The regions of special cycle of division of Agapanthus chromosomes

Summary

• (1) In the chromosomes ofAgapanthus umbellatus the kinetochore can be seen to be divided as early as pro-metaphase II. At this stage, the two longitudinal halves of the kinetochore are individualized and separated from each other. The region of the chromosome body that appears to be responsible for holding together the two sister chromatids of each chromosome until the beginning of anaphase II is the most proximal region of the arms and not the kinetochore.

• (2) At the second division of meiosis, three regions with distinct cycles of division may be distinguished in the chromosomes of this species: (1) the kinetochore which can be seen to be divided from prometaphase II, (2) the most proximal regions of the arms which only divide at the beginning of anaphase II and (3) the rest of the chromosome arms which appears to be divided from early prophase II. The concept of division is revised.

• (3) The kinetochore has its own cycle of division but does not appear to take direct part in holding together the two sister chromatids of each chromosome until the beginning of anaphase II. The fact that this last property appears to be situated in the arms has several implications. The behaviour of t-ends and of the regions of the B chromosome types responsible for the phenomenon of non-disjunction can be better understood in the light of this finding. In addition, the particular type of division of the mitotic chromosomes ofUlophysema öresundense can be more easily interpreted.