Pre and Post-Zygotic Infertility:
When the experts talk about infertility they speak of pre-zygotic and post-zygotic mechanisms. A zygote means a yoke of oxen, but in this case it means the fertilized egg. So a problem with fertility can arise before the sperm reaches the egg or it can occur after the offspring has emerged. For instance if a female fruit fly rejects a suitor (and they do sometimes) that would be pre-zygotic and if a mule fails to mature into a reproductively competent adult (which is usual) that would be post-zygotic.
It seems very clear and unambiguous, but in my mind it seems like one generation’s post-zygotic mechanism could be the next generation’s pre-zygotic. And if you take a perfectly normal sperm which is unable to produce an offspring after meeting a perfectly normal egg from the same species for the simple fact that it is the wrong sperm for that egg, is it pre-zygotic or post-zygotic?
But the terms seem useful. So what are we talking about here?
I have pointed out recently that deer mouse sperm if they are kin can form a flying wedge, the better to make their way to the egg. They are unlikely to do this with non kin. Since sperm can thus recognize kindred sperm, it is no stretch to think they can recognize kindred egg. The sperm reaches the egg, does not attach or at least does not perform some critical step and there is no zygote. That means it is pre-zygotic pure and simple, end of story, nothing left to do but run down the mechanism.
But there is a catch. As I have mentioned before, male development the world over is in decline. Sperm counts are lower. Organ sizes are smaller. Partially feminized anatomy is on the rise. That would be post-zygotic.
So I think one can definitively say it is either or both or the term does not strictly apply.
Anyway, it’s something.
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