Channeling (development)

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Channeling is the robustness of the phenotype with changes in genetic and environmental factors. A phenotypic trait therefore remains relatively invariant, even if individuals vary genetically or if they are exposed to changing environmental influences.

Concept history

The term canalization was introduced by Conrad Hal Waddington in 1942. Waddington formulates that development ( ontogenesis ) reacts to certain changes caused by external stimuli or genetic mutation in such a way that the phenotypic output remains unchanged. The development readjusts or channels the "disturbance". Independently of Waddington, the Russian evolutionary biologist Iwan Iwanowitsch Schmalhausen came up with similar ideas almost at the same time, which, however, only later became known in the western world due to political barriers and the Russian language. After decades of relatively little resonance in science, the concept received renewed attention in the age of genomics (since the 1980s). Today, channeling is often summarized in related terms (buffering, developmental stability, homeorhesis, tolerance, etc.) as "robustness".

Observation: Wild types are robust against variation

The concept of channeling is based on the observation that wild types are less susceptible to interference than breeding lines to genetic mutations. Waddington explained this with the fact that the genetic diversity of the wild type is greater and thus a channeling to the existing phenotype is more likely. One possible reason is that the wild type is exposed to many generations of stabilizing selection compared to the mutant or breeding type . Stabilizing selection can thus reduce the variability of genetic mutations or environmental influences with regard to the phenotype. However, stability may also arise simply as a by-product of the complex set of rules of the genetic factors that control development. Regulatory RNA sequences were also brought into play as an explanation. The exact reasons for the phenomenon are therefore not yet clearly clarified in science. It is very likely that different explanations will be correct in different cases.

Example Hsp90 chaperone

An example of channeling described in the more recent literature is the chaperone Hsp90 . A chaperone is a protein that helps other, newly synthesized proteins to fold optimally. In this function, the Hsp90 is of fundamental importance in development. Hsp90 takes its name from the term heat shock protein ( heat shock proteins ). In addition to the chaperone function, the name suggests another function: under stress, and not only in heat, Hsp90 is increasingly produced and then ensures that accumulated genetic mutations do not also lead to phenotypic variation; it buffers them. Hsp90 thus acts as a channeling factor . Conversely, if Hsp90 is artificially reduced, masked genetic mutations come to light. The effect of Hsp90 can therefore be imagined as analogous to the effect of a capacitor in a circuit: genetic and ecological variation is normally suppressed, this damping is only lost when stress is high, so that the variation becomes visible and stands for selective Customizations available.

Genetic assimilation

In a series of classic experiments on the fruit fly Drosophila , Waddington tried to show that the abolition of normal channeling can lead to a genetic fixation of traits that were initially pronounced in response to environmental factors. As a consequence, this means an apparently "Lamarckian" inheritance of acquired traits (which Waddington was able to explain within the framework of the synthetic theory of evolution). He called the whole process "genetic assimilation".

Waddington first observed that when Drosophila fly pupae were subjected to heat shock, sometimes a certain cross vein in the wing veins that is normally present is missing. The failure of this vein had previously been observed as an effect of various mutations. The stress of the heat shock can thus produce effects similar to those of a mutation (other biologists such as Goldschmidt or Landauer had already noticed similar effects in other species and were called "phenocopy" by them). Waddington now divided his fly population in half and selected one half for failure of the vein and the other half for retention of the vein even in the event of heat shock (i.e. a classic breeding experiment). After 12 generations, the proportion of individuals without this in the line selected for failure of the vein had increased significantly. In addition, some of the individuals did not have this vein, even if they had not been exposed to any heat shock. This means that a variation in response to an environmental stimulus (heat shock) now also occurred without this stimulus. According to Waddington's explanation, no inheritance of an acquired trait is necessary for the effect, even if it may appear that way at first. Rather, the variability that can bring about the modified phenotype is already present in the initial population, but is not pronounced due to the channeling. By selecting for those phenotypes that respond most strongly to the stimulus, individuals with alleles are selected that tend to have a lower threshold value for triggering the change. If the (in this case: artificial) selection reinforces this selection over many generations, alleles that express the effect even with a low stimulus become more and more common in the gene pool. As a result, variants can ultimately be enriched which express the characteristic even without any stimulus, i.e. This means that existing redundant, internal, genetic / epigenetic mechanisms are overwritten and the system is thus genetically fixed. Waddington later said: The developmental change triggered by the stressor can be genetically assimilated ( genetic assimilation ). The system then “works” without any external impetus. It is directed towards the same phenotype . As at the beginning of the variation, this is ensured by gene combinations and expression patterns that can cause similar variation and that are always present in many different ways in the organism.

A change occurs in the development, for example, when threshold values ​​are exceeded: As long as a threshold value is not exceeded, the development is channeled; if it is exceeded, the development leaves the channelized path and follows a new path, which is now re-channeled can. Threshold values ​​are known in the development of the limbs, in which a morphogen (progression gradient) such as SHH or several different lengths and / or different intensities act on cells in their environment and are thus involved in the identification of certain fingers, depending on how long or how strong the morphogen is.

Genetic variability is masked

As a direct consequence of channeling, one can see that there is accumulated genetic variability that does not show up in the phenotype. Such hidden, masked or phenotypically cryptic genetic variability or hidden developmental paths only come to light when the canalisation path is decanalized ( unmasked ). Masked cumulative mutations arise because selection cannot eliminate the individual mutations. Selection can only attack what phenotypically emerges.

Genotype buffering

The diverse combinations of genes and epigenetic development pathways present in the organism that lead to the same or very similar phenotypic result are referred to as buffering of the genotype. The buffering is always a relative quantity (ratio of the variability in the genome to the variation of the phenotypes) and is therefore a property of the population, not of the individual. A buffered feature, i.e. H. a genetically varying trait with no effect on the phenotype is referred to as neutral , a mutation that acts accordingly is referred to as a neutral mutation . As with the variability of the phenotypes themselves, buffering can start at the genetic level or the level of environmental factors. In the case of the environment, a distinction is made between the "macro-environment", that is, the actual environmental factors that act on the organism from outside, and the "micro-environment", that is the diverse influences of other parts of the organism itself on a characteristic at a certain point , distinguished. These micro-environmental factors are in particular the effects of organizing factors such as transcription factors or hormones during the development of an embryo, micro-environmental factors and development factors are therefore more or less synonymous. The influence of these development factors on the expression of the phenotype is channeling in the sense of Waddington. The other factors that contribute to the robustness of the phenotype are more or less independent of it. In recent literature, for example, channeling is used in both respects, namely genetic channeling, environmental channeling and (in Waddington's sense) developmental channeling.

Molecular Mechanisms

Waddington himself has not described any mechanisms that lead to channeling. Today we know such. Modularity, for example, promotes channeling. Modules can be defined as highly integrated units that are independent of other units. Modules can be seen on different levels of development: copied genes are modules, cell structures and cells themselves, but fingers are also examples of development modules on different phenotypic levels. Higher-order modules obtain their high degree of integration primarily through complex gene networks that are primarily polygenetic in nature (many genes involved in one trait) than are pleiotropic in nature (one gene involved in many traits). Genetic changes in a particular module then usually have no effect on the functioning of other modules. But this is compatible with the mentioned buffering or channeling.

Threshold value effects are an important subject of investigation by Evo-Devo . Threshold values can be established with the help of morphogens , which are progressive gradients (proteins) which, on the one hand, have different degrees of intensity depending on the distance from their place of origin, and on the other hand, can have different lengths of effect. Such forms of action can cause different reactions of cells or the tissue to be developed. The formation of the extremities (fingers) in vertebrates depends on the mode of action of morphogens. As long as threshold values ​​are not exceeded, there is no variation. The development is channeled. However, if threshold values ​​are exceeded or fallen below, this means decanalization or abandoning the development path. Günter P. Wagner on this: “Organisms maintain their functional organization against attacks by genetic mutations by equipping the phenotype with a physiological safety limit, which makes many small effects insignificant. But the effects that become visible as soon as the organism loses its safety limit in a developmental process are not necessarily the same kind of mutations as those that natural selection has to deal with when it creates adaptations ”.

Empirical evidence and measurement methods

In 1953 Waddington provided empirical evidence for his theses in the essay "Genetic Assimilation of an Acquired Character" and showed there how the veins in fly wings disappear, triggered by brief heat shocks of the fly eggs repeated over several generations, and how the veins finally also in some animals Stay away without the heat shocks. In the development of the flies, the change is channeled and later genetically assimilated. A similar experiment is repeated for the first time 50 years later by Fred Nijhout, USA, on tobacco hawks. The very short-term evolution of the beak forms of Darwin's finches , as described by Peter and Rosemary Grant, is associated with changes in development, especially with changes in the protein Hsp90 (see also Evo-Devo ). Likewise, the decades-long attempt by the Russian geneticist Dmitry Belyaev to tame silver foxes shows many changes in development that are now interpreted as channeling in the sense of Waddington (see Evo-Devo ).

Cryptic genetic variation can be provoked, for example, by P-element insertion or by the accumulation of genetic mutation. As described above, environmental stress for the detection of canalization can be achieved by heat shocks, but also by chemical stressors. One then compares different phenotypic lines with or without stressors.

Open questions and criticism

Difficulties arise in demonstrating canalization as an adaptively evolved property, i.e. in proving that canalization arises through natural selection, or in proving that species would be less adapted without canalization. Channeling may be an intrinsic property of gene networks or development pathways. To this end, future research must provide extensive, solid empirical material. Other methods are also required to measure sewerage.

Individual evidence

  1. ↑ a b c d e f Flatt, Thomas (2005): The Evolutionary Genetics of Canalization. Quarterly review of biology 80 (3): 287-316
  2. ↑ Waddington, CH canalization of Development and the Inheritance of Acquired character. 1942 Nature 3811 pp. 563-565
  3. ^ Waddington, CH: Genetic Assimilation of an Acquired Character. In: evolution . Volume 7, 1953, pp. 118-126
  4. ^ Andreas Wagner: Robustness and Evolvability in Living Systems. Princeton University Press 2005
  5. ↑ a b Waddington, CH (1942) p. 563
  6. ^ Mark L. Siegal & Aviv Bergman: Waddington's canalization revisited: Developmental stability and evolution. Proceedings of the national academy of science 99 (16) (2002): 10528-10532
  7. ↑ Eran Hornstein & Noam Shomron: Canalization of development by microRNAs. Nature Genetics 38 (Supplement) (2006): S20-S24
  8. ^ Rutherford, SL & Lindquist, S. (1996): Hsp90 as a capazitor for morphological evolution. Nature 396,336-342
  9. ↑ Gilbert, Scott F. & Epel, David (2009): Ecological Developmental Biology. Integrating Epigenetics, Medicine and Evolution. Sinauer p. 379
  10. ^ Jablonka, eva & Lamb, Mation (2005): Evolution in four dimensions. Genetic, Epigenetic, Behavioral, and Symbolic Variation in the History of Life pp. 265ff MIT Press
  11. ↑ Waddington, CH (1953): The genetic assimilation of an aquired character. Evolution 7: 118-126
  12. ↑ Hermisson J. and Wagner GP (2005): Evolution of phenotypic robustness.in: Robust Design: A Repertoire from Biology, Ecology, and Engineering, E. Jen (ed.), Oxford University Press, Oxford.
  13. ↑ For different definitions of modularity in evolution see z. B.:Callebaut, Werner & Raskin-Gutman, Diego (2005): Modularity - Understanding Development and Evolution of Natural Complex Systems MIT Press
  14. ^ Wagner, Günter P. (2003): Evolutionary Genetics: The Nature of Hidden Genetic Variation Unveiled. Current Biology Vol.13
  15. ↑ Yuichiro Suzuki, H. Federic Nihjout: Genetic basis of adaptive evolution of a polyphenism by genetic accommodation . In: Journal of Evolutionary Biology . 21, No. 1, 2008, pp. 57-66. doi : 10.1111 / j.1420-9101.2007.01464.x
  16. ↑ Peter R. Grant, B. Rosemary Grant: Genetics and the origin of bird species . In: Proceedings of the National Academy of Sciences . Volume 94, No. 15, pp. 7768-7775, July 1997; Online PDF
  17. ↑ Trut, Ludmila N .: Early Canid Domestication: The Farm-Fox Experiment. American Scientist Vol.87 1999

See also

Evolutionary developmental biology