Origin of Sex Chromosomes
The accepted hypothesis of XY and ZW sex chromosome evolution is that they evolved at the same time, in two different branches. However, there is some evidence to suggest that there could have been transitions between ZW and XY, such as in Xiphophorus maculatus, which have both ZW and XY systems in the same population, despite the fact that ZW and XY have different gene locations. A recent theoretical model raises the possibility of both transitions between the XY/XX and ZZ/ZW system and environmental sex determination The platypus' genes also back up the possible evolutionary link between XY and ZW, because they have the DMRT1 gene possessed by birds on their X chromosomes. Regardless, XY and ZW follow a similar route. All sex chromosomes started out as an original autosome of an original reptile that relied upon temperature to determine the sex of offspring. After the mammals separated, the branch further split into Lepidosauria and Archosauromorpha. These two groups both evolved the ZW system separately, as evidenced by the existence of different sex chromosomal locations. In mammals, one of the autosome pair, now Y, mutated its SOX3 gene into the SRY gene, causing that chromosome to designate sex. After this mutation, the SRY-containing chromosome inverted and was no longer completely homologous with its partner. The regions of the X and Y chromosomes that are still homologous to one another are known as the pseudoautosomal region. Once it inverted, Y chromosome continued to receive both positive and negative mutations throughout the years, and without a partner to combine with, it slowly decayed. However, because both the chromosomes were necessary for reproduction, the Y chromosome remained, despite its inability to pair with the X. This is becoming a problem, due to the fact that the Y chromosome is steadily shrinking and losing its genes. Without the ability to swap genes with its pair, it is racking up with mutations and may stop functioning in 10 million years.
There are some species, such as the medaka fish that evolved sex chromosomes separately, whose Y chromosome never inverted and can still swap genes with the X. These species are still in an early phase of evolution of their sex chromosomes. Because the Y does not have male-specific genes and can interact with the X, XY and YY females can be formed as well as XX males.
Read more about this topic: Sex-determination System
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