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 disappear 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 recent study published in
Proceedings of the National Academy of Science demonstrates how the
male-determining gene in the spiny rat has developed.
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| Image by Darwin Laganzon from Pixabay |
How
the Y chromosome determines human sex
Like other mammals, humans have two
X chromosomes for females and one X and a little Y chromosome for males. The X
stood for "unknown," therefore the names have nothing to do with
their shape.
About 900 genes in the X perform
various tasks unrelated to sex. However, there are only about 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, ensure 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, despite not being located on the
sex chromosomes, is essential for the determination of male gender in all
vertebrates.
The disappearing 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 to 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 over 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 that amounts to. In 11 million years, at this rate, the final 55
genes will be extinct.
Our claim that the human Y
chromosome was on the verge of extinction sparked a controversy, and estimates
of the Y chromosome's expected lifespan range from infinity to a few thousand
years.
Rodents with no 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 dose, the X chromosome is
still present in both sexes.
A team led by Hokkaido University
scientist Asato Kuroiwa has had more 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 on 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.
Finally, a successful identification
has been reported in PNAS. The researchers identified sequences in the genomes
of male rats but not female rats. They then refined these sequences and tested
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 tiny
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 means for the future of
men
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 exclusively found in females,
to create eggs from their own genes. 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 men and sperm are required
for reproduction, the loss of the Y chromosome may signal the end of the human
race.
The latest discovery lends support
to a different hypothesis, namely that humans could develop a new sex-specific
gene. Phew!
However, there are dangers
associated with the emergence of a new sex gene. What if different regions of
the earth experience the emergence of multiple new systems?
The split of new species as a result
of a "battle" of the sex genes has already occurred in mole voles and
spiny rats.
Thus, if someone travelled to Earth
in 11 million years, they might not find any people there or find a number of
distinct human species, each with its own technique for determining sex.
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