TRANSIENT LOCALIZATION OF γ‐TUBULIN AROUND THE CENTRIOLES IN THE NUCLEAR DIVISION OF BOERGESENIA FORBESII (SIPHONOCLADALES, CHLOROPHYTA)

Mitosis in Boergesenia forbesii (Harvey) Feldman was studied by immunofluorescence microscopy using anti‐β–tubulin, anti‐γ–tubulin, and anti‐centrin antibodies. In the interphase nucleus, one, two, or rarely three anti‐centrin staining spots were located around the nucleus, indicating the existence of centrioles. Microtubules (MTs) elongated randomly from the circumference of the nuclear envelope, but distinct microtubule organizing centers could not be observed. In prophase, MTs located around the interphase nuclei became fragmented and eventually disappeared. Instead, numerous MTs elongated along the nuclear envelope from the discrete anti‐centrin staining spots. Anti‐centrin staining spots duplicated and migrated to the two mitotic poles. γ–Tubulin was not detected at the centrioles during interphase but began to localize there from prophase onward. The mitotic spindle in B. forbesii was a typical closed type, the nuclear envelope remaining intact during nuclear division. From late prophase, accompanying the chromosome condensation, spindle MTs could be observed within the nuclear envelope. A bipolar mitotic spindle was formed at metaphase, when the most intense staining of γ‐tubulin around the centrioles could also be seen. Both spindle MT poles were formed inside the nuclear envelope, independent of the position of the centrioles outside. In early anaphase, MTs between separating daughter chromosomes were not detected. Afterward, characteristic interzonal spindle MTs developed and separated both sets of the daughter chromosomes. From late anaphase to telophase, γ‐tubulin could not be detected around the centrioles and MT radiation from the centrioles became diminished at both poles. γ‐Tubulin was not detected at the ends of the interzonal spindle fibers. When MTs were depolymerized with amiprophos methyl during mitosis, γ‐tubulin localization around the centrioles was clearly confirmed. Moreover, an influx of tubulin molecules into the nucleus for the mitotic spindle occurred at chromosome condensation in mitosis.