Behavior of Centromeres during Restitution of the First Meiotic Division in a Wheat–Rye Hybrid

In first division restitution (FDR)-type meiosis, univalents congregate on the metaphase I plate and separate sister chromatids in an orderly fashion, producing dyads with somatic chromosome numbers. The second meiotic division is abandoned. The separation of sister chromatids requires separation of otherwise fused sister centromeres and a bipolar attachment to the karyokinetic spindle. This study analyzed packaging of sister centromeres in pollen mother cells (PMCs) in a wheat–rye F(1) hybrid with a mixture of standard reductional meiosis and FDR. No indication of sister centromere separation before MI was observed; such separation was clearly only visible in univalents placed on the metaphase plate itself, and only in PMCs undergoing FDR. Even in the FDR, PMCs univalents off the plate retained fused centromeres. Both the orientation and configuration of univalents suggest that some mechanism other than standard interactions with the karyokinetic spindle may be responsible for placing univalents on the plate, at which point sister centromeres are separated and normal amphitelic interaction with the spindle is established. At this point it is not clear at all what univalent delivery mechanism may be at play in the FDR.