Polyspermy
In biology one calls polyspermy the fertilization of a Ovule by more Spermatozoïde. The organizations diploïdes normally contain two copies of each chromosome, one of each relative. The cell generated by polyspermy contains on the contrary three copies of each Chromosome or more - one of the ovule and one of each multiple spermatozoon. Usually, the zygote which results from it is not viable.
Worked out protections against the polyspermy
The ovules of organizations to sexuée reproduction are adapted to avoid such a situation. These defenses are very characteristic in the marine sea urchin, which after the acceptance of a spermatozoon prevents that another succeeds in penetrating the ovule. Similar defenses exist at other eucaryotes.The prevention of the polyspermy in the sea urchins of sea rests on a change in the electric charge through the surface of the ovule, which is caused by fusion with the ovule of the first spermatozoon. The ovules of not fertilized sea urchins of sea have a negative charge, but this one becomes positive after fecundation. When the spermatozoon of sea urchin of sea meets an ovule with positive load, fusion spermatozoon-ovule is blocked. Thus, after the first spermatozoon came into contact with and caused the change, the spermatozoa which will follow will be prevented from amalgamating. It is thought that this “electric blocking of the polyspermy” comes owing to the fact that a molecule positively charged in the surface membrane with the spermatozoon is pushed back by the positive load on the surface of the ovule.
Electric blockings of the polyspermy take place in many animal species, including frogs, the marine clams and worms, but not in the various mammals which were studied (hamster, rabbit, mouse). In the species without electric blocking, the polyspermy is usually prevented by the secretion of a substance which establishes a mechanical barrier against it. The animals as the sea urchins of sea have a strategy of prevention of the polyspermy in two stages, with a fast but transitory electric blocking, replaced after the first minute or about by a permanent but slower mechanical blocking to develop. One believes that electric blockings are the result of the evolution at the species where it is necessary to block the polyspermy very quickly, because of the presence of a great number of spermatozoa which arrive at the same time at the surface of the ovule, like that occurs in the animals like the sea urchins of sea. On their premises, fecundation takes place in an external way in the ocean, so that of the hundreds of spermatozoa can come into contact with the ovule in several seconds. In the mammals, where fecundation is internal, more a small number of spermatozoon reaches the site of fecundation in the oviduct.
An additional blocking against the polyspermy also exists, which seems to be an additional degradation of nuclei in the zygote lately formed after the first fusion of the cores, but the mechanism of this blocking is not yet known.
Sources
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