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Lyu L, Asghar U, Fu J, Gao Y, Zhang X, Al-Farraj SA, Chen Z, Gao F. Comparative analysis of single-cell genome sequencing techniques toward the characterization of germline and somatic genomes in ciliated protists. Eur J Protistol 2023; 88:125969. [PMID: 36822126 DOI: 10.1016/j.ejop.2023.125969] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2022] [Revised: 01/31/2023] [Accepted: 02/05/2023] [Indexed: 02/12/2023]
Abstract
Ciliated protists contain both germline micronucleus (MIC) and somatic macronucleus (MAC) in a single cytoplasm. Programmed genome rearrangements occur in ciliates during sexual processes, and the extent of rearrangements varies dramatically among species, which lead to significant differences in genomic architectures. However, genomic sequences remain largely unknown for most ciliates due to the difficulty in culturing and in separating the germline from the somatic genome in a single cell. Single-cell whole genome amplification (WGA) has emerged as a powerful technology to characterize the genomic heterogeneity at the single-cell level. In this study, we compared two single-cell WGA, multiple displacement amplification (MDA) and multiple annealing and looping-based amplification cycles (MALBAC) in characterizing the germline and somatic genomes in ciliates with different genomic architectures. Our results showed that: 1) MALBAC exhibits strong amplification bias towards MAC genome while MDA shows bias towards MIC genome of ciliates with extensively fragmented MAC genome; 2) both MDA and MALBAC could amplify MAC genome more efficiently in ciliates with moderately fragmented MAC genome. Moreover, we found that more sample replicates could help to obtain more genomic data. Our work provides a reference for selecting the appropriate method to characterize germline and somatic genomes of ciliates.
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Affiliation(s)
- Liping Lyu
- Institute of Evolution and Marine Biodiversity, Ocean University of China, Qingdao 266003, China; Key Laboratory of Mariculture (OUC), Ministry of Education, Qingdao 266003, China
| | - Usman Asghar
- Institute of Evolution and Marine Biodiversity, Ocean University of China, Qingdao 266003, China; Key Laboratory of Mariculture (OUC), Ministry of Education, Qingdao 266003, China
| | - Jinyu Fu
- Institute of Evolution and Marine Biodiversity, Ocean University of China, Qingdao 266003, China; Key Laboratory of Mariculture (OUC), Ministry of Education, Qingdao 266003, China
| | - Yunyi Gao
- Institute of Evolution and Marine Biodiversity, Ocean University of China, Qingdao 266003, China; Key Laboratory of Mariculture (OUC), Ministry of Education, Qingdao 266003, China
| | - Xue Zhang
- Institute of Evolution and Marine Biodiversity, Ocean University of China, Qingdao 266003, China; Key Laboratory of Mariculture (OUC), Ministry of Education, Qingdao 266003, China
| | - Saleh A Al-Farraj
- Zoology Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
| | - Zigui Chen
- Institute of Evolution and Marine Biodiversity, Ocean University of China, Qingdao 266003, China.
| | - Feng Gao
- Institute of Evolution and Marine Biodiversity, Ocean University of China, Qingdao 266003, China; Key Laboratory of Mariculture (OUC), Ministry of Education, Qingdao 266003, China; Laoshan Laboratory, Qingdao 266237, China.
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Kloc M, Kubiak JZ, Ghobrial RM. Natural genetic engineering: A programmed chromosome/DNA elimination. Dev Biol 2022; 486:15-25. [DOI: 10.1016/j.ydbio.2022.03.008] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2021] [Revised: 02/16/2022] [Accepted: 03/17/2022] [Indexed: 11/03/2022]
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Gene-rich germline-restricted chromosomes in black-winged fungus gnats evolved through hybridization. PLoS Biol 2022; 20:e3001559. [PMID: 35213540 PMCID: PMC8906591 DOI: 10.1371/journal.pbio.3001559] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2021] [Revised: 03/09/2022] [Accepted: 01/28/2022] [Indexed: 12/20/2022] Open
Abstract
Germline-restricted DNA has evolved in diverse animal taxa and is found in several vertebrate clades, nematodes, and flies. In these lineages, either portions of chromosomes or entire chromosomes are eliminated from somatic cells early in development, restricting portions of the genome to the germline. Little is known about why germline-restricted DNA has evolved, especially in flies, in which 3 diverse families, Chironomidae, Cecidomyiidae, and Sciaridae, carry germline-restricted chromosomes (GRCs). We conducted a genomic analysis of GRCs in the fungus gnat Bradysia (Sciara) coprophila (Diptera: Sciaridae), which has 2 large germline-restricted “L” chromosomes. We sequenced and assembled the genome of B. coprophila and used differences in sequence coverage and k-mer frequency between somatic and germline tissues to identify GRC sequence and compare it to the other chromosomes in the genome. We found that the GRCs in B. coprophila are large, gene rich, and have many genes with divergent homologs on other chromosomes in the genome. We also found that 2 divergent GRCs exist in the population we sequenced. GRC genes are more similar in sequence to genes from another Dipteran family (Cecidomyiidae) than to homologous genes from Sciaridae. This unexpected finding suggests that these chromosomes likely arose in Sciaridae through hybridization with a related lineage. These results provide a foundation from which to answer many questions about the evolution of GRCs in Sciaridae, such as how this hybridization event resulted in GRCs and what features on these chromosomes cause them to be restricted to the germline. Germ-line restricted chromosomes are eliminated from all somatic tissues while being retained in the germline. This study of the evolutionary origin of such chromosomes in fungus gnats reveals that they are most similar to a fly of a different family, suggesting an ancient allopolyploidization origin for these peculiar chromosomes.
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Drotos KH, Zagoskin MV, Kess T, Gregory TR, Wyngaard GA. Throwing away DNA: programmed downsizing in somatic nuclei. Trends Genet 2022; 38:483-500. [DOI: 10.1016/j.tig.2022.02.003] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2021] [Revised: 01/30/2022] [Accepted: 02/01/2022] [Indexed: 11/25/2022]
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5
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Programmed DNA elimination: silencing genes and repetitive sequences in somatic cells. Biochem Soc Trans 2021; 49:1891-1903. [PMID: 34665225 DOI: 10.1042/bst20190951] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2021] [Revised: 09/25/2021] [Accepted: 09/28/2021] [Indexed: 12/30/2022]
Abstract
In a multicellular organism, the genomes of all cells are in general the same. Programmed DNA elimination is a notable exception to this genome constancy rule. DNA elimination removes genes and repetitive elements in the germline genome to form a reduced somatic genome in various organisms. The process of DNA elimination within an organism is highly accurate and reproducible; it typically occurs during early embryogenesis, coincident with germline-soma differentiation. DNA elimination provides a mechanism to silence selected genes and repeats in somatic cells. Recent studies in nematodes suggest that DNA elimination removes all chromosome ends, resolves sex chromosome fusions, and may also promote the birth of novel genes. Programmed DNA elimination processes are diverse among species, suggesting DNA elimination likely has evolved multiple times in different taxa. The growing list of organisms that undergo DNA elimination indicates that DNA elimination may be more widespread than previously appreciated. These various organisms will serve as complementary and comparative models to study the function, mechanism, and evolution of programmed DNA elimination in metazoans.
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Hodson CN, Ross L. Evolutionary Perspectives on Germline-Restricted Chromosomes in Flies (Diptera). Genome Biol Evol 2021; 13:evab072. [PMID: 33890671 PMCID: PMC8245193 DOI: 10.1093/gbe/evab072] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 04/06/2021] [Indexed: 12/24/2022] Open
Abstract
In some eukaryotes, germline soma differentiation involves elimination of parts of the genome from somatic cells. The portions of the genome restricted to the germline often contain genes that play a role in development and function of the germline. Lineages with germline-restricted DNA are taxonomically diverse, and the size of the germline-restricted genome varies substantially. Unfortunately, few of these lineages have been studied in detail. As a result, we understand little about the general evolutionary forces that drive the origin and maintenance of germline-restricted DNA. One of the taxonomic groups where germline-restricted DNA has been poorly studied are the flies (Diptera). In three Dipteran families, Chironomidae, Cecidomyiidae, and Sciaridae, entire chromosomes are eliminated from somatic cells early in embryonic development. Germline-restricted chromosomes are thought to have evolved independently in the Dipteran families and their size, number, and transmission patterns vary between families. Although there is a wealth of cytological studies on these chromosomes in flies, almost no genomic studies have been undertaken. As a result, very little is known about how and why they evolved and what genes they encode. This review summarizes the literature on germline-restricted chromosomes in Diptera, discusses hypotheses for their origin and function, and compares germline-restricted DNA in Diptera to other eukaryotes. Finally, we discuss why Dipteran lineages represent a promising system for the study of germline-restricted chromosomes and propose future avenues of research on this topic.
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Affiliation(s)
- Christina N Hodson
- Institute of Evolutionary Biology, University of Edinburgh, United Kingdom
| | - Laura Ross
- Institute of Evolutionary Biology, University of Edinburgh, United Kingdom
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7
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Danzmann RG, Norman JD, Rondeau EB, Messmer AM, Kent MP, Lien S, Igboeli O, Fast MD, Koop BF. A genetic linkage map for the salmon louse (Lepeophtheirus salmonis): evidence for high male:female and inter-familial recombination rate differences. Mol Genet Genomics 2018; 294:343-363. [PMID: 30460550 DOI: 10.1007/s00438-018-1513-7] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2018] [Accepted: 10/15/2018] [Indexed: 01/28/2023]
Abstract
A salmon louse (Lepeophtheirus salmonis salmonis) genetic linkage map was constructed to serve as a genomic resource for future investigations into the biology of this important marine parasitic copepod species, and to provide insights into the inheritance patterns of genetic markers in this species. SNP genotyping of 8 families confirmed the presence of 15 linkage groups based upon the assignment of 93,773 markers. Progeny sample size weight adjusted map sizes in males (with the exception of SL12 and SL15) ranged in size from 96.50 cM (SL11) to 134.61 cM (SL06), and total combined map steps or bins ranged from 143 (SL09) to 203 (SL13). The SL12 male map was the smallest linkage group with a weight-averaged size of 3.05 cM with 6 recombination bins. Male:female specific recombination rate differences are 10.49:1 and represent one of the largest reported sex-specific differences for any animal species. Recombination ratio differences (M:F) ranged from 1.0 (SL12) to 29:1 (SL15). The number of markers exhibiting normal Mendelian segregation within the sex linkage group SL15 was extremely low (N = 80) in comparison to other linkage groups genotyped [range: 1459 (SL12)-10206 markers (SL05)]. Re-evaluation of Mendelian inheritance patterns of markers unassigned to any mapping parent according to hemizygous segregation patterns (models presented) identified matches for many of these markers to hemizygous patterns. The greatest proportion of these markers assigned to SL15 (N increased to 574). Inclusion of the hemizygous markers revised SL15 sex-specific recombination rate differences to 28:1. Recombination hot- and coldspots were identified across all linkage groups with all linkage groups possessing multiple peaks. Nine of 13 linkage groups evaluated possessed adjacent domains with hot-coldspot transitional zones. The most common pattern was for one end of the linkage to show elevated recombination in addition to internal regions. For SL01 and SL06, however, a terminal region with high recombination was not evident while a central domain possessing extremely high-recombination levels was present. High levels of recombination were weakly coupled to higher levels of SNP variation within domains, but this association was very strong for the central domains of SL01 and SL06. From the pooled paternal half-sib lots (several virgin females placed with 1 male), only 1 or two surviving family lots were obtained. Surviving families possessed parents where both the male and female possessed either inherently low or high recombination rates. This study provides insight into the organization of the sea louse genome, and describes large differences in recombination rate that exist among individuals of the same sex, and between the sexes. These differences in recombination rate may be coupled to the capabilities of this species to adapt to environmental and pharmaceutical treatments, given that family survivorship appears to be enhanced when parents have similar recombination levels.
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Affiliation(s)
- Roy G Danzmann
- Department of Integrative Biology, College of Biological Sciences, University of Guelph, 50 Stone Road East, Guelph, ON, N1G 2W1, Canada.
| | - Joseph D Norman
- Department of Integrative Biology, College of Biological Sciences, University of Guelph, 50 Stone Road East, Guelph, ON, N1G 2W1, Canada.,The Hospital for Sick Children, 686 Bay St., Toronto, ON, M5G 0A4, Canada
| | - Eric B Rondeau
- Department of Biology, Centre for Biomedical Research, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8W 3N5, Canada
| | - Amber M Messmer
- Department of Biology, Centre for Biomedical Research, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8W 3N5, Canada
| | - Matthew P Kent
- Department of Animal and Aquacultural Sciences, Centre for Integrative Genetics (CIGENE), Norwegian University of Life Sciences, 1432, As, Norway
| | - Sigbjørn Lien
- Department of Animal and Aquacultural Sciences, Centre for Integrative Genetics (CIGENE), Norwegian University of Life Sciences, 1432, As, Norway
| | - Okechukwu Igboeli
- Department of Pathology and Microbiology, Atlantic Veterinary College, University of Prince Edward Island, 550 University Ave., Charlottetown, PEI, C1A 4P3, Canada
| | - Mark D Fast
- Department of Pathology and Microbiology, Atlantic Veterinary College, University of Prince Edward Island, 550 University Ave., Charlottetown, PEI, C1A 4P3, Canada
| | - Ben F Koop
- Department of Biology, Centre for Biomedical Research, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8W 3N5, Canada
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8
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Grishanin AK, Zagoskin MV. Chromatin Diminution in Cyclops kolensis Lill. (Copepoda, Crustacea) as a Radical Way to Inactivate Redundant Genome in Somatic Cells. Cytogenet Genome Res 2018; 156:165-172. [PMID: 30376670 DOI: 10.1159/000494157] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 05/03/2018] [Indexed: 12/21/2022] Open
Abstract
Chromatin diminution (CD) is a phenomenon of programmed DNA elimination which takes place in early embryogenesis in some eukaryotes. The mechanism and biological role of CD remain largely unknown. During CD in the freshwater copepod Cyclops kolensis, the genome of cells of the somatic lineage is reorganized and reduced in size by more than 90% without affecting the genome of germline cells. Although the diploid chromosome number is unchanged, chromosome size is dramatically reduced by CD. The eliminated DNA consists primarily of repetitive sequences and localizes within granules during the elimination process. In this review, we provide an overview of CD in C. kolensis including both cytological and molecular studies.
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9
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Qiu GH, Huang C, Zheng X, Yang X. The protective function of noncoding DNA in genome defense of eukaryotic male germ cells. Epigenomics 2018; 10:499-517. [PMID: 29616594 DOI: 10.2217/epi-2017-0103] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Peripheral and abundant noncoding DNA has been hypothesized to protect the genome and the central protein-coding sequences against DNA damage in somatic genome. In the cytosol, invading exogenous nucleic acids may first be deactivated by small RNAs encoded by noncoding DNA via mechanisms similar to the prokaryotic CRISPR-Cas system. In the nucleus, the radicals generated by radiation in the cytosol, radiation energy and invading exogenous nucleic acids are absorbed, blocked and/or reduced by peripheral heterochromatin, and damaged DNA in heterochromatin is removed and excluded from the nucleus to the cytoplasm through nuclear pore complexes. To further strengthen the hypothesis, this review summarizes the experimental evidence supporting the protective function of noncoding DNA in the genome of male germ cells. Based on these data, this review provides evidence supporting the protective role of noncoding DNA in the genome defense of sperm genome through similar mechanisms to those of the somatic genome.
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Affiliation(s)
- Guo-Hua Qiu
- Fujian Provincial Key Laboratory for the Prevention & Control of Animal Infectious Diseases & Biotechnology; Key Laboratory of Preventive Veterinary Medicine and Biotechnology, Fujian Province University; College of Life Sciences, Longyan University, Longyan 364012, Fujian, PR China
| | - Cuiqin Huang
- Fujian Provincial Key Laboratory for the Prevention & Control of Animal Infectious Diseases & Biotechnology; Key Laboratory of Preventive Veterinary Medicine and Biotechnology, Fujian Province University; College of Life Sciences, Longyan University, Longyan 364012, Fujian, PR China
| | - Xintian Zheng
- Fujian Provincial Key Laboratory for the Prevention & Control of Animal Infectious Diseases & Biotechnology; Key Laboratory of Preventive Veterinary Medicine and Biotechnology, Fujian Province University; College of Life Sciences, Longyan University, Longyan 364012, Fujian, PR China
| | - Xiaoyan Yang
- Fujian Provincial Key Laboratory for the Prevention & Control of Animal Infectious Diseases & Biotechnology; Key Laboratory of Preventive Veterinary Medicine and Biotechnology, Fujian Province University; College of Life Sciences, Longyan University, Longyan 364012, Fujian, PR China
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10
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Wang J, Gao S, Mostovoy Y, Kang Y, Zagoskin M, Sun Y, Zhang B, White LK, Easton A, Nutman TB, Kwok PY, Hu S, Nielsen MK, Davis RE. Comparative genome analysis of programmed DNA elimination in nematodes. Genome Res 2017; 27:2001-2014. [PMID: 29118011 PMCID: PMC5741062 DOI: 10.1101/gr.225730.117] [Citation(s) in RCA: 69] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2017] [Accepted: 10/12/2017] [Indexed: 12/20/2022]
Abstract
Programmed DNA elimination is a developmentally regulated process leading to the reproducible loss of specific genomic sequences. DNA elimination occurs in unicellular ciliates and a variety of metazoans, including invertebrates and vertebrates. In metazoa, DNA elimination typically occurs in somatic cells during early development, leaving the germline genome intact. Reference genomes for metazoa that undergo DNA elimination are not available. Here, we generated germline and somatic reference genome sequences of the DNA eliminating pig parasitic nematode Ascaris suum and the horse parasite Parascaris univalens. In addition, we carried out in-depth analyses of DNA elimination in the parasitic nematode of humans, Ascaris lumbricoides, and the parasitic nematode of dogs, Toxocara canis. Our analysis of nematode DNA elimination reveals that in all species, repetitive sequences (that differ among the genera) and germline-expressed genes (approximately 1000–2000 or 5%–10% of the genes) are eliminated. Thirty-five percent of these eliminated genes are conserved among these nematodes, defining a core set of eliminated genes that are preferentially expressed during spermatogenesis. Our analysis supports the view that DNA elimination in nematodes silences germline-expressed genes. Over half of the chromosome break sites are conserved between Ascaris and Parascaris, whereas only 10% are conserved in the more divergent T. canis. Analysis of the chromosomal breakage regions suggests a sequence-independent mechanism for DNA breakage followed by telomere healing, with the formation of more accessible chromatin in the break regions prior to DNA elimination. Our genome assemblies and annotations also provide comprehensive resources for analysis of DNA elimination, parasitology research, and comparative nematode genome and epigenome studies.
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Affiliation(s)
- Jianbin Wang
- Department of Biochemistry and Molecular Genetics, RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, Colorado 80045, USA
| | - Shenghan Gao
- Department of Biochemistry and Molecular Genetics, RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, Colorado 80045, USA.,Beijing Institute of Genomics, Chinese Academy of Sciences, Beijing 100101, China
| | - Yulia Mostovoy
- Cardiovascular Research Institute, UCSF School of Medicine, San Francisco, California 94158, USA
| | - Yuanyuan Kang
- Department of Biochemistry and Molecular Genetics, RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, Colorado 80045, USA
| | - Maxim Zagoskin
- Department of Biochemistry and Molecular Genetics, RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, Colorado 80045, USA
| | - Yongqiao Sun
- Beijing Institute of Genomics, Chinese Academy of Sciences, Beijing 100101, China
| | - Bing Zhang
- Beijing Institute of Genomics, Chinese Academy of Sciences, Beijing 100101, China
| | - Laura K White
- Department of Biochemistry and Molecular Genetics, RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, Colorado 80045, USA
| | - Alice Easton
- Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Disease, National Institutes of Health, Bethesda, Maryland 20892, USA
| | - Thomas B Nutman
- Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Disease, National Institutes of Health, Bethesda, Maryland 20892, USA
| | - Pui-Yan Kwok
- Cardiovascular Research Institute, UCSF School of Medicine, San Francisco, California 94158, USA
| | - Songnian Hu
- Beijing Institute of Genomics, Chinese Academy of Sciences, Beijing 100101, China
| | - Martin K Nielsen
- Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky 40546, USA
| | - Richard E Davis
- Department of Biochemistry and Molecular Genetics, RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, Colorado 80045, USA
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Clower MK, Holub AS, Smith RT, Wyngaard GA. EMBRYONIC DEVELOPMENT AND A QUANTITATIVE MODEL OF PROGRAMMED DNA ELIMINATION IN MESOCYCLOPS EDAX (S. A. FORBES, 1891) (COPEPODA: CYCLOPOIDA). JOURNAL OF CRUSTACEAN BIOLOGY : A QUARTERLY OF THE CRUSTACEAN SOCIETY FOR THE PUBLICATION OF RESEARCH ON ANY ASPECT OF THE BIOLOGY OF CRUSTACEA 2016; 36:661-674. [PMID: 27857452 PMCID: PMC5110247 DOI: 10.1163/1937240x-00002473] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
The highly programmed fragmentation of chromosomes and elimination of large amounts of nuclear DNA from the presomatic cell lineages (i.e., chromatin diminution), occurs in the embryos of the freshwater zooplankton Mesocyclops edax (S. A. Forbes, 1891) (Crustacea: Copepoda). The somatic genome is reorganized and reduced to a size five times smaller even though the germline genome remains intact. We present the first comprehensive, quantitative model of DNA content throughout embryogenesis in a copepod that possesses embryonic DNA elimination. We used densitometric image analysis to measure the DNA content of polar bodies, germline and somatic nuclei, and excised DNA "droplets." We report: 1) variable DNA contents of polar bodies, some of which do not contain the amount corresponding to the haploid germline genome size; 2) presence of pronuclei in newly laid embryo sacs; 3) gonomeric chromosomes in the second to fourth cleavage divisions and in the primordial germ cell and primordial endoderm cell during the fifth cleavage division; 4) timing of early embryonic cell stages, elimination of DNA, and divisions of the primordial germ cell and primordial endoderm cell at 22°C; and 5) persistence of a portion of the excised DNA "droplets" throughout embryogenesis. DNA elimination is a trait that spans multiple embryonic stages and a knowledge of the timing and variability of the associated cytological events with DNA elimination will promote the study of the molecular mechanisms involved in this trait. We propose the "genome yolk hypothesis" as a functional explanation for the persistence of the eliminated DNA that might serve as a resource during postdiminution cleavage divisions.
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Affiliation(s)
- Michelle K. Clower
- North Carolina Coastal Federation, 128 Grenville St., Manteo, NC 27954, USA
| | - Ashton S. Holub
- Department of Biology, MSC 7801, James Madison University, Harrisonburg, VA 22807, USA
| | - Rebecca T. Smith
- Eastern Shore Laboratory, Virginia Institute of Marine Science, Wachapreague, VA 23480, USA
| | - Grace A. Wyngaard
- Department of Biology, MSC 7801, James Madison University, Harrisonburg, VA 22807, USA
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Abstract
The ciliate Oxytricha is a microbial eukaryote with two genomes, one of which experiences extensive genome remodeling during development. Each round of conjugation initiates a cascade of events that construct a transcriptionally active somatic genome from a scrambled germline genome, with considerable help from both long and small noncoding RNAs. This process of genome remodeling entails massive DNA deletion and reshuffling of remaining DNA segments to form functional genes from their interrupted and scrambled germline precursors. The use of Oxytricha as a model system provides an opportunity to study an exaggerated form of programmed genome rearrangement. Furthermore, studying the mechanisms that maintain nuclear dimorphism and mediate genome rearrangement has demonstrated a surprising plasticity and diversity of noncoding RNA pathways, with new roles that go beyond conventional gene silencing. Another aspect of ciliate genetics is their unorthodox patterns of RNA-mediated, epigenetic inheritance that rival Mendelian inheritance. This review takes the reader through the key experiments in a model eukaryote that led to fundamental discoveries in RNA biology and pushes the biological limits of DNA processing.
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13
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Wang J, Davis RE. Programmed DNA elimination in multicellular organisms. Curr Opin Genet Dev 2014; 27:26-34. [PMID: 24886889 DOI: 10.1016/j.gde.2014.03.012] [Citation(s) in RCA: 100] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2014] [Revised: 03/17/2014] [Accepted: 03/25/2014] [Indexed: 10/25/2022]
Abstract
Genetic information typically remains constant in all cells throughout the life cycle of most organisms. However, there are exceptions where DNA elimination is an integral, developmental program for some organisms, associated with generating distinct germline versus somatic genomes. Programmed DNA elimination occurs in unicellular ciliates and diverse metazoa ranging from nematodes to vertebrates. DNA elimination can occur through chromosome breakage and selective loss of chromosome regions or the elimination of individual chromosomes. Recent studies provide compelling evidence that DNA elimination is a novel form of gene silencing, dosage compensation, and sex determination. Further identification of the eliminated sequences, genome changes, and in depth characterization of this phenomenon in diverse metazoans is needed to shed new light on the functions and mechanisms of this regulated process.
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Affiliation(s)
- Jianbin Wang
- Department of Biochemistry and Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80045, United States.
| | - Richard E Davis
- Department of Biochemistry and Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80045, United States.
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14
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Billions of basepairs of recently expanded, repetitive sequences are eliminated from the somatic genome during copepod development. BMC Genomics 2014; 15:186. [PMID: 24618421 PMCID: PMC4029161 DOI: 10.1186/1471-2164-15-186] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2013] [Accepted: 03/03/2014] [Indexed: 12/15/2022] Open
Abstract
Background Chromatin diminution is the programmed deletion of DNA from presomatic cell or nuclear lineages during development, producing single organisms that contain two different nuclear genomes. Phylogenetically diverse taxa undergo chromatin diminution — some ciliates, nematodes, copepods, and vertebrates. In cyclopoid copepods, chromatin diminution occurs in taxa with massively expanded germline genomes; depending on species, germline genome sizes range from 15 – 75 Gb, 12–74 Gb of which are lost from pre-somatic cell lineages at germline – soma differentiation. This is more than an order of magnitude more sequence than is lost from other taxa. To date, the sequences excised from copepods have not been analyzed using large-scale genomic datasets, and the processes underlying germline genomic gigantism in this clade, as well as the functional significance of chromatin diminution, have remained unknown. Results Here, we used high-throughput genomic sequencing and qPCR to characterize the germline and somatic genomes of Mesocyclops edax, a freshwater cyclopoid copepod with a germline genome of ~15 Gb and a somatic genome of ~3 Gb. We show that most of the excised DNA consists of repetitive sequences that are either 1) verifiable transposable elements (TEs), or 2) non-simple repeats of likely TE origin. Repeat elements in both genomes are skewed towards younger (i.e. less divergent) elements. Excised DNA is a non-random sample of the germline repeat element landscape; younger elements, and high frequency DNA transposons and LINEs, are disproportionately eliminated from the somatic genome. Conclusions Our results suggest that germline genome expansion in M. edax reflects explosive repeat element proliferation, and that billions of base pairs of such repeats are deleted from the somatic genome every generation. Thus, we hypothesize that chromatin diminution is a mechanism that controls repeat element load, and that this load can evolve to be divergent between tissue types within single organisms.
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Elliott TA, Stage DE, Crease TJ, Eickbush TH. In and out of the rRNA genes: characterization of Pokey elements in the sequenced Daphnia genome. Mob DNA 2013; 4:20. [PMID: 24059783 PMCID: PMC3849761 DOI: 10.1186/1759-8753-4-20] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2013] [Accepted: 08/29/2013] [Indexed: 11/16/2022] Open
Abstract
Background Only a few transposable elements are known to exhibit site-specific insertion patterns, including the well-studied R-element retrotransposons that insert into specific sites within the multigene rDNA. The only known rDNA-specific DNA transposon, Pokey (superfamily: piggyBac) is found in the freshwater microcrustacean, Daphnia pulex. Here, we present a genome-wide analysis of Pokey based on the recently completed whole genome sequencing project for D. pulex. Results Phylogenetic analysis of Pokey elements recovered from the genome sequence revealed the presence of four lineages corresponding to two divergent autonomous families and two related lineages of non-autonomous miniature inverted repeat transposable elements (MITEs). The MITEs are also found at the same 28S rRNA gene insertion site as the Pokey elements, and appear to have arisen as deletion derivatives of autonomous elements. Several copies of the full-length Pokey elements may be capable of producing an active transposase. Surprisingly, both families of Pokey possess a series of 200 bp repeats upstream of the transposase that is derived from the rDNA intergenic spacer (IGS). The IGS sequences within the Pokey elements appear to be evolving in concert with the rDNA units. Finally, analysis of the insertion sites of Pokey elements outside of rDNA showed a target preference for sites similar to the specific sequence that is targeted within rDNA. Conclusions Based on the target site preference of Pokey elements and the concerted evolution of a segment of the element with the rDNA unit, we propose an evolutionary path by which the ancestors of Pokey elements have invaded the rDNA niche. We discuss how specificity for the rDNA unit may have evolved and how this specificity has played a role in the long-term survival of these elements in the subgenus Daphnia.
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Affiliation(s)
- Tyler A Elliott
- Department of Integrative Biology, University of Guelph, Guelph, ON N1G 2W1, Canada
| | - Deborah E Stage
- Department of Biology, University of Rochester, Rochester, NY 14627, USA.,Department of Biology, Butler County Community College, Butler, PA 16002, USA
| | - Teresa J Crease
- Department of Integrative Biology, University of Guelph, Guelph, ON N1G 2W1, Canada
| | - Thomas H Eickbush
- Department of Biology, University of Rochester, Rochester, NY 14627, USA
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