Genomic imprinting
Genomic imprinting is an epigenetic phenomenon that causes genes to be expressed or not, depending on whether they are inherited from the female or male parent.
[3] In diploid organisms (like humans), the somatic cells possess two copies of the genome, one inherited from the male and one from the female.
Although imprinting accounts for a small proportion of mammalian genes, they play an important role in embryogenesis particularly in the formation of visceral structures and the nervous system.
[16][17][18] That imprinting might be a feature of mammalian development was suggested in breeding experiments in mice carrying reciprocal chromosomal translocations.
[19] Nucleus transplantation experiments in mouse zygotes in the early 1980s confirmed that normal development requires the contribution of both the maternal and paternal genomes.
However, in 2004, experimental manipulation by Japanese researchers of a paternal methylation imprint controlling the Igf2 gene led to the birth of a mouse (named Kaguya) with two maternal sets of chromosomes, though it is not a true parthenogenone since cells from two different female mice were used.
[12][27][28][29] Hybrid offspring of two species may exhibit unusual growth due to the novel combination of imprinted genes.
[31] An intriguing study surveying the transcriptome of murine brain tissues revealed over 1300 imprinted gene loci (approximately 10-fold more than previously reported) by RNA-sequencing from F1 hybrids resulting from reciprocal crosses.
[33][34] In domesticated livestock, single-nucleotide polymorphisms in imprinted genes influencing foetal growth and development have been shown to be associated with economically important production traits in cattle, sheep and pigs.
[37][38] The generation of a series of such uniparental disomies, which together span the entire genome, allowed the creation of an imprinting map.
[39] Those regions which when inherited from a single parent result in a discernible phenotype contain imprinted gene(s).
This process of erasure and reprogramming[45] is necessary such that the germ cell imprinting status is relevant to the sex of the individual.
In both plants and mammals there are two major mechanisms that are involved in establishing the imprint; these are DNA methylation and histone modifications.
Among the hypothetical explanations for this novel phenomenon, two possible mechanisms have been proposed: either a histone modification that confers imprinting at novel placental-specific imprinted loci or, alternatively, a recruitment of DNMTs to these loci by a specific and unknown transcription factor that would be expressed during early trophoblast differentiation.
The grouping of imprinted genes within clusters allows them to share common regulatory elements, such as non-coding RNAs and differentially methylated regions (DMRs).
[54][55] The father's genes that encode for imprinting gain greater fitness through the success of the offspring, at the expense of the mother.
The mother's evolutionary imperative is often to conserve resources for her own survival while providing sufficient nourishment to current and subsequent litters.
A small number of imprinted genes are fast evolving under positive Darwinian selection possibly due to antagonistic co-evolution.
[58] The majority of imprinted genes display high levels of micro-synteny conservation and have undergone very few duplications in placental mammalian lineages.
[59] Another hypothesis proposed is that some imprinted genes act coadaptively to improve both fetal development and maternal provisioning for nutrition and care.
Statistical frameworks and mapping models are used to identify imprinting effects on genes and complex traits.
[67] These models will show classic quantitative genetics and the effects of dominance of the imprinted genes.
[citation needed] In vitro fertilisation, including ICSI, is associated with an increased risk of imprinting disorders, with an odds ratio of 3.7 (95% confidence interval 1.4 to 9.7).
[74] Therefore, if uniparental disomy occurs and a person inherits both chromosomes from the mother, the gene will not be expressed and the individual is put at a greater risk for breast and ovarian cancer.
In some insects the entire paternal genome is silenced in male offspring, and thus is involved in sex determination.
The imprinting produces effects similar to the mechanisms in other insects that eliminate paternally inherited chromosomes in male offspring, including arrhenotoky.
[77] In social honey bees, the parent of origin and allele-specific genes has been studied from reciprocal crosses to explore the epigenetic mechanisms underlying aggressive behavior.
[78] In placental species, parent-offspring conflict can result in the evolution of strategies, such as genomic imprinting, for embryos to subvert maternal nutrient provisioning.
[88] It has been suggested that these imprinted genes are responsible for the triploid block effect in flowering plants that prevents hybridization between diploids and autotetraploids.
