Origins and evolution Y chromosome

1 origins , evolution

1.1 before y chromosome
1.2 origin
1.3 recombination inhibition
1.4 degeneration

1.4.1 high mutation rate
1.4.2 inefficient selection
1.4.3 genetic drift


1.5 gene conversion
1.6 future evolution
1.7 1:1 sex ratio

origins , evolution
before y chromosome

many ectothermic vertebrates have no sex chromosomes. if have different sexes, sex determined environmentally rather genetically. of them, reptiles, sex depends on incubation temperature; others hermaphroditic (meaning contain both male , female gametes in same individual).

origin

the x , y chromosomes thought have evolved pair of identical chromosomes, termed autosomes, when ancestral animal developed allelic variation, so-called sex locus – possessing allele caused organism male. chromosome allele became y chromosome, while other member of pair became x chromosome. on time, genes beneficial males , harmful (or had no effect on) females either developed on y chromosome or acquired through process of translocation.

until recently, x , y chromosomes thought have diverged around 300 million years ago. however, research published in 2010, , particularly research published in 2008 documenting sequencing of platypus genome, has suggested xy sex-determination system not have been present more 166 million years ago, @ split of monotremes other mammals. re-estimation of age of therian xy system based on finding sequences on x chromosomes of marsupials , eutherian mammals present on autosomes of platypus , birds. older estimate based on erroneous reports platypus x chromosomes contained these sequences.

recombination inhibition

recombination between x , y chromosomes proved harmful—it resulted in males without necessary genes formerly found on y chromosome, , females unnecessary or harmful genes found on y chromosome. result, genes beneficial males accumulated near sex-determining genes, , recombination in region suppressed in order preserve male specific region. on time, y chromosome changed in such way inhibit areas around sex determining genes recombining @ x chromosome. result of process, 95% of human y chromosome unable recombine. tips of y , x chromosomes recombine. tips of y chromosome recombine x chromosome referred pseudoautosomal region. rest of y chromosome passed on next generation intact. because of disregard rules y chromosome such superb tool investigating recent human evolution.

degeneration

by 1 estimate, human y chromosome has lost 1,393 of 1,438 original genes on course of existence, , linear extrapolation of 1,393-gene loss on 300 million years gives rate of genetic loss of 4.6 genes per million years. continued loss of genes @ rate of 4.6 genes per million years result in y chromosome no functional genes – y chromosome lose complete function – within next 10 million years, or half time current age estimate of 160 million years. comparative genomic analysis reveals many mammalian species experiencing similar loss of function in heterozygous sex chromosome. degeneration may fate of non-recombining sex chromosomes, due 3 common evolutionary forces: high mutation rate, inefficient selection, , genetic drift.

however, comparisons of human , chimpanzee y chromosomes (first published in 2005) show human y chromosome has not lost genes since divergence of humans , chimpanzees between 6–7 million years ago, , scientific report in 2012 stated 1 gene had been lost since humans diverged rhesus macaque 25 million years ago. these facts provide direct evidence linear extrapolation model flawed , suggest current human y chromosome either no longer shrinking or shrinking @ slower rate 4.6 genes per million years estimated linear extrapolation model.

high mutation rate

the human y chromosome particularly exposed high mutation rates due environment in housed. y chromosome passed exclusively through sperm, undergo multiple cell divisions during gametogenesis. each cellular division provides further opportunity accumulate base pair mutations. additionally, sperm stored in highly oxidative environment of testis, encourages further mutation. these 2 conditions combined put y chromosome @ greater risk of mutation rest of genome. increased mutation risk y chromosome reported graves factor 4.8. however, original reference obtains number relative mutation rates in male , female germ lines lineage leading humans.

inefficient selection

without ability recombine during meiosis, y chromosome unable expose individual alleles natural selection. deleterious alleles allowed hitchhike beneficial neighbors, propagating maladapted alleles in next generation. conversely, advantageous alleles may selected against if surrounded harmful alleles (background selection). due inability sort through gene content, y chromosome particularly prone accumulation of junk dna. massive accumulations of retrotransposable elements scattered throughout y. random insertion of dna segments disrupts encoded gene sequences , renders them nonfunctional. however, y chromosome has no way of weeding out these jumping genes . without ability isolate alleles, selection cannot act upon them.

a clear, quantitative indication of inefficiency entropy rate of y chromosome. whereas other chromosomes in human genome have entropy rates of 1.5–1.9 bits per nucleotide (compared theoretical maximum of 2 no redundancy), y chromosome s entropy rate 0.84. means y chromosome has lower information content relative overall length; more redundant.

genetic drift

even if adapted y chromosome manages maintain genetic activity avoiding mutation accumulation, there no guarantee passed down next generation. population size of y chromosome inherently limited 1/4 of autosomes: diploid organisms contain 2 copies of autosomal chromosomes while half population contains 1 y chromosome. thus, genetic drift exceptionally strong force acting upon y chromosome. through sheer random assortment, adult male may never pass on y chromosome if has female offspring. thus, although male may have adapted y chromosome free of excessive mutation, may never make in next gene pool. repeat random loss of well-adapted y chromosomes, coupled tendency of y chromosome evolve have more deleterious mutations rather less reasons described above, contributes species-wide degeneration of y chromosomes through muller s ratchet.

gene conversion

as has been mentioned, y chromosome unable recombine during meiosis other human chromosomes; however, in 2003, researchers mit discovered process may slow down process of degradation. found human y chromosome able recombine itself, using palindrome base pair sequences. such recombination called gene conversion.

in case of y chromosomes, palindromes not noncoding dna; these strings of bases contain functioning genes important male fertility. of sequence pairs greater 99.97% identical. extensive use of gene conversion may play role in ability of y chromosome edit out genetic mistakes , maintain integrity of relatively few genes carries. in other words, since y chromosome single, has duplicates of genes on instead of having second, homologous, chromosome. when errors occur, can use other parts of template correct them.

findings confirmed comparing similar regions of y chromosome in humans y chromosomes of chimpanzees, bonobos , gorillas. comparison demonstrated same phenomenon of gene conversion appeared @ work more 5 million years ago, when humans , non-human primates diverged each other.

future evolution

in terminal stages of degeneration of y chromosome, other chromosomes increasingly take on genes , functions formerly associated it. finally, y chromosome disappears entirely, , new sex-determining system arises. several species of rodent in sister families muridae , cricetidae have reached these stages, in following ways:

the transcaucasian mole vole, ellobius lutescens, zaisan mole vole, ellobius tancrei, , japanese spinous country rats tokudaia osimensis , tokudaia tokunoshimensis, have lost y chromosome , sry entirely. tokudaia spp. have relocated other genes ancestrally present on y chromosome x chromosome. both sexes of tokudaia spp. , ellobius lutescens have xo genotype (turner syndrome), whereas ellobius tancrei possess xx genotype. new sex-determining system(s) these rodents remains unclear.
the wood lemming myopus schisticolor, arctic lemming, dicrostonyx torquatus, , multiple species in grass mouse genus akodon have evolved fertile females possess genotype coding males, xy, in addition ancestral xx female, through variety of modifications x , y chromosomes.
in creeping vole, microtus oregoni, females, 1 x chromosome each, produce x gametes only, , males, xy, produce y gametes, or gametes devoid of sex chromosome, through nondisjunction.

outside of rodent family, black muntjac, muntiacus crinifrons, evolved new x , y chromosomes through fusions of ancestral sex chromosomes , autosomes.

1:1 sex ratio

fisher s principle outlines why species using sexual reproduction have sex ratio of 1:1, meaning in case of humans, 50% of offspring receive y chromosome, , 50% not. w. d. hamilton gave following basic explanation in 1967 paper on extraordinary sex ratios , given condition males , females cost equal amounts produce:

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