Chromosome pairing and synaptonemal complex formation in wheat-rye hybrids

Abstract

The effect of chromosome 5B on chromosome pairing and chiasma formation in hybrids between allohexaploid wheat and diploid rye has been analyzed by ultrastructural investigations of synaptonemal complexes in spread silver stained microsporocytes of hybrids nullisomic for chromosome 5B, euploid hybrids and hybrids carrying an isochromosome for the long arm of chromosome 5B. At metaphase I, there were a mean number of 6.9 chiasmata per cell in the nullisomic 5B hybrids and 0.7 chiasmata per cell in the euploid hybrids, while the monoisosomic 5BL hybrids were virtually achiasmatic, with the exception of the isochromosomes where chiasmata formed regularly between the homologous/identical arms. Up to 40–50% of the complement paired with a synaptonemal complex in the three genotypes, giving rise to multiple associations often involving nearly all lateral components. In the nullisomic 5B hybrids the number of pairing partner exchanges per lateral component engaged in synapsis at zygotene was twice that found in the two other genotypes. It is concluded that nullisomy for chromosome 5B in wheat-rye hybrids results both in a lower stringency of synapsis and in permissiveness for crossing over between homoeologues.