Canalisation (genetics)
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Canalisation is a measure of the ability of a population to produce the same
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
regardless of variability of its environment or
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
. It is a form of evolutionary robustness. The term was coined in 1942 by
C. H. Waddington Conrad Hal Waddington (8 November 1905 – 26 September 1975) was a British developmental biologist, paleontologist, geneticist, embryologist and philosopher who laid the foundations for systems biology, epigenetics, and evolutionary devel ...
to capture the fact that "developmental reactions, as they occur in organisms submitted to
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
...are adjusted so as to bring about one definite end-result regardless of minor variations in conditions during the course of the reaction". He used this word rather than robustness to take into account that biological systems are not robust in quite the same way as, for example, engineered systems. Biological robustness or canalisation comes about when developmental pathways are shaped by evolution. Waddington introduced the concept of the epigenetic landscape, in which the state of an organism rolls "downhill" during development. In this metaphor, a canalised trait is illustrated as a valley (which he called a creode) enclosed by high ridges, safely guiding the phenotype to its "fate". Waddington claimed that canals form in the epigenetic landscape during evolution, and that this heuristic is useful for understanding the unique qualities of biological robustness.


Genetic assimilation

Waddington used the concept of canalisation to explain his experiments on genetic assimilation. In these experiments, he exposed ''
Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
''
pupa A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in thei ...
e to heat shock. This environmental disturbance caused some flies to develop a crossveinless
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
. He then selected for crossveinless. Eventually, the crossveinless phenotype appeared even without heat shock. Through this process of genetic assimilation, an environmentally induced phenotype had become inherited. Waddington explained this as the formation of a new canal in the epigenetic landscape. It is, however, possible to explain genetic assimilation using only
quantitative genetics Quantitative genetics deals with phenotypes that vary continuously (such as height or mass)—as opposed to discretely identifiable phenotypes and gene-products (such as eye-colour, or the presence of a particular biochemical). Both branches u ...
and a threshold model, with no reference to the concept of canalisation. However, theoretical models that incorporate a complex
genotype–phenotype map The genotype–phenotype map is a conceptual model in genetic architecture. Coined in a 1991 paper by Pere Alberch, it models the interdependency of genotype (an organism's full hereditary information) with phenotype (an organism's actual obser ...
have found evidence for the evolution of phenotypic robustness contributing to genetic assimilation, even when selection is only for developmental stability and not for a particular phenotype, and so the quantitative genetics models do not apply. These studies suggest that the canalisation heuristic may still be useful, beyond the more simple concept of robustness.


Congruence hypothesis

Neither canalisation nor robustness are simple quantities to quantify: it is always necessary to specify which trait is canalised (robust) to which perturbations. For example, perturbations can come either from the environment or from mutations. It has been suggested that different perturbations have congruent effects on development taking place on an epigenetic landscape. This could, however, depend on the molecular mechanism responsible for robustness, and be different in different cases.


Evolutionary capacitance

The canalisation metaphor suggests that some phenotypic traits are very robust to small perturbations, for which development does not exit the canal, and rapidly returns down, with little effect on the final outcome of development. But perturbations whose magnitude exceeds a certain threshold will break out of the canal, moving the developmental process into uncharted territory. For instance, the study of an allelic series for
Fgf8 Fibroblast growth factor 8 (FGF-8) is a protein that in humans is encoded by the ''FGF8'' gene. Function The protein encoded by this gene is a member of the fibroblast growth factor (FGF) family. FGF family members possess broad mitogenic and ...
, an important gene for craniofacial development, with decreasing levels of gene expression demonstrated that the phenotype remains canalised as long as the expression level is above 40% of the wild-type expression. Strong robustness up to a limit, with little robustness beyond, is a pattern that could increase
evolvability Evolvability is defined as the capacity of a system for adaptive evolution. Evolvability is the ability of a population of organisms to not merely generate genetic diversity, but to generate ''adaptive'' genetic diversity, and thereby evolve throu ...
in a fluctuating environment. Canalisation of a large set of genotypes into a limited phenotypic space has been suggested as a mechanism for the accumulation, in a neutral manner, of mutations that could otherwise be deleterious. Genetic canalisation could allow for
evolutionary capacitance Evolutionary capacitance is the storage and release of variation, just as electric capacitors store and release charge. Living systems are robust to mutations. This means that living systems accumulate genetic variation without the variation havi ...
, where genetic diversity accumulates in a population over time, sheltered from
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
because it does not normally affect phenotypes. This hidden diversity could then be unleashed by extreme changes in the environment or by molecular switches, releasing previously cryptic genetic variation that can then contribute to a rapid burst of evolution, a phenomenon termed decanalisation. Cycles of canalization-decanalization could explain the alternating periods of stasis, where genotypic diversity accumulates without morphological changes, followed by rapid morphological changes, where decanalization releases the phenotypic diversity and becomes subject to
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
, in the
fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
, thus providing a potential developmental explanation for the
punctuated equilibrium In evolutionary biology, punctuated equilibrium (also called punctuated equilibria) is a Scientific theory, theory that proposes that once a species appears in the fossil record, the population will become stable, showing little evolution, evol ...
.


HSP90 and decanalisation

In 1998,
Susan Lindquist Susan Lee Lindquist, ForMemRS (June 5, 1949 – October 27, 2016) was an American professor of biology at MIT specializing in molecular biology, particularly the protein folding problem within a family of molecules known as heat-shock proteins ...
discovered that ''
Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
'' ''hsp83'' heterozygous mutants exhibit a large diversity of phenotypes (from sexual combs on the head, to scutoid-like and notched wings phenotypes). She showed that these phenotypes could be passed on to the next generation, suggesting a genetic basis for those phenotypes. The authors hypothesized that ''
Hsp90 Hsp90 (heat shock protein 90) is a chaperone protein that assists other proteins to fold properly, stabilizes proteins against heat stress, and aids in protein degradation. It also stabilizes a number of proteins required for tumor growth, ...
'' (the gene mutated in ''hsp83''), as a chaperone protein, plays a pivotal role in the folding and activation of many proteins involved in developmental signaling pathways, thus buffering against genetic variation in those pathways. ''hsp83'' mutants would therefore release the cryptic genetic variation, resulting in a diversity of phenotypes. In 2002, Lindquist showed that pharmacological inhibition of
HSP90 Hsp90 (heat shock protein 90) is a chaperone protein that assists other proteins to fold properly, stabilizes proteins against heat stress, and aids in protein degradation. It also stabilizes a number of proteins required for tumor growth, ...
in ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...
'' also lead to a wide range of phenotypes, some of which could be considered adaptive, further supporting the canalising role of
HSP90 Hsp90 (heat shock protein 90) is a chaperone protein that assists other proteins to fold properly, stabilizes proteins against heat stress, and aids in protein degradation. It also stabilizes a number of proteins required for tumor growth, ...
. Finally, the same type of experiment in the cavefish ''Astyanax mexicanus'' yielded similar results. This species encompasses two populations: an eyed population living under the water surface and an eye-less blind population living in caves. Not only is the cave population eye-less but it also displays a largely reduced
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
size. HSP90 inhibition leads to an increased variation in orbit size that could explain how this trait could evolve in just a few generations. Further analysis showed that low
conductivity Conductivity may refer to: *Electrical conductivity, a measure of a material's ability to conduct an electric current **Conductivity (electrolytic), the electrical conductivity of an electrolyte in solution ** Ionic conductivity (solid state), ele ...
in the cave water induces a
stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
response mimicking the inhibition of
HSP90 Hsp90 (heat shock protein 90) is a chaperone protein that assists other proteins to fold properly, stabilizes proteins against heat stress, and aids in protein degradation. It also stabilizes a number of proteins required for tumor growth, ...
, providing a mechanism for decanalisation. It is worth noting that interpretation of the original ''
Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
'' paper is now subject to controversy. Molecular analysis of the ''hsp83'' mutant showed that
HSP90 Hsp90 (heat shock protein 90) is a chaperone protein that assists other proteins to fold properly, stabilizes proteins against heat stress, and aids in protein degradation. It also stabilizes a number of proteins required for tumor growth, ...
is required for
piRNA Pirna (; hsb, Pěrno; ) is a town in Saxony, Germany and capital of the administrative district Sächsische Schweiz-Osterzgebirge. The town's population is over 37,000. Pirna is located near Dresden and is an important district town as well as ...
biogenesis, a set of small RNAs repressing
transposons A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Tran ...
in the germline. , causing massive
transposon A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Transpo ...
insertional
mutagenesis Mutagenesis () is a process by which the genetic information of an organism is changed by the production of a mutation. It may occur spontaneously in nature, or as a result of exposure to mutagens. It can also be achieved experimentally using lab ...
that could explain the phenotypic diversification.


Significance of Variability of Components

Understanding variability is an extremely important aspect of understanding natural selection and mutations. Variability can be classified into two categories the first is modulating phenotypic variation and the second is modulating the phenotypes that are produced. While this so call bias found in genetic variability is found the study allows us to further understand how certain phenotypes are more successful into its actual morphology, biochemical makeup, or behavior. It is scientifically known that organisms have to developed systematically into integrated systems in order to thrive in their specific ecosystem. This splays to morphology in the order of variations as well as if the variations are not done in a systematic order the phenotypic mutation will not last as natural selection will occur. This variation effects the speed and rate of evolutionary change due to the selection and modulation of the phenotypic variations This ultimately lows the amount of diversity seen through evolution as the majority of phenotypes never make it more than a couple generations due to their inferior basis of either morphology, biochemical makeup, or physical movement or appearance.


See also

*
Developmental noise Developmental noise is a concept within developmental biology in which the phenotype varies between individuals even though both the genotypes and the environmental factors are the same for all of them. Contributing factors include stochastic gene ...
* Phenotypic integration *
Phenotypic plasticity Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompa ...
*
Developmental systems theory Developmental systems theory (DST) is an overarching theoretical perspective on biological development, heredity, and evolution. It emphasizes the shared contributions of genes, environment, and epigenetic factors on developmental processes. DST ...
*
Gene regulatory network A gene (or genetic) regulatory network (GRN) is a collection of molecular regulators that interact with each other and with other substances in the cell to govern the gene expression levels of mRNA and proteins which, in turn, determine the fun ...
*
Systems biology Systems biology is the computational modeling, computational and mathematical analysis and modeling of complex biological systems. It is a biology-based interdisciplinary field of study that focuses on complex interactions within biological syst ...


References

{{DEFAULTSORT:Canalisation (Genetics) Developmental biology Extended evolutionary synthesis Population genetics