(Human male chromosome set. Image via genomicseducation.uk)
Staff Writer: Maya Arruda
Email: marruda7@umassd.edu
The Y chromosome is slowly disappearing from the human genome.
The already notably small sex chromosome has been continuously shrinking for over 160 million years. The current estimate is that it will shrink into non-existence within the next five million years.
For those who haven’t taken biology since high school, humans have an XY sex chromosome-determining system. The Y chromosome has an SRY gene that directs male embryonic development. If there is no SRY gene present, the embryo follows a female development pattern. Though there are a multitude of different genes and genetic elements responsible for the development of the sex phenotype, the SRY gene plays a significant role in sex determinism.
According to research reported in Nature, the Y chromosome has lost 97% of its ancestral genes because of functional specialization after multiple genetic inversions that prevented recombination with the X chromosome (though they rarely still occur). The lack of recombination has contributed to genetic decay on the small sex chromosome.
In Layman’s terms, a lack of recombination is bad for chromosomes because it shuts down an important DNA repair mechanism for double-stranded DNA damage, which leads to an accumulation of mutations with ill effects.
Recombination is used for the Homologous Recombination Repair mechanism, as the name suggests, which is a better alternative than Nonhomologous End Joining (which involves sticking the two ends back together) that can lead to a higher amount of deletions of the DNA sequence.
In other mammalian species, the Y chromosome has already disappeared from their genome. Enter the Amami spiny rat, which has no Y sex chromosome anymore, only X. The SRY gene has left along with the Y chromosome.
However, despite not having a male sex chromosome, the Amami spiny rat still has males. A gene next in the SRY signaling pathway SOX9, is still clinging for life on Chromosome 3 with a duplicated region affecting SOX9 expression. If you have the region on your chromosome, SOX9 levels increase, and you become a male rat.
While it is likely that if humankind is around in five million years to mourn the loss of the Y chromosome (admittedly unlikely at this rate—I think the planet would combust first and take all of us along with it), we would most likely just have a different sex-determining system like the Amami spiny rat, and males would still continue to exist in the species (assuming that the species hasn’t gone extinct yet, which is doubtful).
Male phenotype enthusiasts can rest at least: while the Y chromosome may shrink into non-existence, males most likely will still be around, one way or another.
Humanity will not end with males, but interestingly, males may be obsolete by the time the Y chromosome loses its free trial of existence—and I am not referring to the extinction of humanity via rampant greed and poor decision-making this time.
Two years ago, researchers were able to induce parthenogenesis (making babies without sperm) in mice via epigenetic modifications. The process was able to produce living mice who were perfectly fertile. Parthenogenesis is common in many species, mainly in birds, lizards, and invertebrates. However, it is not common in mammals in the wild.
But really, who needs the wild if we can do it in a lab?
If we somehow survive another five million years at the rate we’re going (unlikely) and males are entirely wiped from the genome with no other sex determinism method adopted instead (even more unlikely); we’ve got a viable backup plan to propagate the human race.