OOGENESIS IN HABROTROCHA TRIDENS (MILNE) W. SIANG HSU Zoology Department, University of Washington, Seattle, Washington The bdelloid rotifers of about 200 species, classified into 19 genera and 4 families, reproduce exclusively by parthenogenesis, males being unknown in this group. It is therefore interesting to study the behavior of their chromosomes during oogenesis. I have reported such a study on one of them, Philodina roseola (1956). Some of the findings reported in that paper are as follows: 1. In Philo-dina roseola, there are two maturation divisions, both equational. 2. No indica-tion of synapsis has been observed between any two of the chromosomes. Indi-vidual chromosomes even in the earliest oocytes were observed to be in a con-densed state. The anaphase chromosomes of the oogonial division do not de-spiralize in forming the nuclei found in the syncytial ovary. The chromosomes, after the last oogonial division, remain condensed, and, by progressive packing together, they form first a ring and then a homogeneous and spherical mass of chromatin occupying the center of the nucleus. When one of these nuclei is iso-lated by the ovary to form an oocyte, its condensed chromosomes do not despira-lize into leptotene threads, but persist in a condensed state. As the germinal vesicle increases in size they separate from each other until finally 13 condensed chromosomes can be easily counted. 3. The zygoid chromosome number in this rotifer is 13. Three of the 13 chromosomes, two dot-shaped ones and one that is appreciably longer than the rest, are morphologically distinguishable from one another and from any one of the other ten (Fig. 37). It was suggested that the chromosomes in this group of animals may have lost their homology. The present paper reports observations made on Habrotrocha tridcns, which belongs to the family of Habrotrochidae. For methods employed in this study reference may be made to my paper dealing with Philodina roseola. OBSERVATIONS As in Philodina roseola, the ovary and its accessory structure, the vitellarium, in Habrotrocha tridcns are syncytial. In mature animals, the ovary consists of about 30 nuclei, with the chromosomes in each nucleus grouped so tightly together that they form a single spherical body of smooth outline. When one of the nuclei is isolated by the ovary to form an oocyte, the individual chromosomes do not go through the meiotic changes characteristic of oocytes in other animals. They sim-ply remove themselves from each other as contracted bodies while the nucleus in-creases in size. Figure 1 illustrates the condition of the chromosomes in the nucleus of a young oocyte. At this stage, it is still difficult to differentiate the individual chromosomes. But as the nucleus enlarges, the chromosomes begin to stand out clearly as con-densed bodies. If the whole history of the chromosomes in the developing egg is 364
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