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Nuclear behavior in the cells of Rhodosporidium toruloides, a heterobasidiomycetous yeast, was followed by microscopic observation. The behavior in the process of both asexual and sexual reproduction of the microorganism was found to agree, in principle, with the description on that of Leucosporidium sp., a heterobasidiomycetous yeast. In the artificially induced sexual differentiation in the cells of mating type a strain by A factor (a sex hormone secreted from the cells of mating type A strain), a complete repression of nuclear division occurs with the induction of mating tube formation. As elongation of the tube proceeds, a nucleus moves into the tube from the cell, followed by further migration to the apex of the tube. In the process of artificially induced dedifferentiation by removal of the factor from the incubation medium, the nucleus which was located in the mating tube by previous incubation with the factor recovers the ability to divide into two nuclei without formation of clamp connection. Then the septum formation occurs, followed by emergence of a bud. The bud grows and matures to a yeast cell.In the previous paper (1), we described the cell behavior as a morphological feature during the early stages of mating process of Rhodosporidium toruloides, which belongs to Heterobasidiomycetes and takes the yeast form of cell in the haploid phase, and demonstrated the presence of biologically active compounds, A and a factors, in the culture media of the cells of mating types A and a, respectively. These factors were characterized as sex hormones; A and a factors exclusively induce sexual differentiation in the cells of mating types a and A, respectively. A factor, which is constitutively formed, was partially purified and identified as a peptide (2).In this paper, we describe the cell behavior as the nuclear behavior during the life cycle of this microorganism, and the effect of A factor on cell morphology 175

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Phase-lengths in the cell division cycle of the basidiomycetous yeast Rhodosporidium toruloides Banno in an asynchronous culture at 27° were estimated by using three parameters : cell morphology, DNA synthesis, and nuclear behavior. In the yeast-extract-sucrose medium, one division cycle is 160 min, and phase-lengths of GI, S, G2, and M are calculated to be 116 min, 27 min, 10 min, and 7 min, respectively, by applying Powell's distribution function. As a dynamic phenomenon, the cell division cycle was presented in a cell clock.

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