Articles
INTERSPECIFIC HYBRIDIZATION OF FLOWER BULBS: A REVIEW
For flower bulbs, interspecific hybridization is the most important source of genetic variation.
Many of the cultivars have originated from complex species crosses which have given rise to a broad range of shapes and colours to plants and flowers (e.g., Alstroemeria, Gladiolus, Hippeastrum, Lilium, Narcissus and Tulipa).
Crossing barriers frequently occur when interspecific crosses are attempted.
Sexual barriers preventing interspecific hybridization have been distinguished into pre- and post-fertilization barriers.
The nature of the barrier determines the method to be used to overcome the specific barrier.
A range of techniques such as cut-style pollination, the use of mentor pollen and grafting of the style have been applied successfully to overcome pre-fertilization barriers.
Lilium has been used as a model crop for the development of in vitro methods.
Ovary, ovary slice, ovule and embryo culture are being exploited to overcome post-fertilization blocks which cause endosperm failure and embryo abortion.
An integrated approach of in vitro pollination and fertilization followed by embryo rescue has been applied in many crosses.
Barriers occurring after a successful embryo rescue are hybrid breakdown (e.g., hybrid albinism in Zantedeschia, Yao et al., 1995) and F1-sterility.
Hybrid breakdown results in the loss of the hybrid before flowering and is a result of the unbalanced new genome combinations.
F1-sterility of interspecific hybrids is very common and among others may be the consequence of reduced chromosome pairing during meiosis.
In this case fertility can be restored by polyploidization, enabling pairing of homologous chromosomes in the allopolyploid hybrid.
Breeding at polyploid levels is widely used in interspecific hybridization programmes of many bulb crops such as Alstroemeria, Freesia, Gladiolus, Lilium. Application of 2n-gametes (meiotic polyploidization) in