ABSTRACT
The kiwifruit of commerce are large-fruited selections of Actinidia chinensis var. chinensis and A. chinensis var. deliciosa. The main constraints to kiwifruit breeding include dioecy, the long generation time, the difficulties of genetic analysis in polyploids, and the complexity of some key traits. In Actinidia there is variation in ploidy both within and between taxa. Most taxa are diploid but there are also tetraploids, hexaploids, octoploids and decaploids in diminishing frequency. The high basic chromosome number, x = 29, suggests that diploids of Actinidia are palaeopolyploids. Many taxa contain ploidy races, especially the A. arguta and A. chinensis species complexes. Thus the ploidy of any genotype used in breeding programmes and the ploidy of offspring should be checked: they are not always at the expected ploidy. In wild populations, there appears to have been frequent hybridization and reticulate evolution influenced by geographic patterns of distribution with transitional forms suggesting considerable gene flow between sympatric Actinidia taxa. Controlled crossing has also often been successful although differences in ploidy can be expected to complicate interspecific crosses or even crosses within a taxon. Ploidy in Actinidia can be manipulated by chromosome doubling, plantlet regeneration from pollen or unfertilized embryos, use of numerically unreduced gametes, or culture of endosperm tissue. Protoplast fusion to produce allopolyploids needs further development. Success in kiwifruit breeding programmes will depend on a good understanding of the reproductive biology of Actinidia and an awareness of the commercial requirements of new cultivars.
