Weismann Barrier
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The Weismann barrier, proposed by
August Weismann August Friedrich Leopold Weismann (; 17 January 18345 November 1914) was a German evolutionary biology, evolutionary biologist. Fellow German Ernst Mayr ranked him as the second most notable evolutionary theorist of the 19th century, after Charl ...
, is the strict distinction between the "immortal" germ cell lineages producing gametes and "disposable" somatic cells in animals (but not plants), in contrast to
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
's proposed pangenesis mechanism for inheritance. In more precise terminology, hereditary information is copied only from germline cells to somatic cells. This means that new information from somatic mutation is not passed on to the germline. This ''barrier'' concept implies that somatic mutations are not inherited. Weismann set out the concept in his 1892 book ″Das Keimplasma: eine Theorie der Vererbung″ (German for ''The Germ Plasm: a theory of inheritance''). The use of this theory, commonly in the context of the germ plasm theory of the late 19th century, before the development of better-based and more sophisticated concepts of genetics in the early 20th century, is sometimes referred to as Weismannism. Some authors distinguish Weismannist development (either preformistic or
epigenetic In biology, epigenetics is the study of changes in gene expression that happen without changes to the DNA sequence. The Greek prefix ''epi-'' (ἐπι- "over, outside of, around") in ''epigenetics'' implies features that are "on top of" or "in ...
) that in which there is a distinct germline, from somatic embryogenesis. This type of development is correlated with the evolution of death of the somatic line. The Weismann barrier was of great importance in its day and among other influences it effectively banished certain Lamarckian concepts: in particular, it would make Lamarckian inheritance from changes to the body (the soma) difficult or impossible. It remains important, but has however required qualification in the light of modern understanding of
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
and some other genetic and histological developments.


Immortality of the germline

The Russian biologist and historian Zhores A. Medvedev, reviewing Weismann's theory a century later, considered that the accuracy of
genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
replicative and other synthetic systems alone could not explain the "immortal" germ cell lineages proposed by Weismann. Rather Medvedev thought that known features of the biochemistry and genetics of
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
indicated the presence of unique information maintenance and restoration processes at the different stages of gametogenesis. In particular, Medvedev considered that the most important opportunities for information maintenance of
germ cell A germ cell is any 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 unde ...
s are created by recombination during meiosis and
DNA repair DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is cons ...
; he saw these as processes within the germ cells that were capable of restoring the integrity of
DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
and
chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
s from the types of damage that caused irreversible ageing in
somatic cell In cellular biology, 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. Somatic cells compose the body of an organism ...
s.


Basal animals

Basal animals such as sponges (
Porifera Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a Basal (phylogenetics) , basal clade and a sister taxon of the Eumetazoa , diploblasts. They are sessility (motility) , sessile ...
) and corals (
Anthozoa Anthozoa is one of the three subphyla of Cnidaria, along with Medusozoa and Endocnidozoa. It includes Sessility (motility), sessile marine invertebrates and invertebrates of brackish water, such as sea anemones, Scleractinia, stony corals, soft c ...
) contain multipotent stem cell lineages, that give rise to both somatic and reproductive cells. The Weismann barrier appears to be of a more recent evolutionary origin among animals.


Plants

In plants, genetic changes in somatic lines can and do result in genetic changes in the germ lines, because the germ cells are produced by somatic cell lineages (vegetative
meristem In cell biology, the meristem is a structure composed of specialized tissue found in plants, consisting of stem cells, known as meristematic cells, which are undifferentiated cells capable of continuous cellular division. These meristematic c ...
s), which may be old enough (many years) to have accumulated multiple mutations since seed germination, some of them subject to natural selection. It is noteworthy in this context that, generally speaking, adult, reproducing plants tend to produce many more offspring in number than animal organisms.


See also

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References

{{reflist Genetics Lamarckism 1892 in science 1892 in Germany