NUCLEOLAR CYCLE AND LOCALIZATION OF NORS IN EARLY EMBRYOS OF PARASCARIS UNIVALENS

During early embryogenesis of the nematode Parascaris univalens (2n=2) the processes of chromatin diminution and segregation of the germ and somatic cell lineages take place simultaneously. In this study we ana lyzed the nucleolar cycle in early embryos, both in germinal and somat ic blastomeres, by means of silver staining and antibodies against the nucleolar protein fibrillarin. We observed an identical nucleolar cyc le in both types of blastomeres, hence, the chromatin diminution proce ss has no effect on the nucleolar cycle of somatic blastomeres. We rep ort the existence of outstanding differences between this cycle and th ose previously reported during early embryogenesis of other species. T here is a true nucleolar cycle in early embryos that shows a peculiar nucleolar disorganization at prophase, and a preferential localization of prenucleolar bodies only on the euchromatic regions during nucleol ogenesis. Moreover, fibrillarin does not form a perichromosomal sheath in metaphase or anaphase holocentric chromosomes, probably owing to t heir special centromeric organization. The number and location of nucl eolus organizer regions (NORs) in the chromosomal complement have been determined using silver impregnation, chromomycin A(3)/distamycin A s taining, and fluorescent in situ hybridization using an rDNA probe. Th ere are only two NORs, one per chromosome, and these are located in th e middle of the euchromatic central regions. This location implies tha t no rDNA sequences are lost in blastomeres after chromatin diminution . Moreover, the constant presence of two nucleoli in somatic blastomer es suggests that NORs are not affected during the fragmentation of euc hromatic regions when this process occurs.