Chromosomes and Nucleoli of the Axolotl, Ambystoma Mexicanum

Abstract

Amongst the axolotl's haploid complement of fourteen mitotic chromosomes, one of the four largest, with a greater arm asymmetry than the other three, shows a nucleolar constriction subterminally in its shorter arm. Low-temperature treatment causes further secondary constrictions to appear; these constrictions enable most of the mitotic chromosomes to be identified; the constrictions occur at similar sites in the chromosomes of tail-fin epithelial cells, hepatocytes, and brain cells. Homology between the mitotic and oocyte (lampbrush) nucleolar organizers has been established, and thus the several hundred free nucleoli in oocytes are genetically related to the two nucleoli of diploid somatic interphases. During oocyte development the free nucleoli transform from solid structures to rings and back to solid structures again without detectable increase in number. During the contraction and aggregation of the lampbrush chromosomes within the oocyte nucleus as maturity approaches, in most axolotls the free ring-shaped nucleoli become stretched between the nuclear periphery and central chromosome group, and take on a characteristic beaded appearance. These transformations of the free nucleoli are largely paralleled by forms which nucleoli attached subterminally to the shorter arm of lampbrush chromosome III concurrently assume. The question as to whether fully developed nucleoli detach from the organizer loci and add to the population of free nucleoli in oocytes remains undecided. It may well be that virtually all the DNA-generators of free nucleoli detach from the organizer loci before starting to carry out nucleolar functions, and before there is any significant accumulation of protein and RNA around them. If so, the variability in quantity of attached nucleolar material may not reflect different states in a nucleolar synthesis and detachment cycle, but rather variation in the number of nucleolar DNA Anlagen which happen to remain attached to the organizer loci after the synthesis and detachment of the great majority of the Anlagen has ceased. In occasional oocytes the only chromosomal continuity maintained across the organizer locus consists of a nucleolar ‘double bridge’; this indicates that the genetically persistent (i.e. chromosomal) organizer DNA bears the same structural relationship to neighbouring parts of a lampbrush chromosome as any other chromomere with its attendant pair of lateral loops. The lampbrush chromosomes of the axolotl have been provisionally mapped. The centromeres are represented by short portions of chromosome axis without lateral loops, and there are two spheres close to the centromeres of both chromosome VI and chromosome XIII. Other recognition characters are inconspicuous or not very reliable, and features of the lampbrush chromosomes related to the low-temperature induced secondary constrictions of mitotic chromosomes have not been identified.