Chimera (genetics)

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Grafting ( goat's foot refinement ) in a cherry tree . The resulting plant is a chimera.

In medicine and biology, a chimera is the name given to an organism that is made up of genetically different cells or tissues and yet represents a uniform individual . The different cells of such a chimeric organism come from different fertilized egg cells . Whether the different cells come from individuals of the same species or from different species is irrelevant for the definition. The chimera must be separated from the mosaic, in which genetically different cells also occur, but they all come from the same fertilized egg cell, and from the inter-species hybrid , which normally comes from a single fertilized egg cell but has parents from different species, e.g. B. the mule .

Blood chimeras are known in humans and other mammals , which in cattle are known as freemartins . They can arise in pregnancies with multiple multiples if anastomoses form in the placenta . The blood of the embryos mixes with each other and is therefore based on different blood stem cells , in addition to the blood stem cells of the multiple siblings. In principle, different blood groups are also possible in these cases . Also allografts or xenografts make the organ recipient to the chimera.

In plants, chimeras are created artificially through refinement . If the cells of a chimera have different degrees of ploidy , for example after colchicine treatment, they are called cytochimera. From the study of plant chimera it could be deduced that these cells and tissues can also combine strongly genetically different cells to form a complex organism. According to current doctrine, there is no exchange of genetic information between the genetically different cells of the chimera. For genetically modified tobacco plants, however, it was shown in 2009 that after grafting, the transgenes can be transferred to tissue of the other partner near the grafting point. It remains unclear whether a transfer is also possible in other cases.

Word origin and history

The name chimera is derived from a hybrid being in Greek mythology. The Chimera is a monster snorting fire, a lion in the front, a goat in the middle, a dragon in the back, killed by Bellerophon . In a figurative sense, chimera or chimera also stands for a fantasy , absurdity or a pipe dream .

Chimeras have long been known, at least in plants. In 1674 P. Natus described a Citrus Bizzarria - a chimera made from citron and bitter orange - found in Florence . In 1875 near Paris a chimera from Laburnum anagyroides and Cytisus purpureus became known, of which Eduard Strasburger was able to show in 1907 that it was not a hybrid but a chimera. The term chimera was coined by Hans Winkler in 1908.

Human chimeras

Natural and artificial chimeras

Blood chimeras can occur naturally in humans. Aside from these, different cell lines can also make different organs. In one known case , the uterus came from one cell line and the rest of the body from the other.

Artificial animal-human embryos are also produced in biomedical research . This involves introducing human genome into egg cells from animals. The resulting embryo is called a cytoplasmic hybrid , or cybrid for short , and can be used for stem cell research . This procedure is ethically controversial. The opponents fear abuse, such as the breeding of hybrids from humans and animals. Proponents argue that human egg cells are becoming increasingly scarce and expensive for research. It is difficult to find enough women to donate eggs.

In medicine, organ transplantation involves the transplantation of donor organs from a donor that is genetically different from the recipient. Since cell lines from different fertilized egg cells come together in the recipient's body, the recipient of an organ donation also becomes a chimera by definition. The recipient of a blood transfusion, on the other hand, only becomes a chimera for a short time, as the blood cells received are broken down again after some time.

British Embryo Act 2008

The British House of Commons decided on 22 October 2008 an extensive embryos law that allows the breeding of animal-human chimeras and human-human chimeras. The latter are used, among other things, to generate so-called rescue siblings . After the House of Lords approved the law, it came into effect in November 2008. According to the law, the chimeras must be destroyed after two weeks. The then British Prime Minister Gordon Brown is a strong supporter of the law. His son Fraser suffers from cystic fibrosis , a disease that there is hope that one day it will be better treated with findings from embryo research.

Intracellular chimeras

Intracellular chimeras contain nuclear , plastid or mitochondrial genomes of different species in a cell . This happens experimentally through protoplast fusion , or with hybrid formation in species in which plastids and mitochondria are inherited by both parents.

Gene chimeras

A chimeric gene consists of parts derived from two or more precursor genes. They arise through exon shuffling or through retrotransposition . The first such gene discovered was the jingwei gene in Drosophila teissieri and Drosophila yakuba, which is made up of the three exons of the yellow emperor gene and a fourth exon, 90% of which is the intron -less Adh gene (which codes for alcohol dehydrogenase ) corresponds, exists.

Individual evidence

  1. ↑ a b c Peter Schopfer, Axel Brennicke: Plant Physiology . Elsevier, Munich 2006, ISBN 978-3-8274-1561-5 , pp. 546f.
  2. ^ RR Behringer: Human-animal chimeras in biomedical research . In: Cell Stem Cell . 1, No. 3, September 2007, pp. 259-62. doi : 10.1016 / j.stem.2007.07.021 . PMID 18371360 .
  3. ^ Pschyrembel. ISBN 3-11-007018-9
  4. ↑ a b Gerhard Wagenitz : Dictionary of Botany. The terms in their historical context. 2nd, expanded edition. Spektrum Akademischer Verlag, Heidelberg / Berlin 2003, ISBN 3-8274-1398-2 , p. 60.
  5. ↑ Stegemann S, Bock R: Exchange of genetic material between cells in plant tissue grafts . In: Science . 324, No. 5927, May 2009, pp. 649-51. doi : 10.1126 / science.1170397 . PMID 19407205 .
  6. ^ F. A. Brockhaus: The Volks-Brockhaus . 10th edition. Verlag F. A. Brockhaus, Leipzig 1943, p. 105.
  7. ↑ N. Yu, MS Kruskall et al. a .: Disputed maternity leading to identification of tetragametic chimerism. In: The New England Journal of Medicine . Volume 346, Number 20, May 2002, pp. 1545-1552, ISSN  1533-4406 . doi : 10.1056 / NEJMoa013452 . PMID 12015394 .
  8. ↑ Lower House approves chimeras in "Märkische Oderzeitung", October 24, 2008, p. 4.
  9. ↑ netzeitung.de British House of Commons has decided: - Chimera research becomes law ( Memento of October 24, 2008 in the Internet Archive )
  10. ^ Regulation of stem cell research in the United Kingdom | Eurostemcell. Retrieved July 10, 2017 (English).
  11. ↑ Chronology of the legislative process
  12. ^ Douglas Futuyma: Evolution . Sinauer, Sunderland 2005, ISBN 0-87893-187-2 , p. 462f.