Centromere association is an unlikely mechanism by which the wheat Ph1 locus regulates metaphase I chromosome pairing between homoeologous chromosomes

Chiasmate pairing between homoeologous chromosomes at metaphase I (MI) of meiosis in wheat is prevented by the activity of the Ph1 locus on chromosome 5B. Several hypotheses have been proposed sharing the assumption that Ph1 regulates MI chromosome pairing by regulating centromere-mediated chromosome alignment before the onset of meiosis. To test the relevance of the putative predetermination of chromosome pairing at MI by the centromere-mediated chromosome association prior to meiosis, a 2BL.2RL homoeoisochromosome was constructed and its MI pairing was assessed in the presence and absence of the Ph1 locus. Although the 2BL and 2RL arms of the homoeoisochromosome paired with each other at MI in the absence of Ph1, they never paired with each other at MI in the presence of Ph1. Since the two arms were permanently associated in the homoeoisochromosome via a common centromere, it is unlikely that Ph1 predetermines MI pairing between homoeologous chromosomes solely by controlling premeiotic association of centromeres. These findings are consistent with the idea that Ph1 determines the chromosome pairing pattern at MI by scrutinizing homology across the entire chromosome.