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Interlocus contest evolution (ICE) is a process of intergenomic conflict by which different
loci Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * ''Locus'' (magazine), science fiction and fantasy magazine ** '' Locus Award ...
within a single
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
antagonistically
coevolve In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as wel ...
. ICE supposes that the Red Queen process, which is characterized by a never-ending antagonistic
evolutionary arms race In evolutionary biology, an evolutionary arms race is an ongoing struggle between competing sets of co-evolving genes, phenotypic and behavioral traits that develop escalating adaptations and counter-adaptations against each other, resembling ...
, does not only apply to
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
but also to
gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s within the genome of a species.Rice & Holland, 1997.
The enemies within: intergenomic conflict, interlocus contest evolution (ICE), and the intraspecific Red Queen
" Behavioral Ecology and Sociobiology 41(1): 1-10.
Because
sexual recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryot ...
allows different gene loci to evolve semi-autonomously, genes have the potential to coevolve antagonistically. ICE occurs when "an allelic substitution at one locus selects for a new allele at the interacting locus, and vice versa." As a result, ICE can lead to a chain reaction of perpetual gene substitution at antagonistically interacting loci, and no stable equilibrium can be achieved. The rate of evolution thus increases at that locus. ICE is thought to be the dominant mode of evolution for genes controlling social behavior. The ICE process can explain many biological phenomena, including intersexual conflict, parent offspring conflict, and interference competition.


Intersexual conflict

A fundamental conflict between the sexes lies in differences in investment: males generally invest predominantly in fertilization while females invest predominantly in offspring. This conflict manifests itself in many traits associated with
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
. Genes expressed in only one sex are selectively neutral in the other sex; male- and female-linked genes can therefore be acted upon separated by selection and will evolve semi-autonomously. Thus, one sex of a species may evolve to better itself rather than better the species as a whole, sometimes with negative results for the opposite sex: loci will antagonistically coevolve to enhance male reproductive success at females’ expense on the one hand, and to enhance female resistance to male coercion on the other. This is an example of
intralocus sexual conflict Intralocus sexual conflict is a type of sexual conflict that occurs when a genetic locus harbours alleles which have opposing effects on the fitness of each sex, such that one allele improves the fitness of males (at the expense of females), while ...
, and is unlikely to be resolved fully throughout the genome. However, in some cases this conflict may be resolved by the restriction of the gene’s expression to only the sex that it benefits, resulting in
sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...
. The ICE theory can explain the differentiation of the human X- and
Y-chromosomes The Y chromosome is one of two sex chromosomes (allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or abs ...
. Semi-autonomous evolution may have promoted genes beneficial to females in the X-chromosome even when detrimental to males, and genes beneficial to males in the Y-chromosome, even when detrimental to females. As the distribution of the X-chromosome is three times as large as the Y-chromosome (the X-chromosome occurs in 3/4 of offspring genes, while the Y-chromosome occurs in only 1/4), the Y-chromosome has a reduced opportunity for rapid evolution. Thus, the Y-chromosome has "shed" its genes to leave only the essential ones (such as the SRY gene), which gives rise to the differences in the X- and Y-chromosomes.


Parent–offspring conflict

A father, mother and offspring may differ in the optimal resource allocation to the offspring. This co-evolutionary conflict can be considered in the context of ICE. Selection will favor genes in the male to maximize female investment in the current offspring, no matter the consequences to the female's reproduction later in life, while selection will favor genes in the female that increase her overall lifetime
fitness Fitness may refer to: * Physical fitness, a state of health and well-being of the body * Fitness (biology), an individual's ability to propagate its genes * Fitness (cereal), a brand of breakfast cereals and granola bars * ''Fitness'' (magazine), ...
. Genes expressed in the offspring will be selected to produce an intermediary level of resource allocation between the male-benefit and female-benefit loci. This three-way conflict again occurs when parents feed their offspring, as the optimum feeding rate and optimum point in time to discontinue feeding differ between father, mother and offspring.


Interference competition

ICE can also explain the theory of
interference competition Competition is an interaction between organisms or species in which both require a resource that is in limited supply (such as food, water, or territory). Competition lowers the fitness of both organisms involved since the presence of one of the ...
, which is most likely to be associated with opposing sets of genes that determine the outcome of
competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, ind ...
between individuals. Different sets of genes may code for signal or receiver
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 prop ...
s, such as in the context of
threat displays Deimatic behaviour or startle display means any pattern of bluffing behaviour in an animal that lacks strong defences, such as suddenly displaying conspicuous eyespots, to scare off or momentarily distract a predator, thus giving the prey anima ...
: when a competing male can win more contests by intimidation, rather than by fighting, selection will favor the accumulation of deceitful genes that may not be honest indicators of the male’s fighting capability. For example, primitive male
elephant seal Elephant seals are very large, oceangoing earless seals in the genus ''Mirounga''. Both species, the northern elephant seal (''M. angustirostris'') and the southern elephant seal (''M. leonina''), were hunted to the brink of extinction for oil ...
s may have used the lowest frequencies in the threat call of a rival as an indication of body size. The elephant seal's enormous nose may have evolved as a resonating device to amplify low frequencies,Bartholomew, GA, NE Collias. 1962. "The role of vocalization in the social behaviour of the northern elephant seal." Animal Behaviour 10(1): 7-14. illustrating selection that favors the production of low-frequency threat vocalizations. However, this counter-selects for receptor systems that provide an increased threshold required for intimidation, which in turn selects for deeper threat vocalizations. The rapid divergence of threat displays among closely related species provides further evidence in support of the co-evolutionary arms race within the genome of a single species, driven by the ICE process.


References

{{Reflist, 32em Genetics Evolution