Intragenomic conflict refers to the
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
ary phenomenon where
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 ba ...
s have phenotypic effects that promote their own transmission in detriment of the transmission of other genes that reside in the same
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 ge ...
. The
selfish gene theory
With gene defined as "not just one single physical bit of DNA utall replicas of a particular bit of DNA distributed throughout the world", the gene-centered view of evolution, gene's eye view, gene selection theory, or selfish gene theory ...
postulates that
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 ...
will increase the frequency of those genes whose phenotypic effects cause their transmission to new organisms, and most genes achieve this by cooperating with other genes in the same genome to build an
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
capable of
reproducing and/or
helping kin to reproduce. The assumption of the prevalence of intragenomic cooperation underlies the organism-centered concept of
inclusive fitness
In evolutionary biology, inclusive fitness is one of two metrics of evolutionary success as defined by W. D. Hamilton in 1964:
* Personal fitness is the number of offspring that an individual begets (regardless of who rescues/rears/supports them ...
. However, conflict among genes in the same genome may arise both in events related to reproduction (a
selfish gene may "cheat" and increase its own presence in
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...
s or
offspring
In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
above the expected according to fair
Mendelian segregation and fair
gametogenesis
Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic d ...
) and altruism (genes in the same genome may disagree on how to value other organisms in the context of helping kin because
coefficients of relatedness diverge between genes in the same genome).
Nuclear genes
Autosomic genes usually have the same mode of transmission in
sexually reproducing species due to the fairness of
Mendelian segregation, but conflicts among
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 chro ...
s of autosomic genes may arise when an allele cheats during
gametogenesis
Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic d ...
(segregation distortion) or eliminates
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s that don't contain it (lethal maternal effects). An allele may also directly convert its rival allele into a copy of itself (homing endonucleases). Finally, mobile genetic elements completely bypass Mendelian segregation, being able to insert new copies of themselves into new positions in the genome (transposons).
Segregation distortion
In principle, the two parental
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 chro ...
s have equal probabilities of being present in the mature
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...
. However, there are several mechanisms that lead to an unequal transmission of parental alleles from parents to offspring. One example is a gene, called a ''segregation distorter'', that "cheats" during meiosis or
gametogenesis
Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic d ...
and thus is present in more than half of the functional gametes. The most studied examples are ''sd'' in ''
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 Ch ...
'' (
fruit fly), ''t'' haplotype in ''
Mus musculus
Mus or MUS may refer to:
Abbreviations
* MUS, the NATO country code for Mauritius
* MUS, the IATA airport code for Minami Torishima Airport
* MUS, abbreviation for the Centre for Modern Urban Studies on Campus The Hague, Leiden University, Neth ...
'' (
mouse
A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...
) and ''sk'' in ''
Neurospora
''Neurospora'' is a genus of Ascomycete fungi. The genus name, meaning "nerve spore" refers to the characteristic striations on the spores that resemble axons.
The best known species in this genus is ''Neurospora crassa'', a common model organi ...
'' spp. (
fungus
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
). Possible examples have also been reported in humans.
Segregation distorters that are present in sexual chromosomes (as is the case with the X chromosome in several ''Drosophila'' species
) are denominated sex-ratio distorters, as they induce a sex-ratio bias in the offspring of the carrier individual.
Killer and target
The simplest model of
meiotic drive
Meiotic drive is a type of intragenomic conflict, whereby one or more loci within a genome will effect a manipulation of the meiotic process in such a way as to favor the transmission of one or more alleles over another, regardless of its phenotyp ...
involves two tightly linked loci: a ''Killer'' locus and a ''Target'' locus. The segregation distorter set is composed by the allele ''Killer'' (in the ''Killer'' locus) and the allele ''Resistant'' (in the ''Target'' locus), while its rival set is composed by the alleles ''Non-killer'' and ''Non-resistant''. So, the segregation distorter set produces a toxin to which it is itself resistant, while its rival is not. Thus, it kills those gametes containing the rival set and increases in frequency. The tight linkage between these loci is crucial, so these genes usually lie on low-recombination regions of the genome.
True meiotic drive
Other systems do not involve gamete destruction, but rather use the asymmetry of
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
in females: the driving allele ends up in the
oocyte
An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...
instead of in the
polar bodies
A polar body is a small haploid cell that is formed at the same time as an egg cell during oogenesis, but generally does not have the ability to be fertilized. It is named from its polar position in the egg.
When certain diploid cells in ani ...
with a probability greater than one half. This is termed true
meiotic drive
Meiotic drive is a type of intragenomic conflict, whereby one or more loci within a genome will effect a manipulation of the meiotic process in such a way as to favor the transmission of one or more alleles over another, regardless of its phenotyp ...
, as it does not rely on a post-meiotic mechanism. The best-studied examples include the
neocentromere
Neocentromeres are new centromeres that form at a place on the chromosome that is usually not centromeric. They typically arise due to disruption of the normal centromere. These neocentromeres should not be confused with “knobs”, which were als ...
s (knobs) of maize, as well as several chromosomal rearrangements in mammals. The general molecular evolution of
centromere
The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...
s is likely to involve such mechanisms.
Lethal maternal effects
The
Medea gene causes the death of progeny from a heterozygous mother that do not inherit it. It occurs in the
flour beetle
Flour beetles are members of the darkling beetle genus, genera ''Tribolium (beetle), Tribolium'' or ''Tenebrio''. They are pests of cereal silos and are widely used as laboratory animals, as they are easy to keep. The flour beetles consume wheat ...
(''Tribolium castaneum'').
Maternal-effect selfish genes have been successfully synthesized in the lab.
Transposons
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. Transpo ...
are autonomous replicating genes that encode the ability to move to new positions in the genome and therefore accumulate in the genomes. They replicate themselves in spite of being detrimental to the rest of the genome.
They are often called 'jumping genes' or parasitic DNA and were discovered by
Barbara McClintock
Barbara McClintock (June 16, 1902 – September 2, 1992) was an American scientist and cytogeneticist who was awarded the 1983 Nobel Prize in Physiology or Medicine. McClintock received her PhD in botany from Cornell University in 1927. There s ...
in 1944.
Homing endonuclease genes
Homing endonuclease genes
The homing endonucleases are a collection of endonucleases encoded either as freestanding genes within introns, as fusions with host proteins, or as self-splicing inteins. They catalyze the hydrolysis of genomic DNA within the cells that synthes ...
(HEG) convert their rival
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 chro ...
into a copy of themselves, and are thus inherited by nearly all meiotic daughter cells of a
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.
Mo ...
cell. They achieve this by encoding an endonuclease which breaks the rival allele. This break is repaired by using the sequence of the HEG as template.
HEGs encode sequence-specific endonucleases. The recognition sequence (RS) is 15–30 bp long and usually occurs once in the genome. HEGs are located in the middle of their own recognition sequences.
Most HEGs are encoded by self-splicing
intron
An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of the cistron .e., gene. ...
s (group I & II) and
inteins. Inteins are internal protein fragments produced from protein splicing and usually contain endonuclease and splicing activities.
The allele without the HEGs are cleaved by the homing endonuclease and the double-strand break are repaired by homologous recombination (gene conversion) using the allele containing HEGs as template. Both chromosomes will contain the HEGs after repair.
B-chromosome
B-chromosome
In addition to the normal karyotype, wild populations of many animal, plant, and fungi species contain B chromosomes (also known as supernumerary, accessory, (conditionally-)dispensable, or lineage-specific chromosomes). By definition, these chro ...
s are nonessential
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s; not
homologous with any member of the normal (A) chromosome set; morphologically and structurally different from the A's; and they are transmitted at higher-than-expected frequencies, leading to their accumulation in progeny. In some cases, there is strong evidence to support the contention that they are simply ''selfish'' and that they exist as
parasitic chromosome Parasitic chromosomes are "selfish" chromosomes that propagate throughout cell divisions, even if they confer no benefit to the overall organism's survival. Parasitic chromosomes can persist even if slightly detrimental to survival, as is characteri ...
s.
They are found in all major taxonomic groupings of both
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
s and
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s.
Cytoplasmic genes
Since nuclear and cytoplasmic genes usually have different modes of transmission, intragenomic conflicts between them may arise. Mitochondria and chloroplasts are two examples of sets of cytoplasmic genes that commonly have exclusive maternal inheritance, similar to endosymbiont parasites in arthropods, like ''Wolbachia''.
Males as dead-ends to cytoplasmic genes
Anisogamy
Different forms of anisogamy: A) anisogamy of motile cells, B) egg_cell.html"_;"title="oogamy_(egg_cell">oogamy_(egg_cell_and_sperm_cell),_C)_anisogamy_of_non-motile_cells_(egg_cell_and_spermatia)..html" ;"title="egg_cell_and_sperm_cell.html" ;" ...
generally produces
zygotes
A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism.
In multicellula ...
that inherit cytoplasmic elements exclusively from the female gamete. Thus, males represent dead-ends to these genes. Because of this fact, cytoplasmic genes have evolved a number of mechanisms to increase the production of female descendants and eliminate offspring not containing them.
Feminization
Male organisms are converted into females by cytoplasmic inherited protists (
Microsporidia
Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore. They were once considered protozoans or prot ...
) or bacteria (''
Wolbachia
''Wolbachia'' is a genus of intracellular bacteria that infects mainly arthropod species, including a high proportion of insects, and also some nematodes. It is one of the most common parasitic microbes, and is possibly the most common reproduct ...
''), regardless of nuclear sex-determining factors. This occurs in
amphipod
Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods range in size from and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far descr ...
and
isopod
Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, an ...
Crustacea
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...
and
Lepidoptera
Lepidoptera ( ) is an order (biology), order of insects that includes butterfly, butterflies and moths (both are called lepidopterans). About 180,000 species of the Lepidoptera are described, in 126 Family (biology), families and 46 Taxonomic r ...
.
Male-killing
Male
embryos
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sper ...
(in the case of cytoplasmic inherited bacteria) or male
larva
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
The ...
e (in the case of Microsporidia) are killed. In the case of embryo death, this diverts investment from males to females who can transmit these cytoplasmic elements (for instance, in ladybird beetles, infected female hosts eat their dead male brothers, which is positive from the viewpoint of the bacterium). In the case of microsporidia-induced larval death, the agent is transmitted out of the male lineage (through which it cannot be transmitted) into the environment, where it may be taken up again infectiously by other individuals. Male-killing occurs in many
insects
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of j ...
. In the case of male embryo death, a variety of bacteria have been implicated, including ''Wolbachia''.
Male-sterility
In some cases
anther
The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10
Morphology and terminology
A stamen typically consists of a stalk called the filam ...
tissue (male
gametophyte
A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...
) is killed by
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
in
monoecious
Monoecy (; adj. monoecious ) is a sexual system in seed plants where separate male and female cones or flowers are present on the same plant. It is a monomorphic sexual system alongside gynomonoecy, andromonoecy and trimonoecy.
Monoecy is conne ...
angiosperms
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
, increasing energy and material spent in developing female gametophytes. This leads to a shift from monoecy to
gynodioecy
Gynodioecy is a rare breeding system that is found in certain flowering plant species in which female and hermaphroditic plants coexist within a population. Gynodioecy is the evolutionary intermediate between hermaphroditism (exhibiting both fem ...
, where part of the plants in the population are male-sterile.
Parthenogenesis induction
In certain
haplodiploid
Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid. Haplodiploidy is sometimes called arrhenotoky.
Haplodiploidy determines the se ...
Hymenoptera
Hymenoptera is a large order (biology), order of insects, comprising the sawfly, sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are Par ...
and
mites
Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear evid ...
, in which males are produced asexually, ''Wolbachia'' and ''
Cardinium
"''Candidatus'' Cardinium" is a genus of Gram-negative parasitic bacteria that reside within cells of some arthropods and nematodes. Although they have not yet been isolated in pure culture (hence the designation ''Candidatus''), they are kno ...
'' can induce duplication of the
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s and thus convert the organisms into females. The cytoplasmic bacterium forces
haploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
cells to go through incomplete mitosis to produce
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
cells which therefore will be female. This produces an entirely female population. If antibiotics are administered to populations which have become asexual in this way, they revert to sexuality instantly, as the cytoplasmic bacteria forcing this behaviour upon them are removed.
Cytoplasmic incompatibility
In many
arthropods
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
, zygotes produced by sperm of infected males and ova of non-infected females can be killed by ''Wolbachia'' or ''Cardinium''.
Evolution of sex
Conflict between chromosomes has been proposed as an element in the
evolution of sex
Sexual reproduction is an adaptive feature which is common to almost all multicellular organisms and various unicellular organisms, with some organisms being incapable of asexual reproduction. Currently the adaptive advantage of sexual repro ...
.
[ See als]
See also
*
Green-beard effect
The green-beard effect is a thought experiment used in evolutionary biology to explain selective altruism among individuals of a species.
The idea of a green-beard gene was proposed by William D. Hamilton in his articles of 1964, and got the ...
References
Further reading
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{{Refend
Evolutionary biology
Selection
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