Spatio-temporal reorganization of intracytoplasmic microtubules is associated with nuclear selection and differentiation during the developmental process in the ciliateTetrahymena thermophila

Abstract

Indirect immunofluorescence has revealed various intracytoplasmic microtubular structures, which are transiently polymerized in specific subcellular locations during the developmental process of conjugation in the ciliateTetrahymena thermophila. These structures include: (1) micronuclear spindles, (2) perimicronuclear microtubules, (3) microtubular baskets surrounding migrating pronuclei, and (4) microtubules interconnecting the pronuclei with the conjugants' junctional zone. Furthermore, a peripheral network of intracytoplasmic microtubules related to the cell cortex is present in both vegetative cells and in conjugants. Comparative observations made on cells undergoing normal conjugation and defective conjugation (occurring either spontaneously or induced by taxol) has revealed some rules governing the pattern of deployment of conjugation-specific microtubules. The presence of perinuclear microtubular arrays during early postmeiotic stages of development is strictly limited to more anteriorly located nuclei which includes the selected haploid nucleus that further divides to form the stationary and migratory pronuclei. These perinuclear microtubules may be involved in the positional control of nuclear fates leading to effective nuclear selection. Microtubular bundles associated with pronuclei and connecting the junctional zone are only formed in the presence of functional pronuclei, and may be involved in the guidance of pronuclei leading to their fusion. The mechanism of cytoplasmic control of nuclear differentiation of derivatives of the zygotic nucleus appear to be associated with a coordinate action of two microtubular arrays: spindle microtubules of the second postzygotic division and the peripheral intracytoplasmic network of microtubules, leading to a proper subcortical positioning of the postzygotic nuclei at opposite poles of the cell.