Interspecific hybridisation and genome size analysis in Buddleja

Interspecific hybrids Buddleja davidii x Buddleja weyeriana, Buddleja weyeriana x Buddleja davidii and Buddleja davidii x Buddleja lindleyana were generated using in vitro embryo rescue 10-11 weeks after manual pollination. The morphological variation within the F1 populations was limited. The F1 progeny of B. davidii x B. lindleyana was almost sterile and no F2 generation was obtained. From the other hybrids, F2 generations were made by self pollinations and back crosses. Hybrid nature of all F1 and F2 seedlings was confirmed by AFLP. Chromosome counting and genome size measurement for B. weyeriana (F2 selection of (diploid) B. globosa x (tetraploid) B. davidii) revealed a higher chromosome number (76 chromosomes) and genome size than expected, indicating 2n-gametes formation occurred during meiosis of B. globosa. The F1 hybrids B. weyeriana x B. davidii (76 chromosomes) had an intermediate genome size compared with the genome size of the parent plants, proving their hybrid nature. However, the F1 and F2 hybrids of B. davidii x B. weyeriana all had 76 chromosomes but had a lower genome size than expected, suggesting the occurrence of chromosome rearrangements in the genome of the hybrids. B. lindleyana had 38 chromosomes, while the F1 hybrids of B. davidii x B. lindleyana had 76 chromosomes. Also genome size measurements revealed that the F1 seedlings B. davidii x B. lindleyana had higher genome sizes than expected. Both the results of chromosome counting and genome size measurement indicate that 2n-gametes formation took place during meiosis of B. lindleyana