Apparently (or so I read in a recent issue of Nature), eggs do this to prevent the embryo having mixed populations of mitochondria. Why would this mix be bad, you ask? Well, mt have genes too, so perhaps it would be detrimental to have organelle diploidy(or has been, for those cells without systems to do this).
How do they do this? Well, the sperm mitochondria are tagged with ubiquitin (by the male, in case you were getting all annoyed) and this triggers the protease pathways in the egg. To do this, the sperm mitochondria have to be protected from destruction in the testes, so that they can power their DNA warhead straight to the point of impact.
Incidentally, there is no truth in the rumour that human sperm do not have mitochondria due to being too small. It takes an awful lot of energy to swim such huge distances (relative to its size!). Also, the tail is apparently not cast off when it enters, as has also been claimed.
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