Conservation of Actinidia chinensis alleles in a Psa environment

The Plant & Food Research Actinidia (kiwifruit) germplasm repository contains over 3000 genotypes from circa 350 accessions. The collection provides genetic diversity for breeding new commercial cultivars. Maintenance of the collection has become a major challenge following the arrival and spread in New Zealand of a bacterial canker disease caused by a virulent pathovar of Pseudomonas syringae pv. actinidiae (Psa). Over 80% of diploid A. chinensis Planch seedlings in the collection have shown severe symptoms of infection and many have died. The potential loss of alleles from the collection is a major concern. A substantial reduction in genetic diversity would constrain the development of new kiwifruit cultivars and undermine the potential of populations to respond to changing environmental conditions. We aim to retain as many as possible of the alleles responsible for various key traits such as fruit size, yield and different flavour profiles by using a carefully designed wide-crossing strategy. To avoid a genetic bottleneck in the A. chinensis germplasm, a crossing program comprising 70 males and 35 females was carried out in 2011. Pollen from the 70 males was bulked into 10 groups each of seven individual male parents and used in polycrosses to produce new breeding lines. A total of 4363 seedlings from these polycrosses were propagated and planted out in our research orchard, with 2185 subsequently lost to Psa. The seedling population is under continuous screening for key traits, including Psa tolerance. Another round of crosses will be made within the population, aiming to maintain allelic diversity. Other strategies for maintaining allelic diversity, such as tissue culture and storing pollen and seed, are also being followed. However, the wide crossing approach has the advantage that the seedlings can be used in breeding while we continue to monitor their performance in a Psa environment.