RESUMEN
This paper reports on a new role for mei-41 in cell cycle control during meiosis. This function is revealed by the requirement of mei-41 for the precocious anaphase observed in crossover-defective mutants. Normally in Drosophila oocytes, tension on the meiotic spindle causes a metaphase I arrest. This tension results because crossovers, and the resulting chiasmata, hold homologs together that are being pulled by kinetochore microtobules toward opposite spindle poles. In the absence of tension, such as in a recombination-defective mutant, metaphase arrest is not observed and meiosis proceeds through the two divisions. Here we show that in some recombination-defective mutants, the precocious anaphase requires the mei-41 gene product. For example, metaphase arrest is not observed in mei-218 mutants because of the severe reduction in crossing over. In mei-41 mei-218 double mutants, however, metaphase arrest was restored. The effect of mei-41 is dependent on double-strand break formation. Thus, in mutants that fail to initiate meiotic recombination the absence of mei-41 has no effect.
Asunto(s)
Anafase/genética , Proteínas de Ciclo Celular/metabolismo , Proteínas de Drosophila , Drosophila/genética , Proteínas Fúngicas/metabolismo , Proteínas de Saccharomyces cerevisiae , Animales , Ciclo Celular/genética , Quinasa 1 Reguladora del Ciclo Celular (Checkpoint 1) , Rotura Cromosómica/genética , Proteínas del Huevo/genética , Femenino , Proteínas Fúngicas/genética , Proteínas Fúngicas/farmacología , Péptidos y Proteínas de Señalización Intracelular , Meiosis/genética , Metafase/genética , Mutagénesis Sitio-Dirigida , Oocitos/efectos de los fármacos , Oocitos/metabolismo , Proteínas Quinasas/genética , Proteínas Serina-Treonina Quinasas , Recombinación Genética , Transducción de Señal/genética , Huso Acromático/genéticaRESUMEN
The segregation of homologous chromosomes from one another is the essence of meiosis. In many organisms, accurate segregation is ensured by the formation of chiasmata resulting from crossing over. Drosophila melanogaster females use this type of recombination-based system, but they also have mechanisms for segregating achiasmate chromosomes with high fidelity. We describe a P-element mutagenesis and screen in a sensitized genetic background to detect mutations that impair meiotic chromosome pairing, recombination, or segregation. Our screen identified two new recombination-deficient mutations: mei-P22, which fully eliminates meiotic recombination, and mei-P26, which decreases meiotic exchange by 70% in a polar fashion. We also recovered an unusual allele of the ncd gene, whose wild-type product is required for proper structure and function of the meiotic spindle. However, the screen yielded primarily mutants specifically defective in the segregation of achiasmate chromosomes. Although most of these are alleles of previously undescribed genes, five were in the known genes alphaTubulin67C, CycE, push, and Trl. The five mutations in known genes produce novel phenotypes for those genes.
Asunto(s)
Elementos Transponibles de ADN/genética , Drosophila melanogaster/genética , Genes de Insecto , Meiosis/genética , Animales , Cromosomas/genética , ADN/genética , Femenino , Heterocromatina , Masculino , Metafase , Mutación , No Disyunción Genética , Fenotipo , Recombinación Genética , Proyectos de Investigación , Cromosoma X/genéticaRESUMEN
BACKGROUND: Drosophila embryogenesis is initiated by 13 rapid syncytial mitotic divisions that do not require zygotic gene activity. This maternally directed cleavage phase of development terminates at the midblastula transition (MBT), at which point the cell cycle slows dramatically, membranes surround the cortical nuclei to form a cellular blastoderm, and zygotic gene expression is first required. RESULTS: We show that embryos lacking Mei-41, a Drosophila homologue of the ATM tumor suppressor, proceed through unusually short syncytial mitoses, fail to terminate syncytial division following mitosis 13, and degenerate without forming cells. A similar cleavage-stage arrest is produced by mutations in grapes, which encodes a homologue of the Checkpoint-1 kinase. We present biochemical, cytological and genetic data indicating that Mei-41 and Grapes are components of a conserved DNA-replication/damage checkpoint pathway that triggers inhibitory phosphorylation of the Cdc2 kinase and mediates resistance to replication inhibitors and DNA-damaging agents. This pathway is nonessential during postembryonic development, but it is required to terminate the cleavage stage at the MBT. Cyclins are required for Cdc2 kinase activity, and mutations in cyclin A and cyclin B bypass the requirement for mei-41 at the MBT. These mutations do not restore wild-type syncytial cell-cycle timing or the embryonic replication checkpoint, however, suggesting that Mei-41-mediated inhibition of Cdc2 has an additional essential function at the MBT. CONCLUSIONS: The Drosophila DNA-replication/damage checkpoint pathway can be activated by externally triggered DNA damage or replication defects throughout the life cycle, and under laboratory conditions this inducible function is nonessential. During early embryogenesis, however, this pathway is activated by developmental cues and is required for the transition from maternal to zygotic control of development at the MBT.
Asunto(s)
Blastocisto/citología , Proteínas de Drosophila , Drosophila melanogaster/embriología , Regulación del Desarrollo de la Expresión Génica/fisiología , Genes de Insecto , Genes Supresores de Tumor , Proteínas de Insectos/fisiología , Proteínas Serina-Treonina Quinasas , Proteínas/fisiología , Factores Asociados con la Proteína de Unión a TATA , Factor de Transcripción TFIID , Factores de Transcripción TFII , Animales , Afidicolina/farmacología , Proteínas de la Ataxia Telangiectasia Mutada , Proteína Quinasa CDC2/antagonistas & inhibidores , Proteína Quinasa CDC2/metabolismo , Ciclo Celular , Proteínas de Ciclo Celular , División Celular , Quinasa 1 Reguladora del Ciclo Celular (Checkpoint 1) , Ciclina A/genética , Ciclina A/fisiología , Ciclina B/genética , Ciclina B/fisiología , Daño del ADN , Replicación del ADN/fisiología , Proteínas de Unión al ADN/genética , Proteínas de Unión al ADN/fisiología , Drosophila melanogaster/genética , Embrión no Mamífero/citología , Femenino , Genes Letales , Humanos , Infertilidad Femenina/genética , Proteínas de Insectos/genética , Masculino , Modelos Biológicos , Proteínas Nucleares , Fosforilación , Proteínas Quinasas/genética , Proteínas Quinasas/fisiología , Procesamiento Proteico-Postraduccional/fisiología , Especificidad de la Especie , Factores de Tiempo , Factor de Transcripción TFIIH , Factores de Transcripción/fisiología , Proteínas Supresoras de TumorRESUMEN
We previously reported evidence that the so-called reactivity level, a peculiar cellular state of oocytes that regulates the frequency of transposition of I factor, a LINE element-like retrotransposon, might be one manifestation of a DNA repair system. In this article, we report data showing that the reactivity level is correlated with the frequency of crossing over, at least on the X chromosome and on the pericentromeric region of the third chromosome. Moreover, a check for X-chromosome losses and recessive lethals produced after gamma irradiation in flies with different reactivity levels, but common genetic backgrounds, brings more precise evidence for the relationship between reactivity levels and DNA repair. Those results support the existence of a repair-recombination system whose efficiency is modulated by endogenous and environmental factors. The implications of this biological system in connecting genomic variability and environment may shed new lights on adaptative mechanisms. We propose to call it VAMOS for variability modulation system.
Asunto(s)
Reparación del ADN/genética , Drosophila melanogaster/genética , Drosophila melanogaster/metabolismo , Oocitos/metabolismo , Recombinación Genética , Envejecimiento/genética , Animales , Intercambio Genético , Drosophila melanogaster/efectos de la radiación , Femenino , Rayos gamma , Genes de Insecto , Genes Letales , Genes Recesivos , Variación Genética , Masculino , Oocitos/efectos de la radiación , Retroelementos , Cromosoma X/genéticaRESUMEN
In the I-R system of hybrid dysgenesis in Drosophila melanogaster, the transposition frequency of I factor, a LINE element-like retrotransposon, is regulated by the reactivity level of the R mother. This reactivity is a cellular state maternally inherited but chromosomally determined, which has been shown to undergo heritable, cumulative and reversible changes with aging and some environmental conditions. We propose the hypothesis that this reactivity level is one manifestation of an inducible repair-recombination system whose biological role might be analogous to the SOS response in bacteria. In this paper, we show that inhibitors of DNA synthesis and gamma rays enhance the reactivity level in a very similar way. This enhancement is heritable, cumulative and reversible.
Asunto(s)
Antimetabolitos/farmacología , Reparación del ADN , Drosophila melanogaster/genética , Metotrexato/farmacología , Recombinación Genética , Uracilo/análogos & derivados , Envejecimiento , Animales , Radioisótopos de Cesio , Reparación del ADN/efectos de los fármacos , Reparación del ADN/efectos de la radiación , Relación Dosis-Respuesta a Droga , Femenino , Fertilidad/genética , Rayos gamma , Cinética , Masculino , Oocitos/fisiología , Oviposición/efectos de la radiación , Recombinación Genética/efectos de los fármacos , Recombinación Genética/efectos de la radiación , Retroelementos , Factores de Tiempo , Uracilo/farmacologíaRESUMEN
In a previous paper, we reported that the reactivity level, which regulates the frequency of transposition of I factor, a LINE element-like retrotransposon, is enhanced by the same agents that induce the SOS response in Escherichia coli. In this report, we describe experimental evidence that, for identical genotypes, the reactivity levels correlate with the sensitivity of oogenesis to gamma rays, measured by the number of eggs laid and by frequency of dominant lethals. This strongly supports the hypothesis that the reactivity level is one manifestation of an inducible DNA repair system taking place in the female germ line of Drosophila melanogaster. The implications of this finding for the understanding of the regulation of I factor are discussed and some other possible biological roles of this system are outlined.