These requirements of normal synapsis between homologous chromosomes are apparently not met in the hybrids. Multiple heterozygosity for chromosomal rearrangements hinders preliminary DSB-independent pairing between homologous chromosomes and increases the incidence of non-homologous associations. Divergence between parental genomes decreases homology at the sequence level, the stability of homologous heteroduplexes, and affects the efficiency of discrimination between correct and ectopic interhomologue interactions. In hybrids, the wide variation between genetically and chromosomally identical oocytes in the ratio of homologously paired to non-homologously paired regions indicates that the choice between homologous and non-homologous synapsis at each of these steps is random. Detailed molecular mechanisms of these processes remain to be elucidated, and interspecies hybrids, such as those reported here, provide an excellent model for future studies.