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Yamashita YM. Asymmetric Stem Cell Division and Germline Immortality. Annu Rev Genet 2023; 57:181-199. [PMID: 37552892 DOI: 10.1146/annurev-genet-022123-040039] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 08/10/2023]
Abstract
Germ cells are the only cell type that is capable of transmitting genetic information to the next generation, which has enabled the continuation of multicellular life for the last 1.5 billion years. Surprisingly little is known about the mechanisms supporting the germline's remarkable ability to continue in this eternal cycle, termed germline immortality. Even unicellular organisms age at a cellular level, demonstrating that cellular aging is inevitable. Extensive studies in yeast have established the framework of how asymmetric cell division and gametogenesis may contribute to the resetting of cellular age. This review examines the mechanisms of germline immortality-how germline cells reset the aging of cells-drawing a parallel between yeast and multicellular organisms.
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Affiliation(s)
- Yukiko M Yamashita
- Whitehead Institute for Biomedical Research, Howard Hughes Medical Institute, and Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA;
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Kindelay SM, Maggert KA. Under the magnifying glass: The ups and downs of rDNA copy number. Semin Cell Dev Biol 2023; 136:38-48. [PMID: 35595601 PMCID: PMC9976841 DOI: 10.1016/j.semcdb.2022.05.006] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2022] [Revised: 04/27/2022] [Accepted: 05/09/2022] [Indexed: 11/22/2022]
Abstract
The ribosomal DNA (rDNA) in Drosophila is found as two additive clusters of individual 35 S cistrons. The multiplicity of rDNA is essential to assure proper translational demands, but the nature of the tandem arrays expose them to copy number variation within and between populations. Here, we discuss means by which a cell responds to insufficient rDNA copy number, including a historical view of rDNA magnification whose mechanism was inferred some 35 years ago. Recent work has revealed that multiple conditions may also result in rDNA loss, in response to which rDNA magnification may have evolved. We discuss potential models for the mechanism of magnification, and evaluate possible consequences of rDNA copy number variation.
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Affiliation(s)
- Selina M Kindelay
- Genetics Graduate Interdisciplinary Program, The University of Arizona, Tucson, AZ 85724, USA
| | - Keith A Maggert
- Genetics Graduate Interdisciplinary Program, The University of Arizona, Tucson, AZ 85724, USA; Department of Cellular and Molecular Medicine, The University of Arizona, Tucson, AZ 85724, USA.
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Nelson JO, Watase GJ, Warsinger-Pepe N, Yamashita YM. Mechanisms of rDNA Copy Number Maintenance. Trends Genet 2019; 35:734-742. [PMID: 31395390 DOI: 10.1016/j.tig.2019.07.006] [Citation(s) in RCA: 54] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2019] [Revised: 06/28/2019] [Accepted: 07/12/2019] [Indexed: 12/21/2022]
Abstract
rDNA, the genes encoding the RNA components of ribosomes (rRNA), are highly repetitive in all eukaryotic genomes, containing 100s to 1000s of copies, to meet the demand for ribosome biogenesis. rDNA genes are arranged in large stretches of tandem repeats, forming loci that are highly susceptible to copy loss due to their repetitiveness and active transcription throughout the cell cycle. Despite this inherent instability, rDNA copy number is generally maintained within a particular range in each species, pointing to the presence of mechanisms that maintain rDNA copy number in a homeostatic range. In this review, we summarize the current understanding of these maintenance mechanisms and how they sustain rDNA copy number throughout populations.
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Affiliation(s)
- Jonathan O Nelson
- Life Sciences Institute, University of Michigan, Ann Arbor, MI, USA; Howard Hughes Medical Institute, University of Michigan, Ann Arbor, MI, USA
| | - George J Watase
- Life Sciences Institute, University of Michigan, Ann Arbor, MI, USA; Howard Hughes Medical Institute, University of Michigan, Ann Arbor, MI, USA
| | - Natalie Warsinger-Pepe
- Life Sciences Institute, University of Michigan, Ann Arbor, MI, USA; Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI, USA
| | - Yukiko M Yamashita
- Life Sciences Institute, University of Michigan, Ann Arbor, MI, USA; Cell and Developmental Biology, University of Michigan, Ann Arbor, MI, USA; Howard Hughes Medical Institute, University of Michigan, Ann Arbor, MI, USA.
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Tritto P, Palumbo V, Micale L, Marzulli M, Bozzetti MP, Specchia V, Palumbo G, Pimpinelli S, Berloco M. Loss of Pol32 in Drosophila melanogaster causes chromosome instability and suppresses variegation. PLoS One 2015; 10:e0120859. [PMID: 25826374 PMCID: PMC4380491 DOI: 10.1371/journal.pone.0120859] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/22/2014] [Accepted: 01/27/2015] [Indexed: 11/29/2022] Open
Abstract
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 mutantssuppress position effect variegation, suggesting a role for Pol32 in chromatin architecture.
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Affiliation(s)
- Patrizia Tritto
- Dipartimento di Biologia, Università degli Studi di Bari “Aldo Moro”, 70125 Bari, Italy
| | - Valeria Palumbo
- Dipartimento di Biologia e Biotecnologie “C. Darwin”, Università degli Studi di Roma “La Sapienza”, 00185 Roma, Italy
| | - Lucia Micale
- IRCCS Casa Sollievo Della Sofferenza Hospital, 71013 San Giovanni Rotondo, Italy
| | - Marco Marzulli
- Department of Microbiology and Molecular Genetics, University of Pittsburgh School of Medicine, Pittsburgh, PA 15219, United States of America
| | - Maria Pia Bozzetti
- Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, Università del Salento, 73100 Lecce, Italy
| | - Valeria Specchia
- Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, Università del Salento, 73100 Lecce, Italy
| | - Gioacchino Palumbo
- Dipartimento di Biologia, Università degli Studi di Bari “Aldo Moro”, 70125 Bari, Italy
| | - Sergio Pimpinelli
- Istituto Pasteur—Fondazione Cenci Bolognetti and Dipartimento di Biologia e Biotecnologie “C. Darwin”, Università degli Studi di Roma “La Sapienza”, 00185 Roma, Italy
| | - Maria Berloco
- Dipartimento di Biologia, Università degli Studi di Bari “Aldo Moro”, 70125 Bari, Italy
- * E-mail:
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Terracol R. Differential magnification of rDNA gene types in bobbed mutants of Drosophila melanogaster. ACTA ACUST UNITED AC 2009; 208:168-76. [PMID: 18814382 DOI: 10.1007/bf00330438] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
In Drosophila melanogaster a partial loss of ribosomal genes leads to the bobbed phenotype. Magnification is a heritable increase in rDNA that may occur in males carrying a deleted X chromosome with a strong bobbed phenotype. The restriction patterns of X chromosome total rDNA, insertions and spacers from magnified bobbed strains were compared with those of the original bobbed mutations. It was found that magnification modifies restriction patterns and differentially affects gene types, increasing specific genes lacking insertions (INS-). Increases in copy number of genes with type I insertions are generally lower than the total number of INS- genes while type II insertion genes are not perceptibly increased. The recovery of homogeneous progeny from a single premagnified male indicates that the magnification event might take place and become stable very early in the germ line, arguing against magnification being due to extrachromosomal amplification. Additionally, some gene types increase 3.5-fold while others are eliminated, indicating that they could not result from a single unequal cross-over. These results are in good agreement with the existence of partial clustering of rDNA genes according to type, and suggest that magnification could result from local amplification of genes.
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Garcia V, Furuya K, Carr AM. Identification and functional analysis of TopBP1 and its homologs. DNA Repair (Amst) 2005; 4:1227-39. [PMID: 15897014 DOI: 10.1016/j.dnarep.2005.04.001] [Citation(s) in RCA: 137] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2005] [Revised: 04/11/2005] [Accepted: 04/11/2005] [Indexed: 01/18/2023]
Abstract
The multiple BRCT-domain protein TopBP1 and its yeast homologs have been implicated in many aspects of DNA metabolism, but their molecular functions remain elusive. In this review, we first summarise how the yeast homologs were identified and characterised. We next review the data available from metazoan systems and finally draw parallels with the yeast models. TopBP1 plays important functions in the initiation of DNA replication in all organisms and participates in checkpoint responses both within S phase and following DNA damage. In metazoan systems there is accumulating evidence for additional roles in transcriptional regulation that have not been reported in yeast. Overall, TopBP1 appears to play a key role in integrating different aspects of DNA metabolism, but the mechanistic basis for this remains to be fully explained.
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Affiliation(s)
- Valerie Garcia
- Genome Damage and Stability Center, University of Sussex, Brighton, Sussex BN1 9RQ, UK
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Yamamoto RR, Axton JM, Yamamoto Y, Saunders RD, Glover DM, Henderson DS. The Drosophila mus101 gene, which links DNA repair, replication and condensation of heterochromatin in mitosis, encodes a protein with seven BRCA1 C-terminus domains. Genetics 2000; 156:711-21. [PMID: 11014818 PMCID: PMC1461266 DOI: 10.1093/genetics/156.2.711] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
The mutagen-sensitive-101 (mus101) gene of Drosophila melanogaster was first identified 25 years ago through mutations conferring larval hypersensitivity to DNA-damaging agents. Other alleles of mus101 causing different phenotypes were later isolated: a female sterile allele results in a defect in a tissue-specific form of DNA synthesis (chorion gene amplification) and lethal alleles cause mitotic chromosome instability that can be observed genetically and cytologically. The latter phenotype presents as a striking failure of mitotic chromosomes of larval neuroblasts to undergo condensation of pericentric heterochromatic regions, as we show for a newly described mutant carrying lethal allele mus101(lcd). To gain further insight into the function of the Mus101 protein we have molecularly cloned the gene using a positional cloning strategy. We report here that mus101 encodes a member of the BRCT (BRCA1 C terminus) domain superfamily of proteins implicated in DNA repair and cell cycle checkpoint control. Mus101, which contains seven BRCT domains distributed throughout its length, is most similar to human TopBP1, a protein identified through its in vitro association with DNA topoisomerase IIbeta. Mus101 also shares sequence similarity with the fission yeast Rad4/Cut5 protein required for repair, replication, and checkpoint control, suggesting that the two proteins may be functional homologs.
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Affiliation(s)
- R R Yamamoto
- CRC Cell Cycle Genetics Group, Department of Genetics, University of Cambridge, Cambridge CB2 3EH, United Kingdom
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Abstract
Preservation of the structural integrity of DNA in any organism is crucial to its health and survival. Such preservation is achieved by an extraordinary cellular arsenal of damage surveillance and repair functions, many of which are now being defined at the gene and protein levels. Mutants hypersensitive to the killing effects of DNA-damaging agents have been instrumental in helping to identify DNA repair-related genes and to elucidate repair mechanisms. In Drosophila melanogaster, such strains are generally referred to as mutagen-sensitive (mus) mutants and currently define more than 30 genetic loci. Whereas most mus mutants have been recovered on the basis of hypersensitivity to the monofunctional alkylating agent methyl methanesulfonate, they nevertheless constitute a phenotypically diverse group, with many mutants having effects beyond mutagen sensitivity. These phenotypes include meiotic dysfunctions, somatic chromosome instabilities, chromatin abnormalities, and cell proliferation defects. Within the last few years numerous mus and other DNA repair-related genes of Drosophila have been molecularly cloned, providing new insights into the functions of these genes. This article outlines strategies for isolating mus mutations and reviews recent advances in the Drosophila DNA repair field, emphasizing mutant analysis and gene cloning.
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Affiliation(s)
- D S Henderson
- Department of Anatomy and Physiology, University of Dundee, Dundee, DD1 4HN, Scotland, United Kingdom
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Sekelsky JJ, Burtis KC, Hawley RS. Damage control: the pleiotropy of DNA repair genes in Drosophila melanogaster. Genetics 1998; 148:1587-98. [PMID: 9560378 PMCID: PMC1460071 DOI: 10.1093/genetics/148.4.1587] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Affiliation(s)
- J J Sekelsky
- Department of Genetics, Section of Molecular and Cellular Biology, University of California, Davis 95616, USA
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Banga SS, Yamamoto AH, Mason JM, Boyd JB. Molecular cloning of mei-41, a gene that influences both somatic and germline chromosome metabolism of Drosophila melanogaster. MOLECULAR & GENERAL GENETICS : MGG 1995; 246:148-55. [PMID: 7862085 DOI: 10.1007/bf00294677] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The mei-41 gene of Drosophila melanogaster plays an essential role in meiosis, in the maintenance of somatic chromosome stability, in postreplication repair and in DNA double-strand break repair. This gene has been cytogenetically localized to polytene chromosome bands 14C4-6 using available chromosomal aberrations. About 60 kb of DNA sequence has been isolated following a bidirectional chromosomal walk that extends over the cytogenetic interval 14C1-6. The breakpoints of chromosomal aberrations identified within that walk establish that the entire mei-41 gene has been cloned. Two independently derived mei-41 mutants have been shown to carry P insertions within a single 2.2 kb fragment of the walk. Since revertants of those mutants have lost the P element sequences, an essential region of the mei-41 gene is present in that fragment. A 10.5 kb genomic fragment that spans the P insertion sites has been found to restore methyl methanesulfonate resistance and female fertility of the mei-41D3 mutants. The results demonstrate that all the sequences required for the proper expression of the mei-41 gene are present on this genomic fragment. This study provides the foundation for molecular analysis of a function that is essential for chromosome stability in both the germline and somatic cells.
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Affiliation(s)
- S S Banga
- Section of Molecular and Cellular Biology, University of California Davis 95616
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Butler DK, Metzenberg RL. Amplification of the nucleolus organizer region during the sexual phase of Neurospora crassa. Chromosoma 1993; 102:519-25. [PMID: 8243164 DOI: 10.1007/bf00368345] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
Previously we have shown that the nucleolus organizer region (NOR) of Neurospora crassa displays frequent size changes during crosses. In these initial studies, we observed that decreases in NOR size are far more common than increases. Here, we have investigated the inheritance of NOR size in a strain with an unusually small NOR. We call this strain SNO for small nucleolus organizer. We found that progeny that inherit their rDNA from SNO receive either an NOR that is larger than that of SNO or, rarely, the same size, but never an NOR that is smaller than that of SNO. The number of progeny that inherit their NOR from SNO is not significantly different from the number that inherit their NOR from the other parent in the cross. This argues against the idea that the failure to find progeny with NORs smaller than that of SNO is due to inviability of spores carrying such an NOR, or that it is due to unconscious bias by the experimenter against isolating such spores. These results can most easily be explained by a combination of unequal sister chromatid exchanges in the rDNA, or sister chromatid conversion, coupled with selection against nuclei harboring small NORs during the premeiotic phase of the Neurospora life cycle. Other, less conventional, explanations are also possible, such as "directed" increase in the target NOR without corresponding loss at some other NOR.
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Affiliation(s)
- D K Butler
- Department of Biomolecular Chemistry, University of Wisconsin, Madison 53706
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Komma DJ, Graves H, Endow SA. Mutant alleles of the meiotic locus, mei-9, differ in degree of effects on rod chromosome magnification and ring chromosome transmission in Drosophila. Genet Res (Camb) 1989; 53:155-61. [PMID: 2504645 DOI: 10.1017/s0016672300028111] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Two mutant alleles of the meiotic locus, mei-9, have been examined for their effect on magnification of a rod Xbb chromosome and transmission of a ring Xbb chromosome under magnifying conditions. Our results indicate that the effects of these two mutations are allele-specific: mei-9a strongly inhibits both rod chromosome magnification and ring chromosome loss under magnifying conditions, while mei-9b has a smaller inhibitory effect on rod chromosome magnification and on the transmission of ring chromosomes under magnifying conditions. These observations can be explained by a difference in leakiness between the two alleles. Our results demonstrate that mutants defective in excision repair and repair replication inhibit ribosomal gene magnification. This suggests that a component of the excision repair pathway is involved in the process of magnification.
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Marcus CH, Zitron AE, Wright DA, Hawley RS. Autosomal modifiers of the bobbed phenotype are a major component of the rDNA magnification paradox in Drosophila melanogaster. Genetics 1986; 113:305-19. [PMID: 3087814 PMCID: PMC1202840 DOI: 10.1093/genetics/113.2.305] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
rDNA magnification in Drosophila melanogaster is defined experimentally as the ability of bb/Ybb- males to produce exceptional progeny that are wild type with respect to rDNA associated phenotypes. Here, we show that some of these bobbed-plus progeny result not from genetic reversion at the bb locus but rather from variants at two or more autosomal loci that ameliorate the bobbed phenotype of rDNA deficient males in Drosophila. In doing so we resolve several aspects of a long-standing paradox concerning the phenomenon of rDNA magnification. This problem arose from the use of two genetic assays, which were presumed to be identical, but paradoxically, produced conflicting data on both the kinetics of reversion and the stability of magnified bb+ chromosomes. We resolve this problem by demonstrating that in one assay bobbed-plus progeny arise primarily by genetic reversion at the bobbed locus, whereas in the other assay bobbed-plus progeny arise both by reversion and by an epistatic effect of autosomal modifiers on the bobbed phenotype. We further show that such modifiers can facilitate the appearance of phenotypically bobbed-plus progeny even under conditions where genetic reversion is blocked by magnification defective mutants. Finally, we present a speculative model relating the action of these modifiers to the large increases in rDNA content observed in males undergoing magnification.
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