Abnormal Nucleolar Cycle in Microsporogenesis of Brachiaria decumbens(Gramineae).

Abstract

Cytogenetic studies carried out on the tetraploid (2n=4x=36) accession of Brachiaria decumbens (BRA007722) revealed an unusual pattern of the nucleolar cycle during microsporogenesis. Nucleolus behavior was normal during prophase and disappeared at the end of diakinesis. After dissolution, it was fractioned into multiple micronucleoli of different sizes at metaphase which persisted during early anaphase. Some entered in a fusion process in late anaphase: they reduced their number and increased in size. Since fusion process continued throughout the cycle, in late telophase all were rejoined into a unique nucleolus in the sister nuclei. They persisted in this condition until the end of prophase II and then disappeared once more. Such nucleolar behavior was also reported in the second division, while at the end of the meiotic process tetrads had 4 microspores with a normal nucleolus. Pollen viability was not affected by this anomalous nucleolar cycle behavior. A mutation affecting some steps of the nucleolar cycle has been suggested.