Breeding polyploid hop cultivars for New Zealand conditions

Hops are dioecious in nature, with the female being the plant of commerce. The main objective of the New Zealand hop-breeding programme is to develop seedless (triploid) hop cultivars with unique brewing properties. Seedlessness is an important requirement for the commercial product, as seeds can impart deleterious flavours during the brewing process. The breeding strategy involves hybridisation between tetraploid and diploid parents. The first step requires the creation of tetraploids, sourced either from somatic doubling techniques or by the identification of sexually derived genotypes via flow cytometry. Recurrent selection procedures are used at both the diploid and tetraploid levels, along with restricted maximum-likelihood statistical procedures, for selection of male and female tetraploid and diploid parental material. Crosses using tetraploids, as both female and male parents, hybridised with diploid genotypes, are described. Sex ratios of the resulting triploid seedlings vary considerably. At the seedling nursery stage, a DNA sex marker is used to identify triploid female genotypes. The economic benefits of using the sex marker are discussed.