125in the embryo and multiple mitotic spindles each induce their own cleavage furrow.
This condition is lethal for the embryo. However, in most urodeles (newts and sala-
manders), polyspermy is the natural mode of fertilization (Fankhauser 1932 ).
Although each sperm gives rise to its own sperm-aster, only the one sperm-aster that
reaches the female pronucleus becomes dominant to span large parts of the cell. The
other sperm-asters disintegrate. It seems necessary that the DNA of the supernumer-
ary asters must also somehow be destroyed later on to prevent polyploidy of subsec-
tions of the embryo. The molecular mechanisms underlying this fascinating
phenomenon are not understood.
In the zebrafish, the sperm-aster has also been shown to facilitate the multimer-
ization of maternally inherited ribonucleoparticles (RNPs) that confer the germ cell
fate, termed the germplasm. This appears to be achieved by the action of sperm-
aster ends on an actin-based network associated with these RNPs, where the radial
growth of sperm-aster microtubules generates a wave of RNP aggregation (Theusch
et al. 2006 ; Nair et al. 2013 ), in anticipation of their accumulation at the furrows
produced during the early cell divisions (Eno and Pelegri 2013 ; reviewed in Eno and
Pelegri 2016 ). A similar function for the sperm-aster in germplasm RNP multimer-
ization in other model systems has not yet been shown, although in some systems,
such as in frogs and chicken, germplasm RNPs exhibit patterns of accumulation at
the furrows that are similar to those occurring in the zebrafish (see Chap. 8 ).
4.3.2.2 Nuclear–Cytoskeletal Attachment During Nuclear Fusion
In the zebrafish, transport of the paternal pronucleus along the sperm-aster has also
been shown to require the function of another maternal-effect gene, futile cycle/
lrmp (Lindeman and Pelegri 2012 ). This gene encodes a KASH-domain protein
with gene products localized to the nuclear envelope of maternal and paternal pro-
nuclei. This protein is thought to mediate a link between the outer nuclear mem-
brane and the microtubule cytoskeleton. Interestingly, the futile cycle/lrmp-dependent
nuclear–microtubule connection is also required to maintain the close attachment of
the paternal pronucleus to the centrosome at the sperm-aster MTOC (Lindeman and
Pelegri 2012 ). Thus, attachment of the outer nuclear membrane to the cytoskeleton
is essential for both long-range transport of the maternal pronucleus toward the
MTOC and local attachment of the paternal pronucleus to the MTOC. This coordi-
nated set of processes drives the movement of the maternal pronucleus toward the
MTOC while keeping the paternal pronucleus close to this structure, thus mediating
the encounter of both pronuclei leading toward their fusion.
Nuclear envelope–cytoskeletal interaction likely continues throughout early
embryonic blastomere divisions, as suggested by the highly localized pattern for
futile cycle/lrmp proteins at the nuclear membrane–centrosome interphase
(Lindeman and Pelegri 2012 ). This nuclear–centrosomal linkage, coupled to the
centering of the (aster-containing) spindle apparatus, guarantees the even distribu-
tion of chromatin content among the newly formed blastomeres.
4 Vertebrate Embryonic Cleavage Pattern Determination