Flow cytometric identification of sexually derived polyploids in hop (Humulus lupulus L.) and their use in hop breeding

The New Zealand hop breeding programme is based solely on the development of seedless triploid cultivars. This relies on the use of tetraploid parents. While asexually derived tetraploid parents have been used successfully, sexually derived tetraploids offer a useful alternative. They may have a higher level of heterozygosity and are easier to obtain. Methods for the identification of tetraploids from seedling populations by flow cytometry are described. Two studies were conducted; one on field-grown plants, the other on plants grown in a glasshouse. Approximately 15% of seedlings in the two studies were identified as tetraploids (2n=4x=40). Between 70 and 80% were identified as either diploid (2n=2x=20) or triploid (2n=3x=30). Remaining plants were haploid, pentaploid or probably aneuploid. Within triploid and tetraploid populations from 40 to 50% of seedlings were female, 30 to 40% were monoecious, with the remainder being males and non-flowering plants. Sexually derived tetraploid parents have been used successfully in the breeding programme, from which several promising triploid selections have subsequently been made.