American biologists by analyzing nutrigenomic conflict from the point of view of individual genes could explain why males of alleles can be fixed in the X-chromosome, of which females have two times more. Simulations showed that the emergence of masculinization of chromosomes contributes to, for example, dose compensation, high life expectancy of males and the low level of inbreeding. Article published in the journal Proceedings of the Royal Society B: Biological Sciences.
Because of sexual dimorphism in the genome arises a contradiction: some alleles are advantageous to males, others to females, but the set of genes they shared. The action of natural selection on such alleles should be balanced so that the benefits to one sex outweigh the reduction in fitness of the other.
Some genes give an unequal contribution to the fitness of males and females, and because of this it may be nutrigenomic conflict. For example, the X-chromosome in the body of females twice more than males, and some scientists suggest that the X-chromosomes accumulate mutations that are beneficial for females (because in their body, they spend twice as much time).
However, mathematical modeling showsthat the benefits of male alleles can be located on the X chromosome more frequently than on autosomes, and their fixation on the X chromosome can occur easier than best female alleles. “Masculinization” X-chromosomes found in humans, aphids and stabilisatoren flies, however, account for the discrepancy between these data and the traditional view vnutrigornye conflicts still failed.
Biologists Thomas Hitchcock (Thomas Hitchcock) and Andy Gardner (Andy Gardner) from St. Andrews University analyzed nutrigenomic the conflict from the perspective of the hospitality gene, not whole genome. They identified the benefits that a mutant allele of one gender brings in homozygous or hemizygous form (when both paired chromosomes carrying this allele in the genome or has only one copy of chromosomes), for S, and reduced fitness for the other sex for T. If the chromosome in question floor-a steam room (as all the autosomes and X chromosome in females), the mutation in one of them will benefit σ = S/2 and adverse effect τ = T/2. And if a chromosome in one set (as the X chromosome in males), then σ = S and τ = T.
Condition of fixation of a gene in the genome is σ > τ. If, as suggested by the traditional view of nutrigenomic conflict, the value of the mutant allele in X chromosomes for females twice that for males, this expression becomes 2σ > τ for best female alleles and σ > 2τ for alleles that benefit males. In both cases, the expression is equivalent to S > T. it Turns out that in the absence of additional factors integration into the genome of the best males or females of alleles are equally probable due to the fact that the effects of changes in contact of googletlogo in heterogametic gender and Vice versa.
The researchers then analyzed the various factors that tip the balance in favor of one of the sexes and favor the emergence of masculinisation or feminized X-chromosomes. The first of these factors — the dose compensation of genes — epigenetic mechanisms that alter the level of gene expression, X-chromosome at different floors, so that the ratio of its expression to the autosomes remain constant (for example, turn off one of the X chromosomes in females).
However, not all organisms the compensation system works the same way. Therefore, the researchers added in their model, the parameter γ — the degree of dose compensation, and the ratio σ : τ was equivalent to S : (1 + γ)T for best female alleles and (1 + γ)S T — best for males. It turned out that the full dose compensation (γ = 1) positive effect on the fitness of the allele beneficial to males is twice higher, while reducing the level of compensation becomes more likely the feminization of the X chromosome.
The second factor discussed by scientists, reproductive value of different genders. In the traditional sense of the conflict vnutricerepnogo females are twice as likely to pass their X-chromosome to the next generation, and therefore feminized alleles spread faster in the population. However, this ratio may change if males and females leave different number of offspring for life. This number is affected by two parameters: the life expectancy and fecundity, so fitness of males and females may rise either due to better survival or a high birth rate. In the first case, the contribution of the sexes to one favorable allele in adaptation will depend on the ratio of mother’s age and father’s (x : y). For best male alleles condition a fixation looks like (y − 1)σ > 2(x − 1)τ, and for best female (x − 1)σ > (y − 1)τ.