Somaclonal variation

Under somaclonal variation (also: Somaclonal variability) refers primarily occurring every deviation from the genetic initial state of an individual. In the in vitro culture, this refers to the arising in the cultivation of plant cells or plant tissue genotypic and phenotypic changes often compared to the original plant. These occur, for example, often after differentiation of plant tissues from callus cells. If in conducting maintenance or proliferation culture in an identical replication of the plant, ie the production of clones desired in vitro culture, somaclonal variants are therefore undesirable.

Initial investigation and definition of terms

During the general and increasingly commercial establishment of Herbal tissue culture in the 1970s and 80s frequently experienced adverse, apparently genetically related changes in plant material. 1981, for the first time proposed to label the genetic changes that occurred in the in vitro cultivation frame as somaclonal variation.

In the meantime, can be found in relevant encyclopaedias the definition that it is occurring variation of properties from in vitro cultures of plant cells, calli or organs in the somaclonal variation. Their causes are often unclear and partly to settle at the genetic or epigenetic level. Known causes of this effect are, for example, gene loss or numerical changes of the karyotype. And the change characteristics in which several genes are involved ( polygeny ) is possible.

Somaclonal in vitro methods and the occurrence of variation in plants

Plants have often already existing prior to in vitro culture genetic differences. The emergence and expression of genetic variation occurs but rarely in the run-up to the in vitro cultivation on, but more often during the implementation of certain in vitro culture methods. By some in vitro cultivation methods and their conditions or external factors such as phytohormones, these differences can be selected and later also occur phenotypically in appearance. Also only during the emerging in vitro cultivation genetic variations are possible.

Especially due to the high rate of cell division in tumor- cell-like plant callus tissue has led to more errors in reading the genetic information. These usually lead to aneuploidy and Ploidiemutation. Be regenerated from this callus tissue later on organogenesis plant organs or whole plants, the now widely copied genetic misinformation occur for the typical characteristics of somaclonal variation are responsible.

In addition, the relationship between genetic stability and the output tissue regeneration occurs during in vitro culture. Thus, a direct regeneration from pre-existing meristematic tissue is considered to be genetically stable. If it comes to the formation of secondary meristem, already first genetic changes occur. Type and quantity of these changes take this all the more, the more moves the regeneration of tissue during in vitro cultivation of the Primärregeneratbildung towards secondary tissue regeneration processes.

In plants, clonal propagation via in vitro culture methods such as shoot tip or meristem culture or the culture of other suitable Explantatformen such as leaf or shoot segments. The aim of clonal propagation is the production of genetically stable and identical offspring. The intention is usually less a quantitative proliferation of clones but - in commercial horticulture or agriculture - the Pathogeneliminierung, consistent with preservation of the species or variety-specific genetic actual condition. The generation of so-called elite material for further conventional propagation (cuttings ) is mainly in horticulture in crops such as Pelargonium, New Guinea Impatiens or Chrysanthemum in the foreground.

In plants the risk of somaclonal variation occurs in the callus, suspension, single-cell - and protoplast culture is high, as in the formation of embryoid under indirect somatic embryogenesis. In general, the somaclonal variation for the maintainer and in vitro cloning is considered undesirable since genetically identical individuals are needed.

Other ways to the emergence of somaclonal variation

In addition to these, just be resettled in changes of the genome causes, also cause complexes come into question. So can also be epigenetically conditioned somaclonal variation. This means that with a constant genome still variations may occur due to causes other than genetic changes. Examples here include changes in the metabolic budget to call, especially in the area of ​​Phytohormonqualität and quantity. For ex vitro plants in addition there is also the possibility of an infectious agent, like in dwarf fruit stocks or highly branched poinsettias. In addition, it comes with respect to adverse changes also on special, rather rarer causes such as "jumping genes " ( transposons ).

Thus are the cause for the emergence of somaclonal variants fixed genetic and non-genetic factors, as well as a variety of fluent transitions between them.

Somaclonal variation in new breeds

In the 80s and 90s of the 20th century, the phenomenon of somaclonal variation in plant cells via in vitro culture as an interesting possibility to acquire new genetic types ( new varieties ) of the starting plants was seen. It was hoped to be able to create targeted and in a short time interesting new genotypes. The selection of these genetic variants should lead to the direct isolation of novel genotypes of cell cultures.

This hope could not be sustained based on practical experience. The generation of interesting genotypes is considered to be very variable and difficult to control, as well, unwanted negative mutations. Especially in relation to the transformed plant cells such genetic instability is undesirable. From somaclonal variants emerged varieties however, there is the case of tomatoes, potatoes and sugar cane. For targeted mutagenesis of plant cells today primarily as opportunities colchicine treatment or irradiation can be used with X-rays, which are much more efficient. Targeted interventions in the genome are now often made ​​with molecular biological methods such as biolistic transformation or derTransformation with agrobacteria.