Spindle formation in karyogamy-blocked zygotes of the isogamous brown algaScytosiphon lomentaria (Scytosiphonales, Phaeophyceae)

The first spindle formation in karyogamy-blocked zygotes in the isogamous b rown alga Scytosiphon lomentaria was studied by electron and immunofluoresc ence microscopy using anti-centrin and anti-beta -tubulin antibodies. Zygot es blocked in karyogamy by incubation at high temperature (22 degreesC) wer e compared with normal zygotes incubated at 14 degreesC. In normal zygotes, after the disappearance of female gamete centrioles. centrioles from the m ale gamete duplicated and migrated to both poles in the metaphase, and a bi polar spindle was formed. The nuclear envelope was almost intact except at the poles. In karyogamy-blocked zygotes of S. lomentaria, both DNA synthesi s and chromosomal condensation occurred in the male and female nuclei. A bi polar mitotic spindle with a pair of centrioles at each of two poles could be observed in the male nucleus, but not in the female nucleus. In some cas es, after a pair of centrioles derived from the male gamete duplicated and separated, one of the two pairs migrated towards the female nucleus. In thi s case, a monopolar spindle was formed independently in each of the female and male nuclei, In addition to the monopolar spindle, a bipolar spindle wa s formed lying across the unfused male and female nuclei. Male and female c hromosomes were intermixed and some of them were incorporated into the spin dle. After cytokinesis of the normal and karyogamy-blocked zygotes, two dau ghter cells, each with one nucleus and one chloroplast, were formed.