Boreskov

OP‐43RISE AND FALL OF THE Y‐CHROMOSOME OR CRONICA DE UNA MUERTEANUNCIADABorodin P.M.Institute of Cytology and Genetic SB RAS, Novosibirsk 630090, RussiaMammalian sex chromosomes are believed to originate from an ancient pair ofautosomes.The first step of their divergence was an occurrence of SRY gene involved into controltestis determination and, consequently, male development of the embryo. SRY is atruncated variant of an ancestral transcription factor SOX3. It controls the testisdetermination indirectly by interfering with the binding of another chromatin remodelingproteins.The chromosome carrying the male specific allele of SRY became the Y, while itshomolog became the X chromosome.The second step of divergence involved an accumulation at the Y chromosome thesexually antagonistic alleles, i.e. the alleles which were beneficial for males and harmful orneutral to females. This was achieved by a selection for such mutations in such genes at theY chromosome and from translocations transposing such genes from the autosomes to theY. At this stage natural selection was directed against recombination between the portionsof the X and Y chromosomes containing sexually antagonistic genes.At the third step of the divergence this selection led to accumulation of variousepigenetic, genetic and chromosomal suppressors of recombination at the Y chromosome,while X chromosomes kept recombining normally in female meiosis. The non‐recombiningportion of the region Y chromosome grew generation by generation. Now in most placentalmammals synapsis and recombination between X and Y chromosomes are restricted by avery small (about 5% of chromosome length) region.Suppression of recombination led to inevitable degradation of the non‐recombining partof the Y chromosome due to mutational meltdown. The Y chromosome as any nonrecombiningchromosome has been doomed to extinction. The human Y‐chromosome has lost1,393 of its 1,438 original genes over the time of it divergence from the X chromosome (about160 mln years). If it will keep degrading with this speed, it gets extinct in about 10 mln years.A hopeful insight into the future of the human Y chromosome may be gained by theanalysis of the species which overrun us in rate the Y chromosome degradation. In somespecies of the grey voles studied in our laboratory the X and Y have completely lost their abilityof pair and recombine. In my report I will discuss the causes and consequences of independentlosses and acquisitions of the X‐Y pairing regions in the evolution of the grey voles.104