Influence of centriole behavior on the first spindle formation in zygotes of the brown alga Fucus distichus (Fucales, Phaeophyceae)

The influence of centrioles, derived from the sperm flagellar basal bodies, and the centrosomal material (MTOCs) on spindle formation in the brown alga Fucus distichus (oogamous) was studied by immunofluorescence microscopy using anti-centrin and anti-beta-tubulin antibodies. In contrast to a bipolar spindle, which is formed after normal fertilization, a multipolar spindle was formed in polyspermic zygote. The number of mitotic poles in polyspermic zygotes was double the number of sperm involved in fertilization. As an anti-centrin staining spot (centrioles) was located at these poles, the multipolar spindles in polyspermic zygotes were produced by the supplementary centrioles. When anucleate egg fragments were fertilized, chromosome condensation and mitosis did not occur in the sperm nucleus. Two anti-centrin staining spots could be detected, microtubules (MTs) radiated from nearby, but the mitotic spindle was never produced. When a single sperm fertilized multinucleate eggs (polygyny), abnormal spindles were also observed. In addition to two mitotic poles containing anti-centrin staining spots, extra mitotic poles without anti-centrin staining spots were also formed, and as a result multipolar spindles were formed. When karyogamy was blocked with colchicine, it became clear that the egg nucleus proceeded independently into mitosis accompanying chromosome condensation. A monoastral spindle could be frequently observed, and in rare cases a barrel-shaped spindle was formed. However, when a sperm nucleus was located near an egg nucleus, the two anti-centrin staining spots shifted to the egg nucleus from the sperm nucleus. In this case, a normal spindle was formed, the egg chromosomes arranged at the equator, and the associated MTs elongated from one pole of the egg spindle toward the sperm chromosomes which were scattered. From these results, it became clear that paternal centrioles derived from the sperm have a crucial role in spindle formation in the brown algae, such as they do during animal fertilization. However, paternal centrioles were not adequate for the functional centrosome during spindle formation. We speculated that centrosomal materials from the egg cytoplasm aggregate around the sperm centrioles and are needed for centrosomal activation.