Editing Germ plasm

{{Short description|Biological concept}}
{{distinguish|text=[[Germplasm]], a collection of genetic resources for an organism (such as seeds or embryos)}}
[[File:Weismann's Germ Plasm.svg|thumb|upright=1.5|[[August Weismann]]'s 1892 germ plasm theory. The hereditary material, the germ plasm, is transmitted only by the [[gonad]]s. Somatic cells (of the body) [[embryology|develop afresh]] in each generation from the germ plasm.]]
[[File:August Weismann.jpg|thumb|upright|August Weismann proposed the germ plasm theory in the 19th century, before the foundation of modern [[genetics]].]]

'''Germ plasm''' ({{Langx|de|Keimplasma}}) 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 [[gonad]]s (ovaries and testes), 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]].

==History==

The term ''Keimplasma'' (germ plasm) was first used by the German biologist, [[August Weismann]] (1834–1914), and described in his 1892 book ''Das Keimplasma: eine Theorie der Vererbung'' (The Germ Plasm: a theory of inheritance).<ref name=Weismann1892>{{cite book |last=Weismann |first=August |authorlink=August Weismann |year=1892 |title=Das Keimplasma: eine Theorie der Vererbung |url=https://archive.org/details/bub_gb_f5U-AAAAYAAJ |publisher=Fischer |location=Jena}}</ref> His theory states that multicellular organisms consist of [[germ cell]]s that contain and transmit heritable information, and [[somatic cells]] which carry out ordinary bodily functions.<ref name=Weismann1892/><ref name="Winther 2001">{{Cite journal |last=Winther |first=Rasmus G. |date=2001 |title=August Weismann on Germ-Plasm Variation |jstor=4331686 |journal=Journal of the History of Biology |volume=34 |issue=3 |pages=517–555 |doi=10.1023/A:1012950826540 |pmid=11859887 |s2cid=23808208 |issn=0022-5010}}</ref> In the germ plasm theory, inheritance in a multicellular organism only takes place by means of the germ cells: the [[gametes]], such as egg cells and sperm cells. Other cells of the body do not function as agents of heredity. The effect is one-way: germ cells produce somatic cells, and more germ cells; the germ cells are not affected by anything the somatic cells learn or any ability the body acquires during its life. Genetic information cannot pass from soma to germ plasm and on to the next generation. This is referred to as the [[Weismann barrier]].<ref>[http://www.esp.org/books/weismann/germ-plasm/facsimile/ Germ-Plasm, a theory of heredity (1893)]</ref> This idea, if true, rules out the [[Lamarckism|inheritance of acquired characteristics]] as proposed by [[Jean-Baptiste Lamarck]], like others before him, and accepted by [[Charles Darwin]] both in ''[[On the Origin of Species#Variation and heredity|On the Origin of Species]]'' and as part of his [[pangenesis]] theory of inheritance.<ref>{{cite book |last=Huxley |first=Julian |authorlink=Julian Huxley |year=1942 |title=Evolution, the Modern Synthesis |page=17 |title-link=Evolution, the Modern Synthesis |jstor=4331686}}</ref>

However, a careful reading of Weismann's work over the span of his entire career shows that he had more nuanced views. He insisted, like Darwin, that a variable environment was necessary to cause variation in the hereditary material.<ref name="Winther 2001"/> Because genetic information cannot pass from soma to germ plasm, these external conditions, he believed, caused different effects on the soma and the germ plasm. Thus, the historian of science Rasmus G. Winther states, Weismann was not a Weismannian, as he, like Darwin, did believe in the inheritance of acquired characteristics, which later came to be known as Lamarckian.<ref name="Winther 2001"/>

The part of Weismann's theory which proved most vulnerable was his notion that the germ plasm (effectively, [[gene]]s) was successively reduced during division of somatic cells. As modern [[genetics]] developed, it became clear that this idea is wrong in most cases.<ref>For example, by studies of [[polytene chromosome]]s in salivary glands (i.e. somatic cells) of larval ''[[Drosophila]]''.</ref> Cases such as [[Dolly (sheep)|Dolly]], the cloned sheep, proved via [[somatic cell nuclear transfer]] that adult cells retain a complete set of information – as opposed to Weismann's increasingly determined gradual loss of genetic information – putting this aspect of Weismann's theory to rest as a general rule of metazoan development. However, genetic information is readily lost by somatic cells in some groups of animals through [[somatic genome processing]]. The best known example is the [[nematode]]s, in which the phenomenon of chromatin diminution was first described by [[Theodor Boveri]] in 1887.<ref name="Streit 2012">{{cite journal |last=Streit |first=Adrian |title=Silencing by Throwing Away: A Role for Chromatin Diminution |journal=Developmental Cell |date=November 2012 |volume=25 |issue=5 |pages=918–919 |doi=10.1016/j.devcel.2012.10.022 |pmid=23153488 |doi-access=free}}</ref>

The idea was to some extent anticipated in an 1865 article by [[Francis Galton]], published in ''[[Macmillan's Magazine]]'', which set out a weak version of the concept. In 1889 Weismann wrote to acknowledge that "You have exposed in your paper an idea which is in one essential point nearly allied to the main idea contained in my theory of the continuity of germ-plasm".<ref>[http://roughguidetoevolution.blogspot.com/2011/08/galton-or-weismann-first-to-continuity.html The Rough Guide to Evolution: Galton or Weismann first to continuity of the germ-plasm?]</ref>

==Modern view==

The idea of the Weismann barrier, namely that changes acquired during an organism's life cannot affect its offspring, is still broadly accepted. This has been extended into molecular terms as the [[central dogma of molecular biology]], which asserts that information written in the form of [[protein]]s cannot be fed back into genetically transmissible information encoded in [[nucleic acid]]s.<ref>{{cite book |author=Turner, J. Scott |editor1=Henning, Brian G.|editor2=Scarfe, Adam Christian |title=Biology's Second Law: Homeostasis, Purpose, and Desire |work=Beyond Mechanism: Putting Life Back Into Biology |url=https://books.google.com/books?id=naQm1_Lutq4C&pg=PA192 |year=2013 |publisher=Rowman and Littlefield |isbn=978-0-7391-7436-4 |page=192 |quote=Where Weismann would say that it is impossible for changes acquired during an organism's lifetime to feed back onto transmissible traits in the germ line, the CDMB now added that it was impossible for information encoded in proteins to feed back and affect genetic information in any form whatsoever, which was essentially a molecular recasting of the Weismann barrier.}}</ref>

The Weismannian notion that the germ cells are unaffected by somatic cells or their environment is however proving not to be absolute. Chemical modification of the [[nucleotide]] bases that constitute the [[genetic code]] such as methylation of [[cytosine]]s as well as modifications of the [[histone]]s around which [[DNA]] is organized into higher-order structures are influenced by the metabolic and physiologic state of the organism and in some cases can be heritable. Such changes are called [[epigenetics|epigenetic]] because they do not alter the nucleotide sequence.<ref>{{cite journal |author1=Berger, S. L. |author2=Kouzarides, T. |author3=Shiekhattar, R. |author4=Shilatifard, A.  | title=An operational definition of epigenetics |journal=[[Genes & Development]] | volume=23 | issue=7 | pages=781–83 | year=2009 | pmid=19339683 | pmc=3959995 | doi=10.1101/gad.1787609 }}</ref>

==References==
{{reflist}}

==External links==
* {{Cite NIE|wstitle=Germ-Plasm |year=1906 |short=x}}

[[Category:Germ cells]]
[[Category:Cytogenetics]]
[[Category:19th century in biology]]