doses, the mutagenized chromosomes that were nonlethal reduced the viability of their carriers by a small amount, but there was no discernable dose-effect relationship. However in every case where a viability effect was seen, the percentage reduction was less than the corresponding frequency of lethals. All the groups of mutagenized nonlethal chromosomes reduced overall fitness by a significant percentage. Wherever a meaningful comparison was possible, this reduction was 2-3 times the reduction in viability, but, as in the viability data, no dose-effect relationship was discernable.     

Global analysis of IL-2 target genes: identification of chromosomal clusters of expressed genes

T lymphocytes play a central role in controlling adaptive immune responses. IL-2 critically regulates both T cell growth and death and is involved in maintaining peripheral tolerance, but the molecules involved in these and other IL-2 actions are only partially known. We now provide a comprehensive compendium of the genes expressed in T cells and of those regulated by IL-2 based on a combination of DNA microarrays and serial analysis of gene expression (SAGE). The newly identified IL-2 target genes include many genes previously linked to apoptosis in other cellular systems that may contribute to IL-2-dependent survival functions. We also studied the mRNA expression of known regulators of signaling pathways for their induction in response to IL-2 in order to identify potential novel positive and/or negative feedback regulators of IL-2 signaling. We show that IL-2 regulates only a limited number of these genes. These include suppressors of cytokine signaling (SOCS) 1, SOCS2, dual-specificity phosphatase (DUSP) 5, DUSP6 and non-receptor type phosphatase-7 (PTPN7). Additionally, we provide evidence that many genes expressed in T cells locate in chromosomal clusters, and that select IL-2-regulated genes are located in at least two clusters, one at 5q31, a known cytokine gene cluster, and the other at 6p21.3, a region that contains genes encoding the tumor necrosis factor (TNF) superfamily members TNF, LT-alpha and LT-beta.