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having large eggs with external fertilization. The Japanese giant salamander,
Andrias japonicus, has eggs as large as 5–8 mm in diameter. The eggs are insemi-
nated following oviposition, after which they undergo polyspermic fertilization
(Iwao 2000 ). It is possible that polyspermic fertilization became necessary to ensure
fertilization of these large eggs because a single sperm did not contain sufficient
amount of sperm factor that could generate a Ca2+ elevation high enough to induce
activation. Multiple sperm were needed to trigger several Ca2+ transients. The large
volume of the egg came with a bonus though: the extra space in the ooplasm may
help to eliminate the accessory sperm nuclei to prevent polyploidy (Iwao 2012 ). The
platypus Ornithorhynchus anatinus, a primitive mammal, lays large yolky eggs of
about 4 mm in diameter (Hughes and Hall 1998 ). The eggs are polyspermic and
several sperm may enter the blastodisk at fertilization (Gatenby and Hill 1924 ).
Another primitive mammal, the marsupial Sminthopsis crassicaudata, has rela-
tively small eggs (about 120 μm in diameter) that contain a yolk mass in the center,
and at fertilization some eggs show polyspermy (Breed and Leigh 1990 ). It seems
that the decrease in egg size and yolk content is associated with the switch in the
mode of fertilization, from polyspermy to monospermy. The sperm-induced Ca2+
changes have not been studied in these primitive mammals, but it is possible that the
ancestor of mammals exhibited polyspermic fertilization that was necessary for
multiple Ca2+ transients and also, for egg activation. Higher eutherians have small
eggs without yolk in the ooplasm, but the need for the repetitive Ca2+ signal remained
for proper activation. The Ca2+ oscillations in the relatively small eutherian eggs
may function in place of the multiple Ca2+ rises induced by more than one sperm in
the ancestral polyspermic eggs.
1.3.6 Resumption of Meiosis
The most important function of the sperm-induced Ca2+ signal is the stimulation of
meiotic resumption. As mentioned above, vertebrate eggs are ovulated while
arrested at metaphase of the second meiotic division. The cell cycle arrest is main-
tained by high MPF activity that is stabilized by CSF (the Mos/MEK/MAPK/p90Rsk
cascade). This is achieved via the phosphorylation of Emi by p90Rsk. Phosphorylated
Emi is bound by the protein phosphatase PP2A, which causes dephosphorylation of
Emi and its interaction with APC, leading to APC inhibition. Cyclin B1 is thus pro-
tected from degradation, resulting in high CDK1 (and thus MPF) activity. This
keeps the chromosomes in a condensed state and stabilizes the meiotic spindle
(Whitaker 1996 ). At this point the cell cycle can proceed only if CDK1 activity
decreases. CSF is sensitive to Ca2+ (Meyerhof and Masui 1977 ), and it would seem
logical that the fertilizing sperm causes the resumption of meiosis by reducing Mos
activity. However, this is not the case. In both Xenopus and mouse eggs, the disap-
pearance of Mos begins 30–45 min after the initiation of the sperm-induced Ca2+
signal, whereas MPF inactivation occurs about 15 min earlier (Lorca et al. 1991 ). It
seems that Mos degradation is not the cause but rather the consequence of the
release from the meiotic arrest. The fertilizing sperm bypasses CSF and acts instead
Z. Machaty et al.