There are six micronuclear divisions during conjugation of Paramecium caudatum
: three prezygotic and three postzygotic
divisions. Four haploid nuclei are formed during the first two meiotic prezygotic divisions. Usually only one meiotic product is
located in the paroral cone (PC) region at the completion of meiosis, which survives and divides mitotically to complete the third
prezygotic division to yield a stationary and a migratory pronucleus. The remaining three located outside of the PC degenerate. The
migratory pronuclei are then exchanged between two conjugants and fuse with the stationary pronuclei to form synkarya, which
undergo three successive divisions (postzygotic divisions). However, little is known about the surviving mechanism of the PC nuclei.
In the current study, stage-specific appearance of cytoplasmic microtubules (cMTs) was indicated during the third prezygotic division
by immunofluorescence labeling with anti-alpha tubulin antibodies surrounding the surviving nuclei, including the PC nuclei and the
two types of prospective pronuclei. This suggested that cMTs were involved in the formation of a physical barrier, whose function
may relate to sequestering and protecting the surviving nuclei from the major cytoplasm, where degeneration of extra-meiotic
products occurs, another important nuclear event during the third prezygotic division.