This is a blog post from
Sample these headlines, all concerning a recent scientific study:
Y chromosome: Why men contribute so little ( >BBC )
Only two genes maketh the man... or mouse ( >New Scientist )
It Takes Two ( >The Scientist )
Why the Y? ( >The New York Times )
Of course, as is the case with most such instances, cleverness comes at the cause of clarity and these headlines serve more as teasers than as information. You’d need to delve deeper into the story to understand that what really happened is the discovery that mice needed just two genes from their Y-chromosome to not just qualify as anatomically male, but also become fathers (albeit with some help).
What makes you male
A chromosome is a long piece of DNA coiled into one package. Every human cell contains 23 pairs of such packages. A normal human has two copies each of chromosome 1 to chromosome 22 (one inherited from each parent) and a pair of sex chromosomes.
High school biology will tell you that the pair of sex chromosomes comprises two copies of the X chromosome in females (XX), and one X chromosome and one Y chromosome (XY) in males. So it is essentially the Y chromosome that determines if you are male -- with some rare exceptions like the platypus which has not two but five pairs of sex chromosomes, and the XY female. (An XY female? We’re coming to that)
You’d think that an individual without a Y chromosome would automatically be classified as non-male and vice-versa, but it turns out that’s not entirely true. DNA analysis of an XY woman more than 15 years back revealed that it’s actually just one gene on the Y chromosome that is responsible for testes development – the SRY gene. While the SRY gene was present on the Y-chromosome of the woman, a mutation had rendered it functionless, which resulted in her developing as a woman.
The discovery that a single gene on the Y-chromosome was enough to make one anatomically male was controversial as much as it was a breakthrough. The presence of the SRY gene was used to disqualify athletes from competing as female in >the Olympics , until compulsory gender verification was banned in 1999 to protect those with Disorders of Sex Development (DSD) from discrimination.
Stripping down Y
The limited usefulness of the Y chromosome has intrigued scientists for decades. So much so that, as was pointed out in a >New York Times> column , it was being painted as “a slovenly beast, sitting in his worn armchair, surrounded by boxes and pizza crusts.”
The same article colourfully quotes evolutionary biologist Dr David Page: “The Y wants to maintain himself but doesn’t know how. He’s falling apart, like the guy who can’t manage to get a doctor’s appointment or clean up the house or apartment unless his wife or girlfriend does it.”
Some context here: Though both the X and the Y chromosome were thought to have evolved from a common sex chromosome, Y began shedding genes at a much faster rate than X. One estimate says that over the 300 million years of existence of the Y chromosome, it has lost 1,393 of its original 1,438 genes. Though this shrinking seems to have tapered off since around a hundred million years ago, chromosome Y is today only one per cent of the total DNA of a human male. The 45 other chromosomes (X + 22 pairs) make up the rest.
Since only one small portion of Y – the SRY gene – is sufficient to give rise to male individuals, scientists found out that male mice could be produced even without a Y chromosome as long as the SRY gene was inserted on another chromosome. However, the resulting male would not be capable of producing functional sperm and father offspring. This confirmed that the other genes on the Y chromosome (14 in mice, more than 50 in humans) are necessary for processes that make reproduction possible.
However, scientists began to wonder – what was the minimal amount of Y chromosome that is necessary for a mouse to be male and father offspring? The same team of scientists in Hawaii had found out earlier that the entire long arm of chromosome Y could be eliminated in male mice and they’d still be capable of becoming fathers.
This narrowed down the search for the minimal genes to the short arm of chromosome Y. That the SRY gene was essential seemed like a given, considering that it was necessary for testes development. But as already shown, that alone was not enough for sperm to be produced. The team placed their hopes on another Y chromosome gene called Eif2s3y. This is a key gene for the initiation of sperm production.
Eif2s3y alone can only produce immature sperm cells called ‘rounded spermatids’ which are usually infertile. However the scientists wanted to find out if assisted reproduction techniques could enable these rounded spermatids to produce live offspring.
They extracted the immature spermatids from the male mice containing only two Y-chromosome genes (SRY and Eif2s3y) and injected them into healthy mouse eggs. The resulting embryos were transplanted into female mice and to their surprise, some (9 per cent, compared to an efficiency of 26 per cent using healthy male mice) of these females did end up delivering healthy offspring!
What does this mean for Y?
Since it has already been established that for an anatomically male mammal, the SRY gene can be present on any chromosome (not necessarily a Y), if the same applies for Eif2s3y or a suitable substitute for it is found on another chromosome, this could mean that the Y chromosome can totally be done without.
However, this possibility isn’t as fantastic as it sounds because a) the Y-less male would still need assisted reproduction which has a pretty low efficiency, and b) we’re still only talking about MICE!
Oh yeah...
So far, we have not identified a human equivalent of the Eif2s3y gene. But that doesn’t mean there are no implications of this study for humans. The most significant result of this study is the success of the assisted reproduction technique used to fertilize eggs with infertile sperm. The technique these scientists used is called round spermatid injection (ROSI) and has been traditionally considered risky for human males because of the fear that the male offspring born may carry the same defect as the father.
This piece of research seems to suggest that those fears may not be founded, and gives hope to men with defective Y-chromosomes who are unable to become fathers.
Back to the beginning
It is unlikely that a headline that was to-the-point, like > Nature ’s (‘Mice with just two 'male' genes father babies’) or the University of Hawaii’s >press release (‘Two Y genes can replace the Y chromosome for assisted reproduction in mice’), would have attracted as many layman readers.
But that’s still better than misleading readers. I found the BBC’s headline “Y chromosome: why men contribute so little” at best ambiguous (is it accurate to equate the Y chromosome with men?), and at worst misleading (this study doesn’t exactly reveal why the Y chromosome contributes so little).
Even New Scientist ’s headline “Only two genes maketh the man... or mouse” is incorrect since it has not yet been determined if only two genes “maketh the man” – the mouse, yes, but the man, we have no idea, yet.
The risk with being clever with a story that is so capable of kindling public imagination is that unless you are doubly careful to compensate with un-decorated information and careful selection of quotes (which, thankfully, most of the above manage to do) within the article itself, the reader – especially if he/she lacks a science background – is probably going to misunderstand the implications.
(Nandita Jayaraj writes about her encounters with the strange and interesting. You can send her feedback at nandita.j@thehindu.co.in. You can also tweet her @nandita_j)
