The Questions of Developmental Biology

Figure 7.20
The prevention of polyspermy
As soon as one sperm has entered the egg, the fusibility of the
egg membrane, which was so necessary to get the sperm inside
the egg, becomes a dangerous liability. In sea urchins, as in most
animals studied, any sperm that enters the egg can provide a haploid
nucleus and a centriole to the egg. In normal monospermy, in
which only one sperm enters the egg, a haploid sperm nucleus and a
haploid egg nucleus combine to form the diploid nucleus of the
fertilized egg (zygote), thus restoring the chromosome number
appropriate for the species. The centriole, which is provided by the
sperm, will divide to form the two poles of the mitotic spindle during
cleavage.
The entrance of multiple sperm polyspermy leads to
disastrous consequences in most organisms. In the sea urchin, fertilization by two sperm results in
a triploid nucleus, in which each chromosome is represented three times rather than twice. Worse,
since each sperm's centriole divides to form the two poles of a mitotic apparatus, instead of a
bipolar mitotic spindle separating the chromosomes into two cells, the triploid chromosomes may
be divided into as many as four cells. Because there is no mechanism to ensure that each of the
four cells receives the proper number and type of chromosomes, the chromosomes would be
apportioned unequally. Some cells receive extra copies of certain chromosomes and other cells
lack them. Theodor Boveri demonstrated in 1902 that such cells either die or develop abnormally
(Figure 7.21).
Species have evolved ways to prevent the union of more than two haploid nuclei. The
most common way is to prevent the entry of more than one sperm into the egg. The sea urchin
egg has two mechanisms to avoid polyspermy: a fast reaction, accomplished by an electric
change in the egg plasma membrane, and a slower reaction, caused by the exocytosis of the
cortical granules (Just 1919).