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 m eiosis in wheat is prevented by the activity of the Phl locus on chromosome 5B. Several hypotheses have been proposed sharing the assumption that Phl regulates MI chromosome pairing by regulating centromere-mediated chromosom e alignment before the onset of meiosis. To test the relevance of the putat ive 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 Phl locus. Although the 2BL and 2RL arms of the homoeoisochromosome pa ired with each other at MI in the absence of Phl, they never paired with ea ch other at MI in the presence of Phl. Since the two arms were permanently associated in the homoeoisochromosome via a common centromere, it is unlike ly that Phl predetermines MI pairing between homoeologous chromosomes solel y by controlling premeiotic association of centromeres. These findings are consistent with the idea that Phl determines the chromosome pairing pattern at MI by scrutinizing homology across the entire chromosome.