Chiasmata In Bivalents Research Articles

Holocentric chromosomes in meiosis. I. Restriction of the number of chiasmata in bivalents

The number of chiasmata in bivalents and the behaviour of chiasmata during the meiotic divisions were studied in Psylla foersteri (Psylloidea, Homoptera). Two chiasmata with a frequency of 97% and one or three chiasmata with frequencies of 2% and 0.9%, respectively, were observed in the largest bivalent in male meiosis. Meiosis was normal for the largest bivalents with one or two chiasmata, whereas bivalents with three chiasmata were not capable of completing anaphase I because of their inability to resolve the chiasma located in the middle. Consequently, the bivalent was seen as a laggard joining together two metaphase II daughter plates. Apparently, cells of this kind are eliminated. Inability to resolve the chiasma situated in the middle is attributed to the condensation process, which is unable to change the spatial orientation of successive chiasma loops in holocentric bivalents so that chiasma loops would be arranged perpendicular to each other at metaphase I. Thus they retain their parallel orientation from diplotene to metaphase I. Consequently, sister chromatid cohesion is exposed for release only in the outermost chiasmata but the chiasma in the middle continues to interlock the chromosomes in the bivalent. The elimination of the cells carrying bivalents with more than two chiasmata creates a strong selection against the formation of more than two chiasmata in holocentric bivalents.

An analysis of co-orientation in mitosis and meiosis

In this paper the type and the process of orientation of chromosomes during prometaphase of mitosis and meiosis are discussed. A theory is proposed suggesting that two principal factors govern the orientation of both mitotic chromosomes (as associated pairs of sister chromatids) and of meiotic bivalents (associated pairs of homologous chromosomes). The first of these factors is that orientation is achieved principally by the presence in either chromosomes or bivalents of two independently acting centromeres linked one to another, the linking being achieved through the chiasmata in bivalents, and the at present little understood substance or structure maintaining the association of chromatids of a mitotic chromosome. Orientation takes place in linked centromere pairs. The second factor is considered to stem from the capacity of one centromere to become influenced and oriented to one of the two spindle poles. Through these two factors stable orientation along the metaphase equator is achieved, with one of the pair of associated centromeres being influenced and directed to one pole, the other to the opposite pole, with the chromosome or bivalent remaining in this position because of the linking device shared between the two members of each. The movements involved in bringing about this orientation are also discussed.