Competence for nuclear replication and the NOR-chromosomes of Allium cepa L

Autotetraploid (4n = 32) cells were induced in Allium cepa L. root meristems by successively treating with a multipolarizing agent in anaphase (carbetamide) and an inhibitor of cell plate formation in telophase (caffeine). This treatment produced cells with their 32 chromosomes distributed in more than two nuclei. During G1, one of the nuclei in the resulting trinucleate cells had a DNA content which was equivalent to an average of 16 chromosomes, while the other 16 chromosomes were randomly distributed in two aneuploid nuclei. In the set of 16 chromosomes forming the onion diploid complement, there are 4 NOR-chromosomes and 5 chromosomes carrying DNA domains providing a nucleus with the competence to replicate, as previously shown. Expected probabilities derived from the different possible models for cosegregation of both kinds of chromosomes in the two aneuploid nuclei of the trinucleate cells were estimated by a computer simulation. These expected values were compared with the recorded frequencies of aneuploid nuclei which were able to organize a nucleolus and to respond to inducers of replication. The present data are compatible with the existence of sequences providing a nucleus with the competence to replicate in three out of the four NOR-bearing chromosomes, as well as in two other chromosomes of this diploid complement. The scarcity of chromosomes bearing early origins able to initiate replication in a nucleus is a striking feature of this huge onion genome.