Dynamic changes of the microtubule system corresponding to the unequal and spiral cleavage modes in the embryo of the zebra mussel, Dreissena polymorpha (Mollusca, Bivalvia).

Unequal cleavage requires a highly organised cytoskeleton. We investigated the localisation of both tubulins and microtubular arrays in Dreissena eggs during and after fertilisation using confocal laser scanning microscopy. Freshly spawned eggs are arrested in metaphase I. A maternal pool of gamma-tubulin is found mainly in the centre of the asters of the meiotic spindle. The paternal pool of gamma-tubulin, present in the fertilising sperm, could not be traced within the egg, but a microtubule-organising centre forms near the male pronucleus at anaphase II. Male and female pronuclei grow as they migrate in the wake of their aster and rendezvous. First cleavage is unequal and starts without pronuclear fusion. At metaphase the two equal-sized asters span the entire egg in a symmetrical arrangement. At late metaphase the spindle shifts along its longitudinal axis into an eccentric position and the peripheral aster takes on an umbrella-like appearance, whereas the central aster remains spherical. The cleavage furrow becomes determined in the circumferential overlap of the asters. The inequality at second cleavage, however, is due to the unequal size of the asters. The third cleavage spindle also has asymmetrical asters and spindle shift was only observed in the D-cell. The spiral character is a result of an asymmetrical organisation of the larger, vegetal aster. Our results show that the arrangement of the gamma-tubulin clusters and of microtubules changes and develops during early development of Dreissena in a way that can explain the axis-generating asymmetries in cell pattern and the spiral sense of cleavage. The major cytological characters expected to direct pattern formation in this phase of development are: size, position, and symmetry or asymmetry of both spindle and asters.