Germ Line-soma Differentiation Research Articles

One Cell, Two Gears: Extensive Somatic Genome Plasticity Accompanies High Germline Genome Stability in Paramecium.

Mutation accumulation (MA) experiments are conventionally employed to study spontaneous germline mutations. However, MA experiments can also shed light on somatic genome plasticity in a habitual and genetic drift-maximizing environment. Here, we revisit an MA experiment that uncovered extraordinary germline genome stability in Paramecium tetraurelia, a single-celled eukaryote with nuclear dimorphism. Our re-examination of isogenic P. tetraurelia MA lines propagated in nutrient-rich medium for >40 sexual cycles reveals that their polyploid somatic genome accrued hundreds of intervening DNA segments (IESs), which are normally eliminated during germline-soma differentiation. These IESs frequently occupy a fraction of the somatic DNA copies of a given locus, producing IES excision/retention polymorphisms, and preferentially fall into a class of epigenetically controlled sequences. Relative to control lines, retained IESs are flanked by stronger cis-acting signals and interrupt an excess of highly expressed coding exons. These findings suggest that P. tetraurelia’s elevated germline DNA replication fidelity is associated with pervasive somatic genome plasticity. They show that MA regimes are powerful tools for investigating the role that developmental plasticity, somatic mutations, and epimutations have in ecology and evolution.

Programmed DNA elimination: silencing genes and repetitive sequences in somatic cells.

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.

Characterization and expression pattern of ZB and PS transposons in zebrafish

Despite comprising much of the genome, transposons were once thought of as junk. However, transposons play many roles in the eukaryotic genome, such as providing new proteins as domesticated genes, expressing during germline-soma differentiation, function in DNA rearrangement in the offspring, and so on. We sought to describe the distribution and structural organization of the two autonomous transposons (ZB and PS) in the zebrafish genome and examine their expression patterns in embryos and adult tissues. The intact copy of ZB and PS was queried by BLAST on NCBI and ENSEMBL using default parameters. Of the copies with coverage and identity, more than 90 % were downloaded to do structural analysis. Spatial and temporal expression patterns were detected by qRT-PCR and Whole-mount in situ hybridization (WISH). There are 19 intact copies of ZB, encoding 341 amino acid residues with DD34E catalytic domain and flanked by 201bp TIRs, and seven intact PS copies, containing 425 amino acid residues with DD35D catalytic domain flanked by 28bp TIRs, were detected in the genome of zebrafish respectively. Analysis of genomic insertions indicated that both ZB and PS transposons are prone to be retained in the intron and intergenic regions of the zebrafish genome. The sense and antisense transcripts of ZB and PS were detected during embryonic development stages and exhibited similar expression patterns. The difference is that the sense strand transcript of ZB was explicitly expressed in midbrain-hindbrain boundary (MHB) and otic vesicle (OV), and pharyngeal arches and pharyngeal pouches (PA&PP) at 48 hpf. In adult zebrafish, the expressions of ZB and PS in muscle and brain are much higher than in other tissues. Our study results indicate that ZB and PS transposons may be involved in the embryonic development and regulation of somatic cells of certain adult tissues, such as the brain and muscle.

How oxygen gave rise to eukaryotic sex.

How did full meiotic eukaryotic sex evolve and what was the immediate advantage allowing it to develop? We propose that the crucial determinant can be found in internal reactive oxygen species (ROS) formation at the start of eukaryotic evolution approximately 2 × 109 years ago. The large amount of ROS coming from a bacterial endosymbiont gave rise to DNA damage and vast increases in host genome mutation rates. Eukaryogenesis and chromosome evolution represent adaptations to oxidative stress. The host, an archaeon, most probably already had repair mechanisms based on DNA pairing and recombination, and possibly some kind of primitive cell fusion mechanism. The detrimental effects of internal ROS formation on host genome integrity set the stage allowing evolution of meiotic sex from these humble beginnings. Basic meiotic mechanisms thus probably evolved in response to endogenous ROS production by the ‘pre-mitochondrion’. This alternative to mitosis is crucial under novel, ROS-producing stress situations, like extensive motility or phagotrophy in heterotrophs and endosymbiontic photosynthesis in autotrophs. In multicellular eukaryotes with a germline–soma differentiation, meiotic sex with diploid–haploid cycles improved efficient purging of deleterious mutations. Constant pressure of endogenous ROS explains the ubiquitous maintenance of meiotic sex in practically all eukaryotic kingdoms. Here, we discuss the relevant observations underpinning this model.

Analysis of global microRNAome profiles of Caenorhabditis elegans oocytes and early embryos

One of the most complex cell transformations in biology is the remodeling of a fertilized oocyte into a totipotent zygote. There is no transcription during this event, and therefore mRNAs and proteins accumulated in the oocyte during oogenesis are utilized until zygotic genome activation. In order to differentiate into somatic cells, the embryo must erase the totipotency signature coming from the oocyte cytoplasm in the somatic blastomeres. In this study, the microRNAome profiles of oocytes and early embryos before and after zygotic genome activation were identified. By using next-generation high-throughput pyrosequencing and miRNA microarray techniques, miRNAome profiles of Caenorhabditis elegans oocytes (mitotic and meiotic) and early embryos before (2-blastomere stage) and after (8-blastomere stage) zygotic genome activation were generated. Besides observing results similar to those of previous reports, new miRNAs were identified whose expression levels changed during this transformation. After selection and polymerase chain reaction validation of the candidate miRNAs, downstream targets were identified by using bioinformatic tools and they were subjected to pathway analysis. Further analysis of the identified miRNAs in this study may provide a better understanding of the regulation of early embryonic development, germline-soma differentiation, oocyte-embryo transition, and maintenance of totipotency status.

Spliced DNA Sequences in the Paramecium Germline: Their Properties and Evolutionary Potential

Despite playing a crucial role in germline-soma differentiation, the evolutionary significance of developmentally regulated genome rearrangements (DRGRs) has received scant attention. An example of DRGR is DNA splicing, a process that removes segments of DNA interrupting genic and/or intergenic sequences. Perhaps, best known for shaping immune-system genes in vertebrates, DNA splicing plays a central role in the life of ciliated protozoa, where thousands of germline DNA segments are eliminated after sexual reproduction to regenerate a functional somatic genome. Here, we identify and chronicle the properties of 5,286 sequences that putatively undergo DNA splicing (i.e., internal eliminated sequences [IESs]) across the genomes of three closely related species of the ciliate Paramecium (P. tetraurelia, P. biaurelia, and P. sexaurelia). The study reveals that these putative IESs share several physical characteristics. Although our results are consistent with excision events being largely conserved between species, episodes of differential IES retention/excision occur, may have a recent origin, and frequently involve coding regions. Our findings indicate interconversion between somatic—often coding—DNA sequences and noncoding IESs, and provide insights into the role of DNA splicing in creating potentially functional genetic innovation.

Cytogenetic Analysis of Partial Elimination of Germ Line Limited Chromosomes from Primary Germ Cells in Acricotopus lucidus (Diptera, Chironomidae)

Unusual mitotic behavior of additional chromosomes present only in the germ line was reported for the dipteran families Cecidomyiidae, Chironomidae and Sciaridae, which are known to have complex chromosome cycles. In the chironomid Acricotopus lucidus all germ line limited chromosomes (=Ks) are eliminated from the future somatic nuclei during germ line-soma differentiation in early, still syncytial, embryonic divisions by remaining in the equatorial plane. But also in the germ line about half of the Ks are eliminated in the first mitotic division of the primary germ cells in the young gonads of first instar larvae. Until now not much is known about the cytogenetic processes in the course of germ line elimination of the Ks. In this study, the cytogenetic changes in the primary germ cells before, during and after the elimination mitosis were analysed in detail with DAPI stained gonad preparations.

Immunocytological and FISH analysis of pole cell formation and soma elimination of germ line-limited chromosomes in the chironomid Acricotopus lucidus.

In the chironomid Acricotopus lucidus, germ line-soma differentiation becomes evident with the formation of the pole cells and the elimination of the germ line-limited chromosomes (Ks) from the future somatic nuclei of the embryo. Unlike in Drosophila, the early nuclear divisions do not proceed synchronously in A. lucidus. Usually, only one nucleus, the future pole nucleus, penetrates into the pole plasm, always at a telophase stage in the course of a regular mitosis. This happens by chance, depending on the orientation of the mitotic spindles of the early syncytial nuclei. Consequently, the time and the cell cycle at which a nucleus reaches the pole plasm, and pole cells arise, vary between embryos of the same oviposition. When entering the first germ line mitosis, while polar plasm and syncytial plasm are still not separated, some future somatic nuclei begin to eliminate their Ks. While the soma chromosomes (Ss) undergo normal anaphasic migration to the opposite poles, the K chromatids do not separate and remain in the equatorial plane, as demonstrated by fluorescence in situ hybridization using germ line-specific DNA probes. The elimination of the Ks does not occur at the same time in all future somatic nuclei. Nondisjunction of Ks was observed in the first mitosis of the pole nucleus, leading to primordial germ cells with different compositions of their K complements. The pattern and timing of elimination mitoses in the embryos indicate that each of the future somatic nuclei seems to regulate the elimination of the Ks autonomously.

Structural evolution of the germ line-limited chromosomes in Acricotopus.

The elimination of chromatin or whole chromosomes from the future somatic nuclei during germ line-soma differentiation in early embryogenesis is a genetic phenomenon found in a wide variety of animal species. Less is known about the origin, structure, and function of the germ line-limited chromosomes. In the chironomid Acricotopus lucidus fluorescence in situ hybridization (FISH) with labeled soma DNA to "Keimbahn" chromosomes (Ks) and soma chromosomes (Ss) of spermatogonial mitoses revealed that each of the nine different K types possesses large S-homologous sections, mostly in the distal parts of both chromosome arms. Painting probes of the three Ss and of each of their chromosome arms were generated by microdissection of polytene salivary gland chromosomes and subsequent amplification by the degenerate oligonucleotide-primed polymerase chain reaction. Multicolor FISH demonstrated that each of the Ks, with the exception of one K type, was painted by only one of the three S probes. Furthermore, in seven Ks, one chromosome arm was painted by the long-arm probe and the other by the short-arm probe of the S concerned. The hybridization pattern strongly suggests that each of these K types is derived from a specific S. One function of the S-homologous K sections is thought to be determination of the regular occurrence of crossover events, with the resulting chiasmata in these sections ensuring correct segregation of the K homologs during meiosis. Reverse chromosome painting on polytene S sets with a probe generated from metaphase Ks corroborates the above results and produces conclusive evidence for the hypothesis that during evolution the Ks have developed from the Ss by endopolyploidization and rearrangements followed by the accumulation of germ line-specific repetitive DNA sequences in the centromeric regions.

Germ Line-Soma Differentiation. W. Hennig

Previous articleNext article No AccessNew Biological BooksGerm Line-Soma Differentiation. W. Hennig Pieter D. NieuwkoopPieter D. Nieuwkoop Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Quarterly Review of Biology Volume 62, Number 2Jun., 1987 Published in association with Stony Brook University Article DOIhttps://doi.org/10.1086/415435 Copyright 1987 Stony Brook Foundation, Inc.PDF download Crossref reports no articles citing this article.

Spermatogenesis in Drosophila.

Spermatogenesis is an intensively studied developmental process in Drosophila melanogaster and Drosophila hydei. Nonetheless, some phases of the development of the germ cells are still hardly understood. The main gap of information is with respect to the germ line-soma differentiation and the formation of the gonad. Our knowledge about pole cell formation and its migration to the testis anlagen is still incomplete, and nearly nothing is known about the genetic control of these processes. In addition, some of these processes may be shared by both spermatogenesis and oogenesis. Therefore, this chapter will deal mainly with later stages of spermatogenesis, i.e., with the genetic control of the spermatogenesis sensu strictu.KeywordsFertility GeneComplementation GroupComplementation TestPrimary SpermatocyteFertility FactorThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Heterochromatin and germ line-restricted DNA.

The three reviews assembled in this volume approach the question of germ line-soma differentiation from the viewpoint of three different organisms. In all three organisms germ line and somatic cells are substantially distinguished in their karyotypes. Entire chromosomes or sections thereof are excluded from the somatic cells. This is achieved by quite different mechanisms. The divergent development of the karyotypes occurs within the germ line (as, for example, in Sciara) or during the earliest development of the organism (as, for example, in Ascaris). In ciliates an even more complex sequence of DNA rearrangements is found.KeywordsNurse CellConstitutive HeterochromatinGerm Cell DevelopmentLampbrush ChromosomeGerm Cell DifferentiationThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Complexity of germline and somatic DNA in Ascaris.

DURING the early determination of the blastomeres in the nematode Ascaris chromosomal segments are eliminated in the presomatic cells (chromatin diminution). In the germline, however, all the chromatin is preserved. Similar genome differentiation phenomena have been published1–3. Since no molecular description of chromatin diminution has been made we have begun to study the chromosomes in germline and somatic cells4. It was found that the amount of DNA occurring only in the germ-line chromosomes is not constant5 and that this DNA is not enriched in rDNA6. There is no evidence that the germline-limited DNA is transcribed in vivo7. The results presented here show that the germline DNA of Ascaris contains highly repetitive components, eliminated by the diminution process. Their possible molecular function in connection with germline–soma differentiation and chromosomal behaviour is discussed.