Why is it so hard for humans to have a baby? A new study by a researcher at the Milner Center for Evolution at the University of Bath suggests that 'selfish chromosomes' are to blame for the early demise of the majority of human embryos. The research, which was published in PLoS Biology, explains w
Why is it so hard for humans to have a baby?suggests that “selfish chromosomes” are to blame for the early demise of the majority of human embryos. The research, which was published in, explains why human embryos often do not survive while fish embryos are fine. The finding also has implications for the treatment of infertility.
The immediate cause of many of these early deaths is that the embryos have the wrong number of chromosomes. Fertilized eggs should have 46 chromosomes, 23 from mom in the eggs, 23 from dad in the sperm. What Hurst noticed was that, in mammals, a selfish mutation that tries to do this but fails, resulting in an egg with one too many or one too few chromosomes, can still be evolutionarily better off. In mammals, because the mother continuously feeds the developing fetus in the womb, it is evolutionarily beneficial for embryos developing from faulty eggs to be lost earlier rather than be carried to full term. This means that the surviving offspring do better than the average.
“Fish and amphibians don’t have this problem,” Hurst commented. “In over 2000 fish embryos not one was found with chromosomal errors from mom.” Rates in birds are also very low, about 1/25th the rate in mammals. This, Hurst notes, is as predicted as there is some competition between nestlings after they hatch, but not before.
Preliminary data shows mammals such as cows, with one embryo at a time seem to have especially high embryo death rates owing to chromosomal errors, while those with many embryos in a brood, like mice and pigs, seem to have somewhat lower rates.
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