COMPETENCE FOR NUCLEAR REPLICATION AND THE NOR-CHROMOSOMES OF ALLIUM-CEPA L

Autotetraploid (4n = 32) cells were induced in Allium cepa L. root mer istems by successively treating with a multipolarizing agent in anapha se (carbetamide) and an inhibitor of cell plate formation in telophase (caffeine). This treatment produced cells with their 32 chromo somes distributed in more than two nuclei. During G(1), one of the nuclei in the resulting trinucleate cells had a DNA content which was equivalen t to an average of 16 chromosomes, while the other 16 chromosomes were randomly distributed in two aneuploid nuclei. In the set of 16 chromo somes forming the onion diploid complement, there are 4 NOR-chromosome s and 5 chromosomes carrying DNA domains providing a nucleus with the competence to replicate, as previously shown. Expected probabilities d erived from the different possible models for cosegregation of both ki nds of chromosomes in the two aneuploid nuclei of the trinucleate cell s were estimated by a computer simulation. These expected values were compared,vith 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 prov iding a nucleus with the competence to replicate in three out of the f our NOR-bearing chromosomes, as well as in two other chromosomes of th is diploid complement. The scarcity of chromosomes bearing early origi ns able to initiate replication in a nucleus is a striking feature of this huge onion genome.