Aegilops × Secale hybrids: the production and cytology of diploid hybrids

Hybrids between four diploid species of Aegilops and species of Secale were obtained by using embryo culture. There was a marked incompatibility in the crosses between Secale species and each of the four species in Section Sitopsis of Aegilops and Ae. mutica. It is suggested that this genetic incompatibility with Secale species is an additional similarity between these species of Aegilops and the diploid species of Triticum.

Most chromosomes of Aegilops (A) and Secale (S) are univalent during meta-anaphase of meiosis in these hybrids, but some appeared to associate and others to pair as apparently normal chiasmate bivalents. Analysis of non-chiasmate and chiasmate associations showed that the frequencies of autosyndetic (AA and SS) and allosyndetic (AS) associations fitted the 3AA: 7AS: 3SS ratio expected if association and pairing is at random. Any deviations from random involved a deficiency rather than an excess of Aegilops-Secale pairing. There is no evidence that the chromosomes of Secale are homologous with those of Ae. caudata, Ae. comosa and Ae. umbel-lulata, and it is suggested that the genome of Secale species does not show any homology with the genomes of the genera Aegilops. This does not preclude the presence of homologous segments. It is suggested that the possibility of random association of chromosomes should be considered when occasional pairing in interspecific hybrids is analysed, and that identification of chromosomes and recognition of chiasmata are required. The possibilities of chiasmata between non-homologous chromosomes, of a genetic mechanism in rye which suppresses the pairing of homoeologous chromosomes, and of other factors causing asynapsis and pseudo-synapsis between genetically similar chromosomes are discussed.