The Relation between Number of Nucleoli and Number of Chromosome Sets in Animal Cells

Abstract

The relation between the number of nucleoli visible in telophase or interphase nuclei and the number of haploid chromosome sets which are present has been studied extensively in plants (cf. the recent review by Gates'). DeMol2 first noted that diploid hyacinths have a maximum of two nucleoli, triploid three, and hypotetraploid four. This relationship became understandable when was shown conclusively by Heitz3 that, during telophase, the nucleoli are formed at specific points on specific chromosomes, usually at secondary constrictions which either mark off a satellite or are located some distance from the end of the chromosome. Since as a rule, each haploid set of chromosomes includes a single chromosome with a organizer, the normal diploid nucleus contains two nucleoli. The number of nucleoli may therefore be used as a criterion for the identification of polyploid individuals or races within a species and, with certain restrictions, for the determination of phylogenetic relationships between different plant species. The relation of nucleolar number to polyploidy has received much less attention from zoologists, largely because, until recently, there was little interest in the study of polyploidy in animals in its various aspects. This has led Gates4 to the conclusion that it is still uncertain in how far the number of nucleoli in animals can be used as an index of the number of sets of chromosomes. However, what evidence there is available in the zoological literature clearly points to the existence of the same constant relationship as obtains in plants, at least in ordinary somatic cells. Under special physiological conditions, however, (e.g., in gland cells and in growing oocytes), the number of nucleoli may be greatly increased, apparently without a corresponding increase in chromosome number. The pertinent information on nucleolar number and heteroploidy will be reviewed briefly in this paper, and new evidence will be added from a study of the nucleoli in an extensive heteroploid series of larvae of the axolotl, Amblystoma mexicanum, and of larvae of mixed tigrinum-mexicanum ancestry. That each haploid set of chromosomes may be associated with a single nucleolus was first indicated by Conklin's classical observations on the fertilization and cleavage of eggs of the gastropod Crepidula.5 Each of the germ nuclei, before their union, contains a single nucleolus, while in the telophase of all the cleavage mitoses two nucleoli appear which may