Survival of the Females
It is well known that many mammals adjust the ratio of male and female young to the surrounding conditions at the time of conception but now growth factors including insulin like growth factor (IGF)-1 has been shown to promote embryonic development by decreasing apoptosis and increasing cell proliferation
A recent study in the group of Christine Aurich at the University of Veterinary Medicine, Vienna has provided important information on how the survival of female embryos may be enhanced under conditions that would otherwise tend to favour the birth of males.
The work is published in the journal Theriogenology.
Because of the process involved in the formation of sperm cells, there should be an equal chance that a mammalian egg will be fertilized by “male” sperm, carrying a Y chromosome, as by a “female” sperm, carrying an X chromosome. The symmetry of the system ensures that roughly the same number of males and females are born, which is clearly helpful for the species’ long-term survival.
Surprisingly, though, many mammals do not produce equal numbers of male and female offspring.
The discrepancy could theoretically be explained by the differential efficiencies of male and female sperm (Y chromosomes are smaller than X chromosomes so perhaps male sperm can swim faster?) or by different rates of survival of male and female foetuses in the uterus. Indeed, it does seem as though male embryos are better able to survive under conditions of high energy intake. But why?
Jana Beckelmann in Christine Aurich’s laboratory at the
University of Veterinary Medicine, Vienna now presents
provocative evidence that a particular protein,
insulin-like growth factor-1 or IGF1, might be involved.
After examining about 30 embryos, Beckelmann noticed
that in early pregnancy (between eight and twelve days
after fertilization) the level of messenger RNA encoding
IGF1 was approximately twice as high in
female embryos as in male embryos.
The difference could relate to the fact that female embryos
have two X chromosomes, which together might produce
more factor required for the expression of the IGF1gene
than the single X chromosome in males can generate.
Beckelmann was also able to confirm that the IGF1
protein was present in the embryos, confirming that the
messenger RNA is actually being translated into protein.
IGF1 is known to have important functions in growth and to inhibit apoptosis, or programmed cell death. As IGF1 treatment of cattle embryos produced in vivo improves their survival, it is likely that the factor has positive effects on the development of the early embryo in the horse. So why should female embryos contain more of the factor than males?
Losses in early pregnancy are unusually high in the horse and it is believed that female embryos are especially prone to spontaneous abortion. Male embryos are known to be better able to survive under high glucose concentrations, so well-nourished mares preferentially give birth to male foals.
“We think the higher IGF1 concentrations in female
embryos might represent a mechanism ensuring the
survival of female embryos under conditions that
would otherwise strongly favour males.”
If this is so, the ratio of the sexes in horses
is the result of a subtle interplay between
environmental and internal factors,
including insulin-like growth factor-1.
The paper "Sex-dependent insulin like growth factor-1 expression in preattachment equine embryos" by Jana Beckelmann, Sven Budik, Magdalena Helmreich, Franziska Palm, Ingrid Walter and Christine Aurich in the journal "Theriogenology" is available online and will appear in print in the issue of January 1, 2013 (Volume 79, Issue 1, 1 January 2013, pp. 193-199).
Original article: http://www.vetmeduni.ac.at/en/research/research-output/press-releases/foals-sex/