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Mahelka V, Kopecký D, Majka J, Krak K. Uniparental expression of ribosomal RNA in × Festulolium grasses: a link between the genome and nucleolar dominance. FRONTIERS IN PLANT SCIENCE 2023; 14:1276252. [PMID: 37790792 PMCID: PMC10544908 DOI: 10.3389/fpls.2023.1276252] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 08/11/2023] [Accepted: 08/30/2023] [Indexed: 10/05/2023]
Abstract
Genome or genomic dominance (GD) is a phenomenon observed in hybrids when one parental genome becomes dominant over the other. It is manifested by the replacement of chromatin of the submissive genome by that of the dominant genome and by biased gene expression. Nucleolar dominance (ND) - the functional expression of only one parental set of ribosomal genes in hybrids - is another example of an intragenomic competitive process which, however, concerns ribosomal DNA only. Although GD and ND are relatively well understood, the nature and extent of their potential interdependence is mostly unknown. Here, we ask whether hybrids showing GD also exhibit ND and, if so, whether the dominant genome is the same. To test this, we used hybrids between Festuca and Lolium grasses (Festulolium), and between two Festuca species in which GD has been observed (with Lolium as the dominant genome in Festulolium and F. pratensis in interspecific Festuca hybrids). Using amplicon sequencing of ITS1 and ITS2 of the 45S ribosomal DNA (rDNA) cluster and molecular cytogenetics, we studied the organization and expression of rDNA in leaf tissue in five hybrid combinations, four generations and 31 genotypes [F. pratensis × L. multiflorum (F1, F2, F3, BC1), L. multiflorum × F. pratensis (F1), L. multiflorum × F. glaucescens (F2), L. perenne × F. pratensis (F1), F. glaucescens × F. pratensis (F1)]. We have found that instant ND occurs in Festulolium, where expression of Lolium-type rDNA reached nearly 100% in all F1 hybrids and was maintained through subsequent generations. Therefore, ND and GD in Festulolium are manifested by the same dominant genome (Lolium). We also confirmed the concordance between GD and ND in an interspecific cross between two Festuca species.
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Affiliation(s)
- Václav Mahelka
- Czech Academy of Sciences, Institute of Botany, Průhonice, Czechia
| | - David Kopecký
- Institute of Experimental Botany of the Czech Academy of Sciences, Centre of Plant Structural and Functional Genomics, Olomouc, Czechia
| | - Joanna Majka
- Institute of Experimental Botany of the Czech Academy of Sciences, Centre of Plant Structural and Functional Genomics, Olomouc, Czechia
| | - Karol Krak
- Czech Academy of Sciences, Institute of Botany, Průhonice, Czechia
- Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Prague, Czechia
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Mlinarec J, Boštjančić LL, Malenica N, Jurković A, Boland T, Yakovlev SS, Besendorfer V. Structure and Methylation of 35S rDNA in Allopolyploids Anemone multifida (2 n = 4 x = 32, BBDD) and Anemone baldensis (2 n = 6 x = 48, AABBDD) and Their Parental Species Show Evidence of Nucleolar Dominance. FRONTIERS IN PLANT SCIENCE 2022; 13:908218. [PMID: 35874014 PMCID: PMC9296772 DOI: 10.3389/fpls.2022.908218] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/30/2022] [Accepted: 06/07/2022] [Indexed: 05/26/2023]
Abstract
Transcriptional silencing of 35S rDNA loci inherited from one parental species is occurring relatively frequently in allopolyploids. However, molecular mechanisms by which it is selected for transcriptional silencing remain unclear. We applied NGS, silver staining and bisulfite sequencing to study the structure, expression and methylation landscape of 35S rDNA in two allopolyploids of common origin, allotetraploid Anemone multifida (2n = 4x = 32, genome composition BBDD) and allohexaploid A. baldensis (2n = 6x = 48, AABBDD), and their genome donors, A. sylvestris (2n = 16, AA), A. cylindrica (2n = 16, BB) and A. parviflora (2n = 16, DD). The size of the recovered 35S rDNA units varied from 10,489 bp in A. cylindrica to 12,084 bp in A. sylvestris. Anemone showed an organization typical of most ribosomal 35S rDNA composed of NTS, ETS, rRNA genes, TTS and TIS with structural features of plant IGS sequences and all functional elements needed for rRNA gene activity. The NTS was more variable than the ETS and consisted of SRs which are highly variable among Anemone. Five to six CpG-rich islands were found within the ETS. CpG island located adjacent to the transcription initiation site (TIS) was highly variable regarding the sequence size and methylation level and exhibited in most of the species lower levels of methylation than CpG islands located adjacent to the 18S rRNA gene. Our results uncover hypomethylation of A. sylvestris- and A. parviflora-derived 35S rDNA units in allopolyploids A. multifida and A. baldensis. Hypomethylation of A. parviflora-derived 35S rDNA was more prominent in A. baldensis than in A. multifida. We showed that A. baldensis underwent coupled A. sylvestris-derived 35S rDNA array expansion and A. parviflora-derived 35S rDNA copy number decrease that was accompanied by lower methylation level of A. sylvestris-derived 35S rDNA units in comparison to A. parviflora-derived 35S rDNA units. These observations suggest that in A. baldensis nucleolar dominance is directed toward A. sylvestris-derived chromosomes. This work broadens our current knowledge of the 35S rDNA organization in Anemone and provides evidence of the progenitor-specific 35S rDNA methylation in nucleolar dominance.
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Affiliation(s)
| | - Ljudevit Luka Boštjančić
- LOEWE Centre for Translational Biodiversity Genomics (LOEWE-TBG), Senckenberg Biodiversity and Climate Research Centre, Senckenberg Gesellschaft für Naturforschung, Frankfurt, Germany
- Department of Computer Science, ICube, UMR 7357, CNRS, Centre de Recherche en Biomédecine de Strasbourg, University of Strasbourg, Strasbourg, France
| | - Nenad Malenica
- Division of Molecular Biology, Department of Biology, University of Zagreb, Horvatovac, Croatia
| | - Adela Jurković
- Division of Molecular Biology, Department of Biology, University of Zagreb, Horvatovac, Croatia
| | - Todd Boland
- Memorial University of Newfoundland’s Botanical Gardens, St. John’s, NL, Canada
| | - Sonja Siljak Yakovlev
- CNRS, AgroParisTech, Ecologie Systématique Evolution, Université Paris-Saclay, Orsay, France
| | - Višnja Besendorfer
- Division of Molecular Biology, Department of Biology, University of Zagreb, Horvatovac, Croatia
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Grabowska AI, Boroń A, Kirtiklis L, Spóz A, Juchno D, Kotusz J. Chromosomal inheritance of parental rDNAs distribution pattern detected by FISH in diploid F 1 hybrid progeny of Cobitis (Teleostei, Cobitidae) species has non-Mendelian character. JOURNAL OF FISH BIOLOGY 2020; 96:261-273. [PMID: 31755097 DOI: 10.1111/jfb.14216] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/26/2019] [Accepted: 11/21/2019] [Indexed: 06/10/2023]
Abstract
This study was conducted to describe the major and the minor rDNA chromosome distribution in the spined loach Cobitis taenia (2n = 48) and the Danubian loach Cobitis elongatoides (2n = 50), and their laboratory-produced diploid reciprocal F1 hybrid progeny. It was tested by fluorescence in situ hybridisation (FISH) whether the number of 28s and 5s rDNA sites in the karyotypes of diploid hybrids corresponds to the expectations resulting from Mendelian ratio and if nucleolar organiser regions (NOR)were inherited from both parents or nucleolar dominance can be observed in the induced F1 hybrid progeny. Ten (females) or twelve (males) 28s rDNA loci were located in nine uniarm chromosomes of C. taenia. Two of such loci terminally bounded on one acrocentric chromosome were unique and indicated as specific for this species. Large 5s rDNA clusters were located on two acrocentric chromosomes. In C. elongatoides of both sexes, six NOR sites in terminal regions on six meta-submetacentric chromosomes and two 5s rDNA sites on large submetacentrics were detected. The F1 hybrid progeny (2n = 49) was characterised by the intermediate karyotype with the sites of ribosome synthesis on chromosomes inherited from both parents without showing nucleolar dominance. 5s rDNA sites were detected on large submetacentric and two acrocentric chromosomes. The observed number of both 28s and 5s rDNAs signals in F1 diploid Cobitis hybrids was disproportionally inherited from the two parental species, showing inconsistency with the Mendelian ratios. The presented rDNA patterns indicate some marker chromosomes that allow the species of the parental male and female to be recognised in hybrid progeny. The 5s rDNA was found to be a particularly effective diagnostic marker of C. elongatoides to partially discern genomic composition of diploid Cobitis hybrids and presumably allopolyploids resulting from their backcrossing with one of the parental species. Thus, the current study provides insight into the extent of rDNA heredity in Cobitis chromosomes and their cytotaxonomic character.
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Affiliation(s)
- Anna I Grabowska
- Department of Zoology, University of Warmia and Mazury, Olsztyn, Poland
| | - Alicja Boroń
- Department of Zoology, University of Warmia and Mazury, Olsztyn, Poland
| | - Lech Kirtiklis
- Department of Zoology, University of Warmia and Mazury, Olsztyn, Poland
| | - Aneta Spóz
- Department of Zoology, University of Warmia and Mazury, Olsztyn, Poland
| | - Dorota Juchno
- Department of Zoology, University of Warmia and Mazury, Olsztyn, Poland
| | - Jan Kotusz
- Museum of Natural History, University of Wroclaw, Wroclaw, Poland
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Sáez-Vásquez J, Delseny M. Ribosome Biogenesis in Plants: From Functional 45S Ribosomal DNA Organization to Ribosome Assembly Factors. THE PLANT CELL 2019; 31:1945-1967. [PMID: 31239391 PMCID: PMC6751116 DOI: 10.1105/tpc.18.00874] [Citation(s) in RCA: 90] [Impact Index Per Article: 18.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/16/2018] [Revised: 05/28/2019] [Accepted: 06/25/2019] [Indexed: 05/11/2023]
Abstract
The transcription of 18S, 5.8S, and 18S rRNA genes (45S rDNA), cotranscriptional processing of pre-rRNA, and assembly of mature rRNA with ribosomal proteins are the linchpins of ribosome biogenesis. In yeast (Saccharomyces cerevisiae) and animal cells, hundreds of pre-rRNA processing factors have been identified and their involvement in ribosome assembly determined. These studies, together with structural analyses, have yielded comprehensive models of the pre-40S and pre-60S ribosome subunits as well as the largest cotranscriptionally assembled preribosome particle: the 90S/small subunit processome. Here, we present the current knowledge of the functional organization of 45S rDNA, pre-rRNA transcription, rRNA processing activities, and ribosome assembly factors in plants, focusing on data from Arabidopsis (Arabidopsis thaliana). Based on yeast and mammalian cell studies, we describe the ribonucleoprotein complexes and RNA-associated activities and discuss how they might specifically affect the production of 40S and 60S subunits. Finally, we review recent findings concerning pre-rRNA processing pathways and a novel mechanism involved in a ribosome stress response in plants.
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Affiliation(s)
- Julio Sáez-Vásquez
- CNRS, Laboratoire Génome et Développement des Plantes, UMR 5096, 66860 Perpignan, France, and Universite Perpignan Via Domitia, Laboratoire Génome et Développement des Plantes, UMR 5096, F-66860 Perpignan, France
| | - Michel Delseny
- CNRS, Laboratoire Génome et Développement des Plantes, UMR 5096, 66860 Perpignan, France, and Universite Perpignan Via Domitia, Laboratoire Génome et Développement des Plantes, UMR 5096, F-66860 Perpignan, France
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Symonová R. Integrative rDNAomics-Importance of the Oldest Repetitive Fraction of the Eukaryote Genome. Genes (Basel) 2019; 10:genes10050345. [PMID: 31067804 PMCID: PMC6562748 DOI: 10.3390/genes10050345] [Citation(s) in RCA: 35] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2019] [Revised: 04/17/2019] [Accepted: 04/25/2019] [Indexed: 02/06/2023] Open
Abstract
Nuclear ribosomal RNA (rRNA) genes represent the oldest repetitive fraction universal to all eukaryotic genomes. Their deeply anchored universality and omnipresence during eukaryotic evolution reflects in multiple roles and functions reaching far beyond ribosomal synthesis. Merely the copy number of non-transcribed rRNA genes is involved in mechanisms governing e.g., maintenance of genome integrity and control of cellular aging. Their copy number can vary in response to environmental cues, in cellular stress sensing, in development of cancer and other diseases. While reaching hundreds of copies in humans, there are records of up to 20,000 copies in fish and frogs and even 400,000 copies in ciliates forming thus a literal subgenome or an rDNAome within the genome. From the compositional and evolutionary dynamics viewpoint, the precursor 45S rDNA represents universally GC-enriched, highly recombining and homogenized regions. Hence, it is not accidental that both rDNA sequence and the corresponding rRNA secondary structure belong to established phylogenetic markers broadly used to infer phylogeny on multiple taxonomical levels including species delimitation. However, these multiple roles of rDNAs have been treated and discussed as being separate and independent from each other. Here, I aim to address nuclear rDNAs in an integrative approach to better assess the complexity of rDNA importance in the evolutionary context.
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Affiliation(s)
- Radka Symonová
- Faculty of Science, Department of Biology, University of Hradec Králové, 500 03 Hradec Králové, Czech Republic.
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Cao L, Qin Q, Xiao Q, Yin H, Wen J, Liu Q, Huang X, Huo Y, Tao M, Zhang C, Luo K, Liu S. Nucleolar Dominance in a Tetraploidy Hybrid Lineage Derived From Carassius auratus red var. () × Megalobrama amblycephala (). Front Genet 2018; 9:386. [PMID: 30319686 PMCID: PMC6166360 DOI: 10.3389/fgene.2018.00386] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2018] [Accepted: 08/27/2018] [Indexed: 01/09/2023] Open
Abstract
Nucleolar dominance is related to the expression of 45S rRNA genes inherited from one progenitor due to the silencing of the other progenitor’s rRNA genes. To investigate nucleolar dominance associated with tetraploidization, we analyzed the changes regarding the genetic traits and expression of 45S rRNA genes in tetraploidy hybrid lineage including F1 allotetraploids (4n = 148) and F2 autotetraploids (4n = 200) derived from the distant hybridization of Carassius auratus red var. (2n = 100) () ×Megalobrama amblycephala (2n = 48) (). Results showed that nucleolar dominance from the females was established in F1 hybrids and it was inherited in F2 hybrids, suggesting that tetraploidization can lead to rapid establishment of nucleolar dominance in the hybrid origin’s tetraploid lineage. These results extend the knowledge of nucleolar dominance in polyploidy hybrid animals, which are of significance for the evolution of hybrids in vertebrates.
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Affiliation(s)
- Liu Cao
- State Key Laboratory of Developmental Biology of Freshwater Fish, Hunan Normal University, Changsha, China.,College of Life Sciences, Hunan Normal University, Changsha, China
| | - QinBo Qin
- State Key Laboratory of Developmental Biology of Freshwater Fish, Hunan Normal University, Changsha, China.,College of Life Sciences, Hunan Normal University, Changsha, China
| | - Qiong Xiao
- State Key Laboratory of Developmental Biology of Freshwater Fish, Hunan Normal University, Changsha, China.,College of Life Sciences, Hunan Normal University, Changsha, China
| | - HongTing Yin
- College of Life Sciences, Hunan Normal University, Changsha, China
| | - Jin Wen
- College of Life Sciences, Hunan Normal University, Changsha, China
| | - QiWen Liu
- State Key Laboratory of Developmental Biology of Freshwater Fish, Hunan Normal University, Changsha, China.,College of Life Sciences, Hunan Normal University, Changsha, China
| | - Xu Huang
- State Key Laboratory of Developmental Biology of Freshwater Fish, Hunan Normal University, Changsha, China.,College of Life Sciences, Hunan Normal University, Changsha, China
| | - YangYang Huo
- State Key Laboratory of Developmental Biology of Freshwater Fish, Hunan Normal University, Changsha, China.,College of Life Sciences, Hunan Normal University, Changsha, China
| | - Min Tao
- State Key Laboratory of Developmental Biology of Freshwater Fish, Hunan Normal University, Changsha, China.,College of Life Sciences, Hunan Normal University, Changsha, China
| | - Chun Zhang
- State Key Laboratory of Developmental Biology of Freshwater Fish, Hunan Normal University, Changsha, China.,College of Life Sciences, Hunan Normal University, Changsha, China
| | - Kaikun Luo
- State Key Laboratory of Developmental Biology of Freshwater Fish, Hunan Normal University, Changsha, China.,College of Life Sciences, Hunan Normal University, Changsha, China
| | - ShaoJun Liu
- State Key Laboratory of Developmental Biology of Freshwater Fish, Hunan Normal University, Changsha, China.,College of Life Sciences, Hunan Normal University, Changsha, China
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Mirzaghaderi G, Abdolmalaki Z, Zohouri M, Moradi Z, Mason AS. Dynamic nucleolar activity in wheat × Aegilops hybrids: evidence of C-genome dominance. PLANT CELL REPORTS 2017; 36:1277-1285. [PMID: 28456843 DOI: 10.1007/s00299-017-2152-x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/01/2017] [Accepted: 04/27/2017] [Indexed: 06/07/2023]
Abstract
NOR loci of C-subgenome are dominant in wheat × Aegilops interspecific hybrids, which may have evolutionary implications for wheat group genome dynamics and evolution. After interspecific hybridisation, some genes are often expressed from only one of the progenitor species, shaping subsequent allopolyploid genome evolution processes. A well-known example is nucleolar dominance, i.e. the formation of cell nucleoli from chromosomes of only one parental species. We studied nucleolar organizing regions (NORs) in diploid Aegilops markgrafii (syn: Ae. caudata; CC), Ae. umbellulata (UU), allotetraploids Aegilops cylindrica (CcCcDcDc) and Ae. triuncialis (CtCtUtUt), synthetic interspecific F1 hybrids between these two allotetraploids and bread wheat (Triticum aestivum, AABBDD) and in F3 generation hybrids with genome composition AABBDDCtCtUtUt using silver staining and fluorescence in situ hybridization (FISH). In Ae. markgrafii (CC), NORs of both 1C and 5C or only 5C chromosome pairs were active in different individual cells, while only NORs on 1U chromosomes were active in Ae. umbellulata (UU). Although all 35S rDNA loci of the Ct subgenome (located on 1Ct and 5Ct) were active in Ae. triuncialis, only one pair (occupying either 1Cc or 5Cc) was active in Ae. cylindrica, depending on the genotype studied. These C-genome expression patterns were transmitted to the F1 and F3 generations. Wheat chromosome NOR activity was variable in Ae. triuncialis × T. aestivum F1 seeds, but silenced by the F3 generation. No effect of maternal or paternal cross direction was observed. These results indicate that C-subgenome NOR loci are dominant in wheat × Aegilops interspecific hybrids, which may have evolutionary implications for wheat group genome dynamics and allopolyploid evolution.
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Affiliation(s)
- Ghader Mirzaghaderi
- Department of Agronomy and Plant Breeding, Faculty of Agriculture, University of Kurdistan, P. O. Box 416, Sanandaj, Iran.
| | - Zinat Abdolmalaki
- Department of Agronomy and Plant Breeding, Faculty of Agriculture, University of Kurdistan, P. O. Box 416, Sanandaj, Iran
| | - Mohsen Zohouri
- Department of Agronomy and Plant Breeding, Faculty of Agriculture, University of Kurdistan, P. O. Box 416, Sanandaj, Iran
| | - Zeinab Moradi
- Department of Agronomy and Plant Breeding, Faculty of Agriculture, University of Kurdistan, P. O. Box 416, Sanandaj, Iran
| | - Annaliese S Mason
- Department of Plant Breeding, IFZ Research Centre for Biosystems, Land Use and Nutrition, Justus Liebig University, Heinrich-Buff-Ring 26-32, 35392, Giessen, Germany
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Vargas AO, Krabichler Q, Guerrero-Bosagna C. An Epigenetic Perspective on the Midwife Toad Experiments of Paul Kammerer (1880-1926). JOURNAL OF EXPERIMENTAL ZOOLOGY PART B-MOLECULAR AND DEVELOPMENTAL EVOLUTION 2016; 328:179-192. [PMID: 27781385 DOI: 10.1002/jez.b.22708] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/15/2016] [Revised: 07/29/2016] [Accepted: 09/24/2016] [Indexed: 12/14/2022]
Abstract
Paul Kammerer was the most outstanding neo-Lamarckian experimentalist of the early 20th century. He reported spectacular results in the midwife toad, including crosses of environmentally modified toads with normal toads, where acquired traits were inherited in Mendelian fashion. Accusations of fraud generated a great scandal, ending with Kammerer's suicide. Controversy reignited in the 1970s, when journalist Arthur Koestler argued against these accusations. Since then, others have argued that Kammerer's results, even if real, were not groundbreaking and could be explained by somatic plasticity, inadvertent selection, or conventional genetics. More recently, epigenetics has uncovered mechanisms by which inheritance can respond directly to environmental change, inviting a reanalysis of Kammerer's descriptions. Previous arguments for mere somatic plasticity have ignored the description of experiments showing heritable germ line modification. Alleged inadvertent selection associated with egg mortality can be discarded, since mortality decreased in a single generation, upon repeated exposures. The challenging implications did not escape the attention of Kammerer's noted contemporary, William Bateson, but he reacted with disbelief, thus encouraging fraud accusations. Nowadays, formerly puzzling phenomena can be explained by epigenetic mechanisms. Importantly, Kammerer described parent-of-origin effects, an effect of parental sex on dominance. Epigenetic mechanisms underlie these effects in genomic imprinting and experiments of transgenerational epigenetic inheritance. In the early 20th century, researchers had no reason to link them with the inheritance of acquired traits. Thus, the parent-of-origin effects in Kammerer's experiments specifically suggest authenticity. Ultimate proof should come from renewed experimentation. To encourage further research, we present a model of possible epigenetic mechanisms.
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Affiliation(s)
- Alexander O Vargas
- Laboratorio de Ontogenia y Filogenia, Departamento de Biología, Facultad de Ciencias Universidad de Chile, Las Palmeras, Ñuñoa, Santiago
| | - Quirin Krabichler
- Chair of Zoology, TUM School of Life Sciences Weihenstephan, Technical University of Munich, Freising, Germany
| | - Carlos Guerrero-Bosagna
- Avian Behavioural Genomics and Physiology Group, IFM Biology, Linköping University, Linköping, Sweden
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Maciak S, Michalak K, Kale SD, Michalak P. Nucleolar Dominance and Repression of 45S Ribosomal RNA Genes in Hybrids between Xenopus borealis and X. muelleri (2n = 36). Cytogenet Genome Res 2016; 149:290-296. [PMID: 27728911 DOI: 10.1159/000450665] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/24/2016] [Indexed: 11/19/2022] Open
Abstract
Nucleolar dominance is a dramatic disruption in the formation of nucleoli and the expression of ribosomal RNA (rRNA) genes, characteristic of some plant and animal hybrids. Here, we report that F1 hybrids produced from reciprocal crosses between 2 sister species of Xenopus clawed frogs, X. muelleri and X. borealis, undergo nucleolar dominance somewhat distinct from a pattern previously reported in hybrids between phylogenetically more distant Xenopus species. Patterns of nucleolar development, 45S rRNA expression, and gene copy inheritance were investigated using a combination of immunostaining, pyrosequencing, droplet digital PCR, flow cytometry, and epigenetic inhibition. In X. muelleri × X. borealis hybrids, typically only 1 nucleolus is formed, and 45S rRNA genes are predominantly expressed from 1 progenitor's alleles, X. muelleri, regardless of the cross-direction. These changes are accompanied by an extensive (∼80%) loss of rRNA gene copies in the hybrids relative to their parents, with the transcriptionally underdominant variant (X. borealis) being preferentially lost. Chemical treatment of hybrid larvae with a histone deacetylase inhibitor resulted in a partial derepression of the underdominant variant. Together, these observations shed light on the genetic and epigenetic basis of nucleolar dominance as an underappreciated manifestation of genetic conflicts within a hybrid genome.
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