Ultrastructural aspects of tetrads with micronuclei in a wheat monosomic

Meiosis of monosomics produces tetrad cells with micronuclei. These micronuclei consist exclusively of one chromosome, the critical chromosome. The electron microscope study of these cells reveals that, in the main nucleus the chromatin is relatively uncondensed with large numbers of perichromatin structures and the nucleolus is almost exclusively fibrillar. This corresponds with a phase of active extranucleolar synthesis and moderate nucleolar activity as reported by other authors using biochemical techniques. The chromatin of the micronucleus is in a more condensed stage than in the main nucleus which, in our opinion, is due to a later beginning of interphase. A nuclear envelope surrounds the micronucleus. In its interior, a patch of nucleolar material can be seen. From the available data on the phylogenetic evolution of this special chromosome which constitutes the micronucleus, we suggest that there must be a partial expression of the nucleolar genes carried by this chromosome due to the absence of the factors which impair their expression in disomic plants.