Complementation (genetics)From Wikipedia, the free encyclopedia

In genetics, complementation refers to a relationship between two genetically distinct strains of an organismwhich both have homozygous recessive mutations that produce the same phenotype (for example, a change inwing structure in flies) but which are not the result of the same allele(s) of a specific gene. If, when these strainsare crossed with each other, some offspring show recovery (often termed "rescue") of the mutant phenotype,and thus a return to the wild-type phenotype, then these strains are said to show "genetic complementation".When this occurs, each strain's haploid genome supplies a single wild-type allele to "complement" the mutatedallele of the other strain's haploid genome, causing the offspring to have heterozygous genotypes in all relatedgenes. Since the mutations are recessive, the offspring will display the wild-type phenotype. Acomplementation test (sometimes called a "cis-trans" test) can be used to test whether two strains of aparticular gene are complements of one another and was developed by American geneticist Edward B. Lewis. Itanswers the question: "Does a wild-type copy of gene X rescue the function of the mutant allele that is believedto define gene X?". If there is an allele with an observable phenotype whose function can be provided by a wildtype genotype (i.e., the allele is recessive), one can ask whether the function that was lost because of therecessive allele can be provided by another mutant genotype. If not, the two alleles must be defective in thesame gene. The convenience and essence of this test is that the trait can serve to describe the gene's functionwithout the exact knowledge of what the gene is doing on a molecular level.[1]

Complementation arises because loss of function in genes responsible for different steps in the same metabolicpathway can give rise to the same phenotype. When strains are bred together, offspring inherit wildtype versionsof each gene from either parent. Because the mutations are recessive, there is a recovery of function in thatpathway, so offspring recover the wild-type phenotype. Thus, the test is used to decide if two independentlyderived recessive mutant phenotypes are caused by mutations in the same gene or in two different genes. If bothparent strains have mutations in the same gene, no normal versions of the gene are inherited by offspring; theyexpress the same mutant phenotype and complementation has failed to occur.

In other words:

If the combination of two haploid genomes containing different recessive mutations yields a mutantphenotype, then there are three possibilities:

Mutations occur in the same gene.1.One mutation affects the expression of the other.2.One mutation may result in an inhibitory product.3.

If the combination of two haploid genomes containing different recessive mutations yields the wild typephenotype, then the mutations must be in different genes.

Contents

1 Example of a Simple Complementation Test2 Complementation tests in fungi and bacteriophage3 Genetic complementation, heterosis and the evolution of sexual reproduction4 Exceptions5 See also

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Example of a complementation test. Two strains offlies are white eyed because of two differentautosomal recessive mutations which interruptdifferent steps in a single pigment-producingmetabolic pathway. Flies from Strain 1 havecomplementary mutations to flies from Strain 2because when they are crossed the offspring areable to complete the full metabolic pathway andthus have red eyes.

6 References

Example of a Simple Complementation Test

For a simple example of a complementation test, suppose ageneticist is interested in studying two strains of white-eyedflies of the species Drosophila melanogaster. In this species,wild type flies have red eyes and eye color is known to berelated to two genes, A and B. Each one of these genes hastwo alleles, a dominant one that codes for a working protein(A and B respectively) and a recessive one that codes for amalfunctioning protein (a and b respectively). Since bothproteins are necessary for the synthesis of red pigmentationin the eyes, if a given fly is homozygous for either a or b, itwill have white eyes.

Knowing this, the geneticist may perform acomplementation test on two separately obtained strains ofpure-breeding white-eyed flies. The test is performed bycrossing two flies, one from each strain. If the resultingprogeny have red eyes, the two strains are said tocomplement; if the progeny have white eyes, they do not.

If the strains complement, we imagine that one strain musthave a genotype aa BB and the other AA bb, which whencrossed yield the genotype AaBb. In other words, eachstrain is homozygous for a different deficiency thatproduces the same phenotype. If the strains do notcomplement, they both must have genotypes aa BB, AA bb,or aa bb. In other words, they are both homozygous for thesame deficiency, which obviously will produce the samephenotype.

Complementation tests in fungi and bacteriophage

Complementation tests can also be carried out with haploid eukaryotes such as fungi, with bacteria and withviruses such as bacteriophage.[2] Research on the fungus Neurospora crassa led to the development of theone-gene-one enzyme concept that provided the foundation for the subsequent development of moleculargenetics.[3][4] The complementation test was one of the main tools used in the early Neurospora work, becauseit was easy to do, and allowed the investigator to determine whether any two nutritional mutants were defectivein the same, or different genes.

The complementation test was also used in the early development of molecular genetics when bacteriophage T4was one of the main objects of study.[5] In this case the test depends on mixed infections of host bacterial cellswith two different bacteriophage mutant types. Its use was key to defining most of the genes of the virus, andprovided the foundation for the study of such fundamental processes as DNA replication and repair, and howmolecular machines are constructed.

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Genetic complementation, heterosis and the evolution of sexualreproduction

Heterosis is the tendency for hybrid individuals to exceed their pure bred parents in size and vigor. Thephenomenon has long been known in animals and plants. Heterosis appears to be largely due to geneticcomplementation, that is the masking of deleterious recessive alleles in hybrid individuals.

In general, the two fundamental aspects of sexual reproduction in eukaryotes are meiosis, and outcrossing.These two aspects have been proposed to have two natural selective advantages, respectively. Meiosis isproposed to be adaptive because it facilitates recombintional repair of DNA damages that are otherwise difficultto repair (seeMeiosis#Theory that DNA repair is the adaptive advantage of meiosis). Outcrossing is proposedto be adaptive because it facilitates complementation, that is the masking of deleterious recessive alleles [6]

(also see Heterosis#Genetic and epigenetic bases of heterosis). The benefit of masking deleterious alleles hasbeen proposed to be a major factor in the maintenance of sexual reproduction among eukaryotes, as summarizedin the article Evolution of sexual reproduction. Further, the selective advantage of complementation that arisesfrom outcrossing may largely account for the general avoidance of inbreeding in nature (e.g see articles Kinrecognition, Inbreeding depression and Incest taboo).

Exceptions

There are exceptions to these rules. Two non-allelic mutants may occasionally fail to complement (called"non-allelic non-complementation" or "unlinked non-complementation"). This situation is rare and is dependenton the particular nature of the mutants being tested. For example, two mutations may be synthetically dominantnegative. Another exception is transvection, in which the heterozygous combination of two alleles withmutations in different parts of the gene complement each other to rescue a wild type phenotype.

See also

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References^ Genetics lectures 1-3 '03 (http://ocw.mit.edu/NR/rdonlyres/Biology/7-03Fall-2004/233B3544-E379-44C5-BC31-EF7D864AE73B/0/lecture2.pdf)

1.

^ Fincham JRS (1966). Genetic Complementation (http://books.google.com.au/books?id=hdc9AAAAIAAJ) .Microbial and molecular biology. 3. W.A. Benjamin. pp. 1–18. ASIN B009SQ0G9C (//www.amazon.com/dp/B009SQ0G9C) . OCLC 239023 (//www.worldcat.org/oclc/239023) . http://books.google.com.au/books?id=hdc9AAAAIAAJ.

2.

^ Beadle GW (2007). "Biochemical genetics: Some recollections" (http://books.google.com.au/books?id=g_JSw4LVKtIC&lpg=PR3&dq=ISBN%3A0879698004&pg=PA23) . In Cairns, J.; Stent, G.S.; Watson,J.D.. Phage and the Origins of Molecular Biology (4th ed.). Cold Spring Harbor Laboratory of Quantitative Biology.pp. 23–32. ISBN 0879698004. http://books.google.com.au/books?id=g_JSw4LVKtIC&lpg=PR3&dq=ISBN%3A0879698004&pg=PA23.

3.

^ Horowitz NH (April 1991). "Fifty years ago: the Neurospora revolution" (http://www.genetics.org/cgi/pmidlookup?view=long&pmid=1827628) . Genetics 127 (4): 631–5. PMC 1204391 (//www.ncbi.nlm.nih.gov/pmc/articles/PMC1204391) . PMID 1827628 (//www.ncbi.nlm.nih.gov/pubmed/1827628) . http://www.genetics.org/cgi/pmidlookup?view=long&pmid=1827628.

4.

^ Epstein RH, Bolle A, Steinberg CM, Kellenberger E, Boy De La Tour E, Chevalley R, Edgar RS, Susman M,5.

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Denhardt GH, Lielausis A (1963). "Physiological studies of conditional lethal mutants of bacteriophage T4D". ColdSpring Harbor Symp. Quant. Biol. 28: 375–394. doi:10.1101/SQB.1963.028.01.053 (http://dx.doi.org/10.1101%2FSQB.1963.028.01.053) .^ Bernstein H, Byerly HC, Hopf FA, Michod RE (September 1985). "Genetic damage, mutation, and the evolution ofsex" (http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=3898363) . Science 229 (4719): 1277–81.PMID 3898363 (//www.ncbi.nlm.nih.gov/pubmed/3898363) . http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=3898363.

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