On the Y chromosome, there is a male-determining gene that determines the
sex of newborn humans and other mammals. But unless we develop a new sex
gene, the human
Y chromosome will likely degenerate
and vanish in a few million years, causing us to go extinct.
The good news is that two rodent species have previously lost their Y
chromosome and survived.
A novel male-determinating gene has emerged in the spiny rat, according to
a recent article published in
Proceedings of the National Academy of Science.
How human sex is determined by the Y chromosome
Like other mammals, humans have two X chromosomes for females and one X and
a little Y chromosome for men. The X stood for "unknown," therefore the
names had nothing to do with their form.
About 900 genes in the X perform various tasks unrelated to sex. However,
there are only around
55 genes
in the Y, and most of the
DNA is non-coding, or just repetitive DNA that doesn't appear to accomplish anything.
However, the Y chromosome is powerful because it has a crucial gene that
initiates male development in the embryo.
This master gene activates other genes that control the growth of a testis
at around 12 weeks after conception. Male hormones (testosterone and its
derivatives), which are produced by the embryonic testis, guarantee that the
infant develops into a boy.
SRY (sex region on the Y), the master sex gene, was discovered in 1990. It
operates by initiating a genetic pathway that begins with the SOX9 gene,
which, while not being located on the sex chromosomes, is essential for the
determination of male gender in all vertebrates.
The vanishing Y
The majority of animals have X and Y chromosomes that are comparable to
ours; the X has many genes, while the Y has SRY and a few additional genes.
Due of the uneven dosage of X genes in males and females, this system has
issues.
How did this peculiar arrangement come to be? The unexpected discovery is
that Australia's platypus has entirely distinct sex chromosomes that are
more similar to those of birds.
The XY pair in
platypus consists of two identical chromosomes, much as other chromosomes. This
indicates that not so long ago, the animal X and Y were a typical pair of
chromosomes.
This implies that throughout the 166 million years that humans and platypus
have been developing independently, the Y chromosome has lost 900–55 active
genes. A loss of five genes every million years is what it amounts to. In
11 million years, at this rate, the final 55 genes will be extinct.
Our assertion that the human Y chromosome will soon disappear sparked a
controversy, and to this day, estimates of the Y chromosome's projected
lifespan range
from infinity to
a few thousand years.
Animals without the Y chromosome
We are aware of two rat lineages that have already lost their Y chromosome
yet are still thriving, which is fantastic news.
Both the Japanese spiny rats and the Eastern European mole voles exhibit
species in which the Y chromosome and SRY have totally vanished. In either a
single or double dosage, the X chromosome is still present in both
sexes.
A team led by Hokkaido University scientist Asato Kuroiwa has had better
success studying the spiny rat, a collection of three species that are all
endangered and found on various Japanese islands. It is still unclear how
mole voles discern sex without the SRY gene.
The majority of the genes in the Y chromosome of spiny rats have been
moved, according to Kuroiwa's study. However, neither SRY nor the gene that
acts as a stand-in were present.
They
reported a successful identification in PNAS in 2022. The researchers identified sequences in the genomes of male rats but not
female rats. They then refined these sequences and examined each individual
rat for the sequence.
What they found was a little variation close to the important sex gene SOX9
on chromosome 3 of the spiny rat. Only 17,000 base pairs out of more than 3
billion base pairs were duplicated, and it was found in all men but not in
any females.
They assert that the switch that typically activates SOX9 in response to
SRY is located inside this little fragment of duplicated DNA. They
discovered that this duplication increases SOX9 activity in mice when they
introduced it, suggesting that the modification would enable SOX9 to
function without SRY.
What this implies for men's future
Future scenarios have been discussed in light of the Y chromosome's
impending extinction, at least in evolutionary terms.
A process known as
parthenogenesis
allows some lizard and snake species, which are only found in females, to
create eggs from their own DNA. However, neither humans nor other mammals
can experience this since we have at least 30 essential genes that are
"imprinted" and only function when passed on from the father through
sperm.
Because males and sperm are required for reproduction, the loss of the Y
chromosome may signal the end of the human race.
The latest discovery lends credence to a different hypothesis, namely that
humans may develop a new sex-specific gene. Phew!
However, there are dangers associated with the emergence of a new sex gene.
What if various regions of the earth see the emergence of many new
systems?
The split of new species as a result of a "war" of the sex genes has
already occurred in mole voles and spiny rats.
Thus, if someone traveled to Earth in 11 million years, they may not find
any people there or find a number of distinct human species, each with its
own technique for determining sex.
Jenny Graves, Distinguished Professor of Genetics and Vice Chancellor's Fellow,
La Trobe University
