Origin, degeneration and detection of sex chromosomes

Sex chromosomes evolved from a pair of autosomes and they are differentiated as a result of supression of recombination. This process leads to a successive degradation of odd sex chromosome (alosome), which is becoming genetically inert finally or even excluded. Fundamental processes taking part in degeneration of alosome are Muller's ratchet, genetic hitchhiking, background selection, accumulation of transposable elements and constitutive heterochromatin. Indeed, these processes take part in either degeneration of both Y or W chromosomes. Remarkably, these alosomes show different rates of degeneration, most probably due to 1) different structure of male and female gonads as well as 2) different course of gametogenesis in both sexes. Furthermore, rate of alosome degeneration is usually lower in plants because they are haploid during the major part of life cycle. Other mechanisms of sex chromosome evolution involve rearrangements between autosomes and gonosomes, nondisjunctions and fissions of original sex chromosomes, transformation of B chromosomes into sex chromosomes or non-random segregation of autosomes with sex chromosomes. Other phenomenon that appears in sex chromosome evolution is transition between XY/XX and ZW/ZZ systems or transition between chromosomal sex determination and epigamy. Actually,..