In modern day mammals sex is determined by the Y chromosome. Females possess two X chromosomes, whereas males possess one X and one Y. Thus it is the Y chromosome that acts as a sex switch, overriding the default sex pathway which is female. It is also therefore the Y chromosome that underlies the physiological differences between males and females. But how did this system come to be?
It is known that mammalian sex chromosomes evolved from ancestral autosomes (non-sex chromosomes), but the nature of the genetic sequences within the Y chromosome has made divulging information on its evolution problematic. A paper published recently in Nature, however, has finally provided some answers.
The researchers used sequence data in order to track down the Y chromosome genes in all of the major mammalian lineages. These are the placentals, for example primates and rodents, marsupials such as kangaroos, and monotremes such as the platypus and echidna. Placentals and marsupials, collectively known as therians, share the same XY system, whereas monotremes evolved sex chromosomes from different autosomes and have multiple X and Y chromosomes. In total 15 different mammals were used for this study.
They developed a subtraction approach using a combination of published sequence data which was available for 5 of the mammals, and sequences collected in this study using large-scale sequencing for the other ten, in order to isolate male-specific genes which were not found within female reference genomes. In total they found 134 protein-coding genes on the Y chromosome within the 10 new species, which was twice as many as previously thought to exist.
The team were also able to trace the evolution of the different sex determining genes for therians and monotremes which are called SRY and AMHY, respectively. Both of these genes are thought to be involved in the development of the testicles. They found that SRY formed in a therian ancestor around 180 million years ago whereas AMHY formed around 175 million years ago, meaning that they emerged “nearly at the same time but in a totally independent way,” according to Henrik Kaessmann, lead researcher of the study.
The Y chromosome has experienced massive decay throughout its history; only around 3% of its original genes still remain today. The team concluded that although some genes would have evolved novel functions over time such as testicular development, the majority of the Y genes that persisted did so because of dosage constraints, meaning that they required two copies to function. This is a similar conclusion to another independent study published very recently.
What determined sex in the ancestors of therians and monotremes remains unclear since they did not possess Y chromosomes like those found in mammals today. It is possible that environmental factors temperature played a role, which is still the case for crocodiles today.