Meiotic restitution in wheat-barley hybrids

Abstract

Meiotic restitution occurs in pollen mother cells (PMCs) of reciprocal F1 hybrids between wheat and barley. In occasional PMCs, all or most of the 28 chromosomes assemble at the equatorial plate at metaphase I, but instead of undergoing anaphase I separation they reform into a mass of chromatin to form a restitution nucleus. Some of these restituted nuclei undergo a regular second division and dyads are produced among other non-restituted cells which have reached the tetrad stage of division. Other restituted nuclei fail to undergo a second division and then the PMCs appear as monads among neighbouring tetrads. Both the monads and dyads are expected to produce microsporocytes with the diploid complement of chromosomes. Chromosomes which fail to become included in the restituted nucleus form separate micronuclei and, depending on whether they undergo a regular second division or not, the PMCs containing them eventually appear as tetrads, triads or dyads. These partially restituted nuclei are expected to produce unreduced gametes, deficient for one or more chromosomes. It is postulated from these observations that restitution in wheatbarley F1 hybrids depends on a high frequency of univalent accumulation at the equatorial plate at metaphase I and the subsequent failure of the chromosomes to undergo anaphase I separation.