Neocentric activity of rye 5RL chromosome in wheat

The neocentric activity of a constriction located on the long arm of rye 5R chromosome (5RL) was analysed. It is not observed in normal rye but it is unusually stretched in bivalents involving 5RL telosomes in wheat-ditelosom ic 5RL addition lines. In 20% of metaphase I cells, the 5RL bivalent presen ts the centromeres oriented to one pole and the constrictions oriented towa rds the opposite pole with a strong tension. In 5% of the cells, the constr iction was able to orient the bivalent to the poles without tension in the centromeres. Sister chromatid cohesion, which is one of the distinct featur es of centromeric function, is persistent at the constriction in delayed 5R L chromosomes at anaphase I. Neither the elongation of the constriction nor the neocentric activity was observed at second meiotic division or mitosis . FISH studies showed that the 5RL constriction lacked detectable quantitie s of two repetitive DNA sequences, CCS1 and the 180-bp knob repeat, present at cereal centromeres and neocentromeres, respectively. We propose that, u nder special conditions, such as the wheat background, the normally non-cen tromeric DNA present at this region of 5RL acquires a specific chromatin st ructure, differentiated as an elongated constriction, which is able to func tion as a centromere.