The most common type of nucleolar behaviour is the "dispersive nucleolus," i.e., the nucleolus dispersing during prophase and reappearing at telophase. In some organisms, Risueño and Medina (Rev. Biol. Cel. 7:1-162, 1976) described a "semipersistent nucleolus" in mitosis. In this case, the nucleolar material remains in the cytoplasm during anaphase and telophase. In most organisms the nucleolus at meiosis is not a visible organelle from diakinesis onwards. The analysis of the meiotic behaviour of individuals belonging to several Bolivian races of maize reveals that the nucleolus initiated the dispersion at prometaphase I but persisted as a discrete structure during metaphase I and anaphase I. However, its size decreased with the progression of meiosis (Fig. 1A). At metaphase the nucleolus was usually located in one of the poles (Fig. 1A). At the end of anaphase I prenucleolar bodies appeared and the nucleolar reorganization occurred in the normal way. The nucleolar behaviour during meiosis II was normal.
This phenomenon was observed in all the studied anthers of some individuals of the races Blando Cruceño and Tuimurú. In other individuals of these and other races (Pisankalla, Jampe Tongo and Duro Amazónico) the "semipersistent nucleolus" was detected only in some of the anthers.
To study this phenomenon young anthers were fixed in 3 parts of absolute alcohol: 1 glacial acetic acid. The squash of anthers was made in a drop of acetic haematoxylin. Feulgen staining was done as indicated in Tito et al. (TAG 83:58-64, 1991) and nucleolar silver staining was carried out according to the technique used by Lacadena et al. (1984).
The absence of DNA in the "semipersistent nucleolus" was indicated by its failure to stain after Feulgen treatment. The presence of proteins was indicated by its positive silver staining (Fig. 1B). These results support the nucleolar nature of this body, although it was not possible to discriminate whether the persistent material belong to the granular or fibrillar component.
A structure called "nucleolar body" was described in several accessions of Bromus and Zea (Walters). This would not be the case in the individuals here studied because in most of the cells only one normal nucleolus was observed until diakinesis and additional nucleolar-like bodies were not detected. The observation of more than 50 cells suggests that the dispersion of the nucleolar material, in this case, is very slow and persists during meiosis I.
In Bolivian races the presence of B chromosomes is frequent. It is interesting to point out that the atypical behaviour of the nucleolus described is not related to the presence of B chromosomes. From a total of 36 studied individuals (growing in the same conditions), 8 presented "semipersistent nucleolus" (6 without B's, 1 with one B and 1 with 2 B's). In general there are no differences in the nucleolar behaviour among individuals with and without B's.
The "semipersistent nucleolus" was also observed in several individuals of alloplasmic lines (lines of maize with cytoplasm of Zea mays ssp. mexicana, Poggio et al., in prep.).
1. Metaphases I, with semipersistent nucleolus. A) Haematoxylin stain;
B) Silver stain. Bar = 10 µm.
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