The first spindle formation in brown algal zygotes

Regulation of the first spindle formation in brown algal zygotes was described. It is well known that there are three types of sexual reproduction in brown algae; isogamy, anisogamy and oogamy. Paternal inheritance of centrioles can be observed in all these cases, similar to animal fertilization. In isogamy and anisogamy, female centrioles (= flagellar basal bodies) selectively disappear and male centrioles remain after fertilization. In a typical oogamy (e.g. fucoid members), liberated egg does not have centrioles, and sperm centrioles are introduced in zygote. Participation of sperm centrioles to the spindle formation in zygotes was also described using Fucus distichus as a model system. Sperm centrioles function as a part of centrosome, namely microtubule organizing center, in zygote. Therefore, they have a crucial role in the spindle formation. Observations on the spindle formation in polygyny and karyogamy-blocked zygotes strongly suggest that egg nucleus can form a mitotic spindle by itself without centrosome, even though the resulting spindles are of abnormal shapes.