Germline

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Cormlets of Watsonia meriana, an example of apomixis Watsonia meriana detail of cormlets on inflorescence IMG 6909.JPG
Cormlets of Watsonia meriana , an example of apomixis
Clathria tuberosa, an example of a sponge that can grow indefinitely from somatic tissue and reconstitute itself from totipotent separated somatic cells Clathria tuberosa (Sponge).jpg
Clathria tuberosa, an example of a sponge that can grow indefinitely from somatic tissue and reconstitute itself from totipotent separated somatic cells

In biology and genetics, the germline is the population of a multicellular organism's cells that pass on their genetic material to the progeny (offspring). In other words, they are the cells that form the egg, sperm and the fertilised egg. They are usually differentiated to perform this function and segregated in a specific place away from other bodily cells. [1]

Contents

As a rule, this passing-on happens via a process of sexual reproduction; typically it is a process that includes systematic changes to the genetic material, changes that arise during recombination, meiosis and fertilization for example. However, there are many exceptions across multicellular organisms, including processes and concepts such as various forms of apomixis, autogamy, automixis, cloning or parthenogenesis. [2] [3] The cells of the germline are called germ cells. [4] For example, gametes such as a sperm and an egg are germ cells. So are the cells that divide to produce gametes, called gametocytes, the cells that produce those, called gametogonia, and all the way back to the zygote, the cell from which an individual develops. [4]

In sexually reproducing organisms, cells that are not in the germline are called somatic cells. According to this view, mutations, recombinations and other genetic changes in the germline may be passed to offspring, but a change in a somatic cell will not be. [5] This need not apply to somatically reproducing organisms, such as some Porifera [6] and many plants. For example, many varieties of citrus, [7] plants in the Rosaceae and some in the Asteraceae, such as Taraxacum produce seeds apomictically when somatic diploid cells displace the ovule or early embryo. [8]

In an earlier stage of genetic thinking, there was a clear distinction between germline and somatic cells. For example, August Weismann proposed and pointed out, a germline cell is immortal in the sense that it is part of a lineage that has reproduced indefinitely since the beginning of life and, barring accident, could continue doing so indefinitely. [9] However, it is now known in some detail that this distinction between somatic and germ cells is partly artificial and depends on particular circumstances and internal cellular mechanisms such as telomeres and controls such as the selective application of telomerase in germ cells, stem cells and the like. [10]

Not all multicellular organisms differentiate into somatic and germ lines, [11] but in the absence of specialised technical human intervention practically all but the simplest multicellular structures do so. In such organisms somatic cells tend to be practically totipotent, and for over a century sponge cells have been known to reassemble into new sponges after having been separated by forcing them through a sieve. [6]

Germline can refer to a lineage of cells spanning many generations of individuals—for example, the germline that links any living individual to the hypothetical last universal common ancestor, from which all plants and animals descend.

Evolution

Plants and basal metazoans such as sponges (Porifera) and corals (Anthozoa) do not sequester a distinct germline, generating gametes from multipotent stem cell lineages that also give rise to ordinary somatic tissues. It is therefore likely that germline sequestration first evolved in complex animals with sophisticated body plans, i.e. bilaterians. There are several theories on the origin of the strict germline-soma distinction. Setting aside an isolated germ cell population early in embryogenesis might promote cooperation between the somatic cells of a complex multicellular organism. [12] Another recent theory suggests that early germline sequestration evolved to limit the accumulation of deleterious mutations in mitochondrial genes in complex organisms with high energy requirements and fast mitochondrial mutation rates. [11]

DNA damage, mutation and repair

Reactive oxygen species (ROS) are produced as byproducts of metabolism. In germline cells, ROS are likely a significant cause of DNA damages that, upon DNA replication, lead to mutations. 8-Oxoguanine, an oxidized derivative of guanine, is produced by spontaneous oxidation in the germline cells of mice, and during the cell’s DNA replication cause GC to TA transversion mutations. [13] Such mutations occur throughout the mouse chromosomes as well as during different stages of gametogenesis.

The mutation frequencies for cells in different stages of gametogenesis are about 5 to 10-fold lower than in somatic cells both for spermatogenesis [14] and oogenesis. [15] The lower frequencies of mutation in germline cells compared to somatic cells appears to be due to more efficient DNA repair of DNA damages, particularly homologous recombinational repair, during germline meiosis.[ citation needed ] Among humans, about five percent of live-born offspring have a genetic disorder, and of these, about 20% are due to newly arisen germline mutations. [14]

Epigenetic alterations

5 methylcytosine methyl highlight. The image shows a cytosine single ring base and a methyl group added on to the 5 carbon. In mammals, DNA methylation occurs almost exclusively at a cytosine that is followed by a guanine. 5 methylcytosine methyl highlight.png
5 methylcytosine methyl highlight. The image shows a cytosine single ring base and a methyl group added on to the 5 carbon. In mammals, DNA methylation occurs almost exclusively at a cytosine that is followed by a guanine.

Epigenetic alterations of DNA include modifications that affect gene expression, but are not caused by changes in the sequence of bases in DNA. A well-studied example of such an alteration is the methylation of DNA cytosine to form 5-methylcytosine. This usually occurs in the DNA sequence CpG, changing the DNA at the CpG site from CpG to 5-mCpG. Methylation of cytosines in CpG sites in promoter regions of genes can reduce or silence gene expression. [16] About 28 million CpG dinucleotides occur in the human genome, [17] and about 24 million CpG sites in the mouse genome (which is 86% as large as the human genome [18] ). In most tissues of mammals, on average, 70% to 80% of CpG cytosines are methylated (forming 5-mCpG). [19]

In the mouse, by days 6.25 to 7.25 after fertilization of an egg by a sperm, cells in the embryo are set aside as primordial germ cells (PGCs). These PGCs will later give rise to germline sperm cells or egg cells. At this point the PGCs have high typical levels of methylation. Then primordial germ cells of the mouse undergo genome-wide DNA demethylation, followed by subsequent new methylation to reset the epigenome in order to form an egg or sperm. [20]

In the mouse, PGCs undergo DNA demethylation in two phases. The first phase, starting at about embryonic day 8.5, occurs during PGC proliferation and migration, and it results in genome-wide loss of methylation, involving almost all genomic sequences. This loss of methylation occurs through passive demethylation due to repression of the major components of the methylation machinery. [20] The second phase occurs during embryonic days 9.5 to 13.5 and causes demethylation of most remaining specific loci, including germline-specific and meiosis-specific genes. This second phase of demethylation is mediated by the TET enzymes TET1 and TET2, which carry out the first step in demethylation by converting 5-mC to 5-hydroxymethylcytosine (5-hmC) during embryonic days 9.5 to 10.5. This is likely followed by replication-dependent dilution during embryonic days 11.5 to 13.5. [21] At embryonic day 13.5, PGC genomes display the lowest level of global DNA methylation of all cells in the life cycle. [20]

In the mouse, the great majority of differentially expressed genes in PGCs from embryonic day 9.5 to 13.5, when most genes are demethylated, are upregulated in both male and female PGCs. [21]

Following erasure of DNA methylation marks in mouse PGCs, male and female germ cells undergo new methylation at different time points during gametogenesis. While undergoing mitotic expansion in the developing gonad, the male germline starts the re-methylation process by embryonic day 14.5. The sperm-specific methylation pattern is maintained during mitotic expansion. DNA methylation levels in primary oocytes before birth remain low, and re-methylation occurs after birth in the oocyte growth phase. [20]

See also

Related Research Articles

Zygote Single diploid eukaryotic cell formed by a fertilization event between two gametes

A zygote is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual organism.

Epigenetics Study of heritable DNA and histone modifications that affect the expression of a gene without a change in its nucleotide sequence.

In biology, epigenetics is the study of heritable phenotype changes that do not involve alterations in the DNA sequence. The Greek prefix epi- in epigenetics implies features that are "on top of" or "in addition to" the traditional genetic basis for inheritance. Epigenetics most often involves changes that affect gene activity and expression, but the term can also be used to describe any heritable phenotypic change. Such effects on cellular and physiological phenotypic traits may result from external or environmental factors, or be part of normal development.

Cellular differentiation Process in which relatively unspecialized cells acquire specialized features

Cellular differentiation is the process in which a cell changes from one cell type to another. Usually, the cell changes to a more specialized type. Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types. Differentiation continues in adulthood as adult stem cells divide and create fully differentiated daughter cells during tissue repair and during normal cell turnover. Some differentiation occurs in response to antigen exposure. Differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals. These changes are largely due to highly controlled modifications in gene expression and are the study of epigenetics. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. Although metabolic composition does get altered quite dramatically where stem cells are characterized by abundant metabolites with highly unsaturated structures whose levels decrease upon differentiation. Thus, different cells can have very different physical characteristics despite having the same genome.

CpG site Region of often-methylated DNA with a cytosine followed by a guanine

The CpG sites or CG sites are regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide in the linear sequence of bases along its 5' → 3' direction. CpG sites occur with high frequency in genomic regions called CpG islands. Cytosines in CpG dinucleotides can be methylated to form 5-methylcytosines. Enzymes that add a methyl group are called DNA methyltransferases. In mammals, 70% to 80% of CpG cytosines are methylated. Methylating the cytosine within a gene can change its expression, a mechanism that is part of a larger field of science studying gene regulation that is called epigenetics.

A somatic cell, or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell.

The term somatic - etymologically from the French word "somatique", from Ancient Greek "σωματικός", from σῶμα - is often used in biology to refer to the cells of the body in contrast to the reproductive (germline) cells, which usually give rise to the egg or sperm. These somatic cells are diploid, containing two copies of each chromosome, whereas germ cells are haploid, as they only contain one copy of each chromosome. Although under normal circumstances all somatic cells in an organism contain identical DNA, they develop a variety of tissue-specific characteristics. This process is called differentiation, through epigenetic and regulatory alterations. The grouping of similar cells and tissues creates the foundation for organs.

DNA methyltransferase Class of enzymes

In biochemistry, the DNA methyltransferase family of enzymes catalyze the transfer of a methyl group to DNA. DNA methylation serves a wide variety of biological functions. All the known DNA methyltransferases use S-adenosyl methionine (SAM) as the methyl donor.

Germ cell Gamete-producing cell

A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they undergo meiosis, followed by cellular differentiation into mature gametes, either eggs or sperm. Unlike animals, plants do not have germ cells designated in early development. Instead, germ cells can arise from somatic cells in the adult, such as the floral meristem of flowering plants.

Weismann barrier distinction between germ cell lineages producing gametes and somatic cells

The Weismann barrier, proposed by August Weismann, is the strict distinction between the "immortal" germ cell lineages producing gametes and "disposable" somatic cells, in contrast to Charles Darwin's proposed pangenesis mechanism for inheritance. In more precise terminology, hereditary information moves only from germline cells to somatic cells. This does not refer to the central dogma of molecular biology, which states that no sequential information can travel from protein to DNA or RNA, but both hypotheses relate to a gene-centric view of life.

DNA methylation Biological process

DNA methylation is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts to repress gene transcription. In mammals, DNA methylation is essential for normal development and is associated with a number of key processes including genomic imprinting, X-chromosome inactivation, repression of transposable elements, aging, and carcinogenesis.

Germline mutation Inherited genetic variation

A germline mutation, or germinal mutation, is any detectable variation within germ cells. Mutations in these cells are the only mutations that can be passed on to offspring, when either a mutated sperm or oocyte come together to form a zygote. After this fertilization event occurs, germ cells divide rapidly to produce all of the cells in the body, causing this mutation to be present in every somatic and germline cell in the offspring; this is also known as a constitutional mutation. Germline mutation is distinct from somatic mutation.

Germ plasm

Germ plasm is a biological concept developed in the 19th century by the German biologist August Weismann. It states that heritable information is transmitted only by germ cells in the gonads, not by somatic cells. The related idea that information cannot pass from somatic cells to the germ line, contrary to Lamarckism, is called the Weismann barrier. To some extent this theory anticipated the development of modern genetics.

In biology, reprogramming refers to erasure and remodeling of epigenetic marks, such as DNA methylation, during mammalian development or in cell culture. Such control is also often associated with alternative covalent modifications of histones.

The cells that give rise to the gametes are often set aside during embryonic cleavage. During development, these cells will differentiate into primordial germ cells, migrate to the location of the gonad, and form the germline of the animal.

DNA demethylation Removal of a methyl group from one or more nucleotides within an DNA molecule.

In mammals, DNA demethylation causes replacement of 5-methylcytosine (5mC) in a DNA sequence by cytosine (C). DNA demethylation can occur by an active process at the site of a 5mC in a DNA sequence or, in replicating cells, by preventing addition of methyl groups to DNA so that the replicated DNA will largely have cytosine in the DNA sequence.

5-Hydroxymethylcytosine Chemical compound

5-Hydroxymethylcytosine (5hmC) is a DNA pyrimidine nitrogen base derived from cytosine. It is potentially important in epigenetics, because the hydroxymethyl group on the cytosine can possibly switch a gene on and off. It was first seen in bacteriophages in 1952. However, in 2009 it was found to be abundant in human and mouse brains, as well as in embryonic stem cells. In mammals, it can be generated by oxidation of 5-methylcytosine, a reaction mediated by TET enzymes. Its molecular formula is C5H7N3O2.

<i>Volvox carteri</i> Species of alga

Volvox carteri is a species of colonial green algae in the order Volvocales. The V. carteri life cycle includes a sexual phase and an asexual phase. V. carteri forms small spherical colonies, or coenobia, of 2000–6000 Chlamydomonas-type somatic cells and 12–16 large, potentially immortal reproductive cells called gonidia. While vegetative, male and female colonies are indistinguishable; however, in the sexual phase, females produce 35-45 eggs and males produce up to 50 sperm packets with 64 or 128 sperm each.

Human epigenome is the complete set of structural modifications of chromatin and chemical modifications of histones and nucleotides. These modifications affect gene expression according to cellular type and development status. Various studies show that epigenome depends on exogenous factors.

A somatic mutation is change in the DNA sequence of a somatic cell of a multicellular organism with dedicated reproductive cells; that is, any mutation that occurs in a cell other than a gamete, germ cell, or gametocyte. Unlike germline mutations, which can be passed on to the descendants of an organism, somatic mutations are not usually transmitted to descendants. This distinction is blurred in plants, which lack a dedicated germline, and in those animals that can reproduce asexually through mechanisms such as budding, as in members of the cnidarian genus Hydra.

TET enzymes

The TET enzymes are a family of ten-eleven translocation (TET) methylcytosine dioxygenases. They are instrumental in DNA demethylation. 5-Methylcytosine is a methylated form of the DNA base cytosine (C) that often regulates gene transcription and has several other functions in the genome.

References

  1. Pieter Dirk Nieuwkoop; Lien A. Sutasurya (1979). Primordial Germ Cells in the Chordates: Embryogenesis and Phylogenesis. CUP Archive. ISBN   978-0-521-22303-4.
  2. Juan J. Tarin; Antonio Cano (14 September 2000). Fertilization in Protozoa and Metazoan Animals: Cellular and Molecular Aspects. Springer. ISBN   978-3-540-67093-3.
  3. Andrew Lowe; Stephen Harris; Paul Ashton (1 April 2009). Ecological Genetics: Design, Analysis, and Application. John Wiley & Sons. pp. 108–. ISBN   978-1-4443-1121-1.
  4. 1 2 Nikolas Zagris; Anne Marie Duprat; Antony Durston (30 November 1995). Organization of the Early Vertebrate Embryo. Springer. pp. 2–. ISBN   978-0-306-45132-4.
  5. C.Michael Hogan. 2010. Mutation. ed. E.Monosson and C.J.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC Archived April 30, 2011, at the Wayback Machine
  6. 1 2 Brusca, Richard C.; Brusca, Gary J. (1990). Invertebrates . Sunderland: Sinauer Associates. ISBN   978-0878930982.
  7. Akira Wakana and Shunpei Uemoto. Adventive Embryogenesis in Citrus (Rutaceae). II. Postfertilization Development. American Journal of Botany Vol. 75, No. 7 (Jul., 1988), pp. 1033-1047 Published by: Botanical Society of America Article Stable URL: https://www.jstor.org/stable/2443771
  8. K V Ed Peter (5 February 2009). Basics Of Horticulture. New India Publishing. pp. 9–. ISBN   978-81-89422-55-4.
  9. August Weismann (1892). Essays upon heredity and kindred biological problems. Clarendon press.
  10. Watt, F. M. and B. L. M. Hogan. 2000 Out of Eden: Stem Cells and Their Niches Science 287:1427-1430.
  11. 1 2 Radzvilavicius, Arunas L.; Hadjivasiliou, Zena; Pomiankowski, Andrew; Lane, Nick (2016-12-20). "Selection for Mitochondrial Quality Drives Evolution of the Germline". PLOS Biology. 14 (12): e2000410. doi:10.1371/journal.pbio.2000410. ISSN   1545-7885. PMC   5172535 . PMID   27997535.
  12. Buss, L W (1983-03-01). "Evolution, development, and the units of selection". Proceedings of the National Academy of Sciences of the United States of America. 80 (5): 1387–1391. Bibcode:1983PNAS...80.1387B. doi: 10.1073/pnas.80.5.1387 . ISSN   0027-8424. PMC   393602 . PMID   6572396.
  13. Ohno M, Sakumi K, Fukumura R, Furuichi M, Iwasaki Y, Hokama M, Ikemura T, Tsuzuki T, Gondo Y, Nakabeppu Y (2014). "8-oxoguanine causes spontaneous de novo germline mutations in mice". Sci Rep. 4: 4689. Bibcode:2014NatSR...4E4689O. doi:10.1038/srep04689. PMC   3986730 . PMID   24732879.
  14. 1 2 Walter CA, Intano GW, McCarrey JR, McMahan CA, Walter RB (1998). "Mutation frequency declines during spermatogenesis in young mice but increases in old mice". Proc. Natl. Acad. Sci. U.S.A. 95 (17): 10015–9. Bibcode:1998PNAS...9510015W. doi: 10.1073/pnas.95.17.10015 . PMC   21453 . PMID   9707592.
  15. Murphey P, McLean DJ, McMahan CA, Walter CA, McCarrey JR (2013). "Enhanced genetic integrity in mouse germ cells". Biol. Reprod. 88 (1): 6. doi:10.1095/biolreprod.112.103481. PMC   4434944 . PMID   23153565.
  16. Bird A (January 2002). "DNA methylation patterns and epigenetic memory". Genes Dev. 16 (1): 6–21. doi: 10.1101/gad.947102 . PMID   11782440.
  17. Lövkvist C, Dodd IB, Sneppen K, Haerter JO (June 2016). "DNA methylation in human epigenomes depends on local topology of CpG sites". Nucleic Acids Res. 44 (11): 5123–32. doi:10.1093/nar/gkw124. PMC   4914085 . PMID   26932361.
  18. Guénet JL (December 2005). "The mouse genome". Genome Res. 15 (12): 1729–40. doi: 10.1101/gr.3728305 . PMID   16339371.
  19. Jabbari K, Bernardi G (May 2004). "Cytosine methylation and CpG, TpG (CpA) and TpA frequencies". Gene. 333: 143–9. doi:10.1016/j.gene.2004.02.043. PMID   15177689.
  20. 1 2 3 4 Zeng Y, Chen T (March 2019). "DNA Methylation Reprogramming during Mammalian Development". Genes (Basel). 10 (4): 257. doi: 10.3390/genes10040257 . PMC   6523607 . PMID   30934924.
  21. 1 2 Yamaguchi S, Hong K, Liu R, Inoue A, Shen L, Zhang K, Zhang Y (March 2013). "Dynamics of 5-methylcytosine and 5-hydroxymethylcytosine during germ cell reprogramming". Cell Res. 23 (3): 329–39. doi:10.1038/cr.2013.22. PMC   3587712 . PMID   23399596.