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.