In this guest blog, Ann Van Soom, currently a Full Professor at the Faculty of Veterinary Medicine, Ghent University, Belgium, writes about male:female sex ratios, and comments on an article published today in BMC Genomics.
When I was a little girl, I asked my father whether there were more boys or girls in the world. “More boys,” he said. “Why?” I questioned him further, with some indignation at being referred to as the less common gender. He answered me with the slightly sexist phrase: “Because boys have to fight for the girls.”
Many years later I discovered he was right, and maybe even for the reason he claimed. The sex ratio of humans at birth is about 1.05 in favour of males. More males need to be born because more are dying as a result of their testosterone-driven search for adventure, risk and…women. It is even more skewed towards males in early pregnancy than it is at birth, at 1.1 in favour.
The human sex ratio, and that of all mammals, is determined at fertilization. The sperm cell is carrying either an X or a Y chromosome, and in combination with the X of the oocyte we turn out to become an XX embryo, which is female, or an XY embryo, which is male.
Since the number of X and Y bearing spermatozoa is equal, the chances of getting a boy or a girl at fertilization are theoretically comparable, but there is a small tilt towards males. Because boys have a higher risk of dying, this sex ratio of 1.1-1.05 ensures that there are equal numbers of males and females at the time of reproductive maturity.
This almost equal ratio can be skewed in some circumstances. It has been demonstrated that some kind of adaptive control of offspring gender may exist in mammals, which is exerted by the mother. It may be beneficial for a mother to produce male offspring, since males can reproduce and spread their genes (and those of their mother) much faster than females.
As an example, within a period of three weeks a single bull can mate with, say, 24 cows, of which 20 will conceive. While the cows are pregnant, the bull can mate with new females, if available, and continue to sire many more offspring. Within the same period of three weeks, a single cow will ovulate only once , and if she mates with a bull she will give birth after nine months, producing one calf per year. So the reproductive advantage of the male is clearly illustrated.
So what circumstances can change the sex ratio in favour of male offspring? Numerous factors such as population density, famine, season, mother’s age and stress are known to influence the sex ratio in mammals. However, how this is brought about is not clear yet.
There are two main hypotheses : the good condition hypothesis, which suggests that females in good condition have predominantly male offspring, and the female dominance hypothesis, which presumes that dominant females have higher testosterone levels, leading to preferential fertilization by Y-spermatozoa. These hypotheses have been corroborated by several studies, but a plausible explanation for this sex selection by the mother has not been found.
This interesting philosophical question which has mesmerized human mankind for centuries, has now been addressed in a study from the Fazeli lab using the most appropriate and state-of-the-art techniques.
The basic research question was whether the oviduct can tell the difference between X- and Y-bearing spermatozoa. The authors have analysed this in sows, which were inseminated in each oviduct with sexed boar semen, consisting either of X or of Y spermatozoa. They then analysed how gene expression was affected in the respective oviducts and they saw that in each sow, the oviduct could tell the difference between X and Y spermatozoa, with about 2% of the oviductal genes being differentially expressed.
This is a first prerequisite: the mother can sense the difference between X- and Y- bearing spermatozoa. Together with changes in the immunological reaction in the reproductive tract, this may allow for only preferred X- or Y-spermatozoa to reach the oocyte. The paper is therefore with very good reason entitled: ‘The battle of the sexes starts in the oviduct: modulation of oviductal transcriptome by X and Y bearing spermatozoa.’