Transient localization of gamma-tubulin around the centrioles in the nuclear division of Boergesenia forbesii (Siphonocladales, Chlorophyta)

Mitosis in Boergesenia forbesii (Harvey) Feldman was studied by immunofluor escence microscopy using anti-beta -tubulin, anti-gamma -tubulin, and anti- centrin antibodies. In the interphase nucleus, one, two, or rarely three an ti-centrin staining spots were located around the nucleus, indicating the e xistence of centrioles. Microtubules (MTs) elongated randomly from the circ umference of the nuclear envelope, but distinct microtubule organizing cent ers could not be observed. In prophase, MTs located around the interphase n uclei became fragmented and eventually disappeared. Instead, numerous MTs e longated along the nuclear envelope from the discrete anti-centrin staining spots. Anti-centrin staining spots duplicated and migrated to the two mito tic poles. gamma -Tubulin was not detected at the centrioles during interph ase but began to localize there from prophase onward. The mitotic spindle i n B. forbesii was a typical closed type, the nuclear envelope remaining int act during nuclear division. From late prophase, accompanying the chromosom e condensation, spindle MTs could be observed within the nuclear envelope. A bipolar mitotic spindle was formed at metaphase, when the most intense st aining of gamma -tubulin around the centrioles could also be seen. Both spi ndle MT poles were formed inside the nuclear envelope, independent of the p osition of the centrioles outside. In early anaphase, MTs between separatin g daughter chromosomes were not detected. Afterward, characteristic interzo nal spindle MTs developed and separated both sets of the daughter chromosom es. From late anaphase to telophase, gamma -tubulin could not be detected a round the centrioles and MT radiation from the centrioles became diminished at both poles. gamma -Tubulin was not detected at the ends of the interzon al spindle fibers. When MTs were depolymerized with amiprophos methyl durin g mitosis, gamma -tubulin localization around the centrioles was clearly co nfirmed. Moreover, an influx of tubulin molecules into the nucleus for the mitotic spindle occurred at chromosome condensation in mitosis.