[Evolutionary role of sex chromosomes (a new concept)]

Sex differentiation provides for testing evolutionary innovations in the male genome before they are transferred to the female genetic system. This is possible with dichronous (asynchronous) evolution, when evolution in males precedes that in females [3-7]. Hence, along with common autosomal genes for stable characters, exclusively male and exclusively female genes must exist. The male genes are already acquired by the male genome but are not yet transferred to the female one. The female genes are already lost by the male genome but still remain in the female genome. They constitute temporary evolutionary genotypic sexual dimorphism. Common genes cannot exhibit genotypic sexual dimorphism; they show only constant phenotypic hormonal sexual dimorphism. On the basis of the interpretation of genotypic sexual dimorphism as a consequence of sex dichronism, the evolutionary role of sex chromatins is clarified and a new concept for them is suggested. According to this concept, the Y chromosome is the "conductor" of ecological information into the genome, the "place of birth" and testing of new genes, the accelerator and regulator of genotypic sexual dimorphism. By contrast, the X chromosome of the heterogametic sex provides the transportation of new genes from the Y chromosome to autosomes. This chromosome stabilizes, relaxes, and suppresses genotypic sexual dimorphism and accumulates genes that will be eliminated. This concept sheds light on many problems: the chromosomal localization of genes and their transfer to other chromosomes, the inactivation of chromosomes, mobile genes, mutation bursts, insertional mutagenesis, the association of the Y chromosome with stress, retroviruses, etc. In particular, it explains why genes "jump," why transpositions of mobile elements depend on ecological stress, why different genes mutate simultaneously, etc.