Underdominance

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In genetics, underdominance, also known as homozygote advantage, heterozygote disadvantage, or negative overdominance," [1] is the opposite of overdominance. It is the selection against the heterozygote, causing disruptive selection [2] and divergent genotypes. Underdominance exists in situations where the heterozygotic genotype is inferior in fitness to either the dominant or recessive homozygotic genotype. Compared to examples of overdominance in actual populations, underdominance is considered more unstable [3] [4] and may lead to the fixation of either allele. [1] [5] [6]

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An example of stable underdominance may occur in individuals who are heterozygotic for polymorphisms that would make them better suited for one of two niches. [7] Consider a situation in which a population is completely homozygotic for an "A" allele, allowing exploitation of a particular resource. Eventually, a polymorphic "a" allele may be introduced into the population, resulting in an individual who is capable of exploiting a different resource. This would result in an "aa" homozygotic invasion of the population due to nonexistent competition of the unexploited resource. The frequency of "aa" individuals would increase until the abundance of the "a" resource begins to decline. Eventually, the "AA" and "aa" genotypes would reach equilibrium with each other, with "Aa" heterozygotic individuals potentially experiencing a reduced fitness compared to those individuals who are homozygotic for utilization of either resource. This example of underdominance is stable because any shift in equilibrium would result in selection for the rare allele due to increased resource abundance. This compensatory selection would ultimately return the dimorphic system to underdominant equilibrium. [7]

Incidence in butterfly populations

An example of stable underdominance can be found in the African butterfly species Pseudacraea eurytus , which utilizes Batesian mimicry to escape predation. This species possesses two alleles which each confer an appearance similar to that of another local butterfly species that is toxic to its predator. Individuals who are heterozygous for this trait appear to be intermediate in appearance and thus experience increased predation and lowered overall fitness. [8]

Uses in pest control

Models of stable underdominance have shown potential in driving the introduction of refractory genes into pest populations that are responsible for the spread of infective diseases such as malaria and dengue fever. [9] A refractory gene alone would not have higher fitness than the native genes, but engineered underdominance may prove effective as a mechanism to spread such a gene. In this model, two genetics constructs are introduced into two non-homologous chromosomes. Each construct is lethal when expressed individually but can be suppressed by the other construct. In this way, individuals with only one of the two constructed genes (heterozygotes) are selected against, but homozygotes with both or neither construct are genetically healthy. Analysis of this model using simple population genetics shows that successful spread of refractory genes using this engineered underdominance is possible with relatively small release of the constructed genotype into the population. [9]

A similar system of manipulation of pest populations was achieved in a population of Drosophila melanogaster by using a knock-down/rescue system. [10] The genetic construct in this system employs a dsRNAi knockdown of a C-reactive protein, RpL14, as well as a rescue element (a complete copy of the wild type RpL14 gene). Individuals that are heterozygous for this construct experience lowered fitness due to limited restoration of the RpL14 gene, which results in reduced female fertility and delayed development, along with various other mutations that ultimately lower fitness by 70-80%. [10] This system of underdominance allowed manipulation of the population and ultimate fixation of the constructed genotype and has potential applications in a number of settings, including agriculture and the reduction of various pest-carried diseases.

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An allele is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. The word is a short form of "allelomorph".

<span class="mw-page-title-main">Dominance (genetics)</span> One gene variant masking the effect of another in the other copy of the gene

In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and the second is called recessive. This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child. Since there is only one copy of the Y chromosome, Y-linked traits cannot be dominant or recessive. Additionally, there are other forms of dominance, such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, and co-dominance, in which different variants on each chromosome both show their associated traits.

Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and population structure.

<span class="mw-page-title-main">Hardy–Weinberg principle</span> Principle in genetics

In population genetics, the Hardy–Weinberg principle, also known as the Hardy–Weinberg equilibrium, model, theorem, or law, states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. These influences include genetic drift, mate choice, assortative mating, natural selection, sexual selection, mutation, gene flow, meiotic drive, genetic hitchhiking, population bottleneck, founder effect,inbreeding and outbreeding depression.

Allele frequency, or gene frequency, is the relative frequency of an allele at a particular locus in a population, expressed as a fraction or percentage. Specifically, it is the fraction of all chromosomes in the population that carry that allele over the total population or sample size. Microevolution is the change in allele frequencies that occurs over time within a population.

<span class="mw-page-title-main">Quantitative genetics</span> Study of the inheritance of continuously variable traits

Quantitative genetics deals with quantitative traits, which are phenotypes that vary continuously —as opposed to discretely identifiable phenotypes and gene-products.

<span class="mw-page-title-main">Polymorphism (biology)</span> Occurrence of two or more clearly different morphs or forms in the population of a species

In biology, polymorphism is the occurrence of two or more clearly different morphs or forms, also referred to as alternative phenotypes, in the population of a species. To be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population.

Heterosis, hybrid vigor, or outbreeding enhancement is the improved or increased function of any biological quality in a hybrid offspring. An offspring is heterotic if its traits are enhanced as a result of mixing the genetic contributions of its parents. The heterotic offspring often has traits that are more than the simple addition of the parents' traits, and can be explained by Mendelian or non-Mendelian inheritance. Typical heterotic/hybrid traits of interest in agriculture are higher yield, quicker maturity, stability, drought tolerance etc.

Balancing selection refers to a number of selective processes by which multiple alleles are actively maintained in the gene pool of a population at frequencies larger than expected from genetic drift alone. Balancing selection is rare compared to purifying selection. It can occur by various mechanisms, in particular, when the heterozygotes for the alleles under consideration have a higher fitness than the homozygote. In this way genetic polymorphism is conserved.

A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. Loci exhibiting heterozygote advantage are a small minority of loci. The specific case of heterozygote advantage due to a single locus is known as overdominance. Overdominance is a rare condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents, and heterozygous individuals have a higher fitness than homozygous individuals.

<span class="mw-page-title-main">Overdominance</span>

Overdominance is a rare condition in genetics where the phenotype of the heterozygote lies outside the phenotypical range of both homozygous parents. Overdominance can also be described as heterozygote advantage regulated by a single genomic locus, wherein heterozygous individuals have a higher fitness than homozygous individuals. However, not all cases of the heterozygote advantage are considered overdominance, as they may be regulated by multiple genomic regions. Overdominance has been hypothesized as an underlying cause for heterosis.

<span class="mw-page-title-main">Chromosomal inversion</span> Chromosome rearrangement in which a segment of a chromosome is reversed

An inversion is a chromosome rearrangement in which a segment of a chromosome becomes inverted within its original position. An inversion occurs when a chromosome undergoes a two breaks within the chromosomal arm, and the segment between the two breaks inserts itself in the opposite direction in the same chromosome arm. The breakpoints of inversions often happen in regions of repetitive nucleotides, and the regions may be reused in other inversions. Chromosomal segments in inversions can be as small as 100 kilobases or as large as 100 megabases. The number of genes captured by an inversion can range from a handful of genes to hundreds of genes. Inversions can happen either through ectopic recombination, chromosomal breakage and repair, or non-homologous end joining.

<span class="mw-page-title-main">Genotype frequency</span>

Genetic variation in populations can be analyzed and quantified by the frequency of alleles. Two fundamental calculations are central to population genetics: allele frequencies and genotype frequencies. Genotype frequency in a population is the number of individuals with a given genotype divided by the total number of individuals in the population. In population genetics, the genotype frequency is the frequency or proportion of genotypes in a population.

Genetic load is the difference between the fitness of an average genotype in a population and the fitness of some reference genotype, which may be either the best present in a population, or may be the theoretically optimal genotype. The average individual taken from a population with a low genetic load will generally, when grown in the same conditions, have more surviving offspring than the average individual from a population with a high genetic load. Genetic load can also be seen as reduced fitness at the population level compared to what the population would have if all individuals had the reference high-fitness genotype. High genetic load may put a population in danger of extinction.

Mutation–selection balance is an equilibrium in the number of deleterious alleles in a population that occurs when the rate at which deleterious alleles are created by mutation equals the rate at which deleterious alleles are eliminated by selection. The majority of genetic mutations are neutral or deleterious; beneficial mutations are relatively rare. The resulting influx of deleterious mutations into a population over time is counteracted by negative selection, which acts to purge deleterious mutations. Setting aside other factors, the equilibrium number of deleterious alleles is then determined by a balance between the deleterious mutation rate and the rate at which selection purges those mutations.

Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding. Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a population bottleneck. In general, the higher the genetic variation or gene pool within a breeding population, the less likely it is to suffer from inbreeding depression, though inbreeding and outbreeding depression can simultaneously occur.

The infinite alleles model is a mathematical model for calculating genetic mutations. The Japanese geneticist Motoo Kimura and American geneticist James F. Crow (1964) introduced the infinite alleles model, an attempt to determine for a finite diploid population what proportion of loci would be homozygous. This was, in part, motivated by assertions by other geneticists that more than 50 percent of Drosophila loci were heterozygous, a claim they initially doubted. In order to answer this question they assumed first, that there were a large enough number of alleles so that any mutation would lead to a different allele ; and second, that the mutations would result in a number of different outcomes from neutral to deleterious.

<span class="mw-page-title-main">Zygosity</span> Degree of similarity of the alleles in an organism

Zygosity is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.

Host–parasite coevolution is a special case of coevolution, where a host and a parasite continually adapt to each other. This can create an evolutionary arms race between them. A more benign possibility is of an evolutionary trade-off between transmission and virulence in the parasite, as if it kills its host too quickly, the parasite will not be able to reproduce either. Another theory, the Red Queen hypothesis, proposes that since both host and parasite have to keep on evolving to keep up with each other, and since sexual reproduction continually creates new combinations of genes, parasitism favours sexual reproduction in the host.

This glossary of genetics and evolutionary biology is a list of definitions of terms and concepts used in the study of genetics and evolutionary biology, as well as sub-disciplines and related fields, with an emphasis on classical genetics, quantitative genetics, population biology, phylogenetics, speciation, and systematics. Overlapping and related terms can be found in Glossary of cellular and molecular biology, Glossary of ecology, and Glossary of biology.

References

  1. 1 2 Doolittle, Donald (1987). Population Genetics- Basic Principles. Germany: Springer-Verlag. pp. 69–72. ISBN   978-3-540-17326-7.
  2. ↑ Gillespie, John (2010). Population Genetics: A Concise Guide. Maryland: JHU Press. pp. 68–69. ISBN   978-0-8018-8009-4.
  3. ↑ Pierce, Benjamin (2008). Genetics: A Conceptual Approach. New York: Macmillan. pp. 699–700. ISBN   978-0-7167-7928-5.
  4. ↑ Newberry, Mitchell; McCandlish, David; Plotkin, Joshua B. (2016-03-16). "Assortative mating can impede or facilitate fixation of underdominant alleles". bioRxiv   10.1101/042192 .
  5. ↑ Altrock, Philipp M.; Traulsen, Arne; Reed, Floyd A. (2011-11-03). "Stability Properties of Underdominance in Finite Subdivided Populations". PLOS Computational Biology. 7 (11): e1002260. Bibcode:2011PLSCB...7E2260A. doi: 10.1371/journal.pcbi.1002260 . ISSN   1553-734X. PMC   3207953 . PMID   22072956.
  6. ↑ Láruson, Áki J.; Reed, Floyd A. (2015-09-07). "Stability of Underdominant Genetic Polymorphisms in Population Networks". Journal of Theoretical Biology. 390: 156–163. arXiv: 1509.02205 . Bibcode:2016JThBi.390..156L. doi:10.1016/j.jtbi.2015.11.023. PMID   26656110. S2CID   6385410.
  7. 1 2 Wilson, David; Turelli, Michael (1986). "Stable Underdominance and the Evolutionary Invasion of Empty Niches". The American Naturalist. 127 (6): 835–850. doi:10.1086/284528. JSTOR   2461418. S2CID   83868190.
  8. ↑ Brauer, Fred; Kribs, Christopher (2015). Dynamical Systems for Biological Modeling: An Introduction. CRC Press. p. 399.
  9. 1 2 Magori, Krisztian; Gould, Fred (2006). "Genetically Engineered Underdominance for Manipulation of Pest Populations: A Deterministic Model". Genetics. 172 (4): 2613–2620. doi:10.1534/genetics.105.051789. PMC   1456375 . PMID   16415364.
  10. 1 2 Reeves, R. Guy; Bryk, Jaroslaw; Altrock, Philipp; Denton, Jai; Reed, Floyd (2014). "First Steps towards Underdominant Genetic Transformation of Insect Populations". PLOS ONE. 9 (5): e97557. Bibcode:2014PLoSO...997557R. doi: 10.1371/journal.pone.0097557 . PMC   4028297 . PMID   24844466.
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