It might look small, but it’s got a lot to offer and it’s here to stay. I am of course talking about the Y chromosome, the stumpy partner of X. This comparatively meager looking chromosome has long been thought to play few roles aside from sex determination in mammals. But a new study, published in the journal Nature, has demonstrated that we shouldn’t judge a chromosome by its size, and that the few genes this chromosome does possess are pivotal to the survival of males.
Mammals possess two sex chromosomes; X and Y. Normal females have two X chromosomes, whereas males have one X and one Y. These chromosomes evolved from non-sex chromosomes, called autosomes, over the last 300 million years. During this time the Y chromosome experienced a spectacular rate of decay, losing 97% of its ancestral genes, leaving a rather dwarfed remnant containing less than 100 genes.
The X chromosome on the other hand retains 98% of its ancestral genes and has today around 2000 genes. Over the last 25 million years, however, the rate of Y chromosome decay has virtually ceased. Are these few remaining genes present purely by chance having escaped decay, or are they there for a reason?
In order to gain some answers, researchers compared the genes found on Y chromosomes across numerous different mammal species to see if there was some commonality between the remaining genes. Having already discerned that humans, chimpanzees and rhesus macaques possess almost identical ancestral gene content on the Y chromosome, the team used five other mammals for sequence comparison. These were the mouse, marmoset, bull, opossum and rat.
The researchers discovered that gene survival was not random as a diverse set of 18 similar genes were found across all eight species, which is highly unlikely to be purely coincidental. Furthermore, most of these genes required two copies to function, meaning that they are dosage-dependent. This is in contrast to numerous other sex chromosome genes that require only one copy; the second gene on the X chromosome is usually switched off in females, whereas it is missing in males.
Considering that the majority of the remaining Y chromosome genes are not actually involved in sex determination, and the fact that they are active in cells all across the body, it is becoming increasingly evident that the Y chromosome plays a much bigger role than once believed. “Evolution is telling us these genes are really important for survival,” said Winston Bellott, lead author of the study. “They’ve been selected and purified over time.”