Direct measurement of recombination frequency in interspecific hybrids between Hordeum vulgare and H. bulbosum using genomic in situ hybridization.

Two different genotypes of diploid Hordeum vulgare x H. bulbosum hybrids, which differ in their pattern of meiotic metaphase pairing behaviour, were investigated at MI and AI by genomic in situ hybridization (GISH). One hybrid, 102C2, showed a high frequency of bivalents at metaphase I whereas the other, 103K5, showed a high frequency of univalents. The GISH analysis of both hybrids established that pairing occurred only between chromosomes of different parental genomes and revealed that pairing frequency greatly exceeded recombination. Hybrid 102C2 had a significantly higher recombination frequency than 103K5, but in both hybrids recombination involved only distal chromosome regions. However, an interesting finding is that the ratio of recombination to pairing frequency in 103K5 (1:8.9) is twice as high compared with 102C2 (1:17). The hybrids also differed in chromosome stability; little chromosome elimination occurred in 102C2 but 103K5 showed extensive chromosome loss. It appears that the high frequency of bound arms at MI favours retention of H. bulbosum chromosomes and maintains stability of chromosome numbers in PMCs. Various ideas are put forward to explain the discrepancy between meiotic pairing frequency and recombination in these hybrids.