Aegilops×Secalehybrids: the production and cytology of diploid hybrids

SUMMARY Hybrids between four diploid species ofAegilopsand species ofSecalewere obtained by using embryo culture. There was a marked incompatibility in the crosses betweenSecalespecies and each of the four species in Section Sitopsis ofAegilopsandAe. mutica. It is suggested that this genetic incompatibility withSecalespecies is an additional similarity between these species ofAegilopsand the diploid species ofTriticum. Most chromosomes ofAegilops(A) andSecale(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 ofAegilops-Secalepairing. There is no evidence that the chromosomes ofSecaleare homologous with those ofAe. caudata,Ae. comosaandAe. umbel-lulata, and it is suggested that the genome ofSecalespecies does not show any homology with the genomes of the generaAegilops. 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.