genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar work ...

, underdominance, also known as homozygote advantage, heterozygote disadvantage, or negative overdominance," is the opposite of

overdominance Introduction 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 sin ...

. It is the selection against the

heterozygote Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () 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. M ...

, causing

disruptive selection Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population ...

and divergent

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 ...

s. Underdominance exists in situations where the heterozygotic

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 and may lead to the fixation of either allele. 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. Consider a situation in which a population is completely homozygotic for an "A"

allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") 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 chrom ...

, 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.

Incidence in butterfly populations

An example of stable underdominance can be found in the African butterfly species '' Pseudacraea eurytus'', which utilizes

Batesian mimicry Batesian mimicry is a form of mimicry where a harmless species has evolved to imitate the warning signals of a harmful species directed at a predator of them both. It is named after the English naturalist Henry Walter Bates, after his work on ...

to escape

predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

. 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.

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 Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. ...

and

dengue fever Dengue fever is a mosquito-borne tropical disease caused by the dengue virus. Symptoms typically begin three to fourteen days after infection. These may include a high fever, headache, vomiting, muscle and joint pains, and a characteristic ...

. 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 chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...

s. 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. A similar system of manipulation of pest populations was achieved in a population of ''

Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the " vinegar fly" or "pomace fly". Starting with ...

'' by using a knock-down/rescue system.{{Cite journal, last1=Reeves, first1=R. Guy, last2=Bryk, first2=Jaroslaw, last3=Altrock, first3=Philipp, last4=Denton, first4=Jai, last5=Reed, first5=Floyd, year=2014, title=First Steps towards Underdominant Genetic Transformation of Insect Populations, journal=PLOS ONE, volume=9, issue=5, pages=e97557, doi=10.1371/journal.pone.0097557, pmid=24844466, pmc=4028297, bibcode=2014PLoSO...997557R, doi-access=free The genetic construct in this system employs a dsRNAi knockdown of a

C-reactive protein C-reactive protein (CRP) is an annular (ring-shaped) pentameric protein found in blood plasma, whose circulating concentrations rise in response to inflammation. It is an acute-phase protein of hepatic origin that increases following interleukin- ...

, RpL14, as well as a rescue element (a complete copy of the

wild type The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, "m ...

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%. 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.

References

External links

Underdominance- ResearchGate

Population Genetics

The Stability of An Equilibrium and the Average Fitness of a Population

Chromosome Inversions, Local Adaptations and Speciation

Chromosomal Rearrangements cause Underdominant Pollen Sterility
Classical genetics

Incidence in butterfly populations

Uses in pest control

See also

References

External links