The Cornell onion breeding program, in cooperation with Lisa Earle, produced a series of doubled haploid (DH) onion lines from long day onions adapted to New York state onion growing conditions. In 2006, the breeding program assessed a large number of unique DH lines, along with related mixaploids and haploids, and controls, for scape development, flower development, pollen viability, and seed set to determine the effect of ploidy on bulb and reproductive characteristics. The largest bulbs and greatest frequency of fertile and fecund bulbs was found in true DH lines, rather than mixaploid or tetraploids. Sizable seed lots were produced for a number of the lines, facilitating replicated trials in 2006. The 2006 trials of pungent and mild lines demonstrated the uniformity of these lines, and the remarkable vigor and relative lack of inbreeding depression in a number of these lines, despite the absolute inbreeding of these lines. Several of the DH lines from the mild program showed good levels of mildness, a trait for which these lines should be fully uniform. Two of the DH lines are being used at cooperating universities for use in testing SSR markers in development and for generation of a core mapping population for onions. Bulbs from the 2006 trials were used to generate large lots of self seed, to facilitate line release, as well as seed of experimental hybrids, which will permit testing of whether DH derived hybrids are superior for uniformity and vigor.

Much of the time and expense of onion breeding is the development of the inbred lines required for hybrid variety creation. The biennial flowering habit of onion slows the inbreeding process, resulting in times for inbred development of 10 to 12 years. Onion is also subject to severe inbreeding depressions, so as lines are inbred, the lines become increasingly less vigorous and produce less seed. Inbred development has been accelerated in other vegetable crops, notably broccoli and other Brassica crops, by the generation of doubled haploid lines. These doubled haploid lines can also be more uniform and vigorous that inbred lines created by traditional means. The ability to produce inbred lines by the creation of DH would therefore be of particular advantage for onions, considering the long development time of this crop species. This would have a major impact on development of improved open pollinated (OP) onion lines as well as hybrids. Production of DH from interspecific hybrids or heterozygous plants derived from such hybrids would also aid the introgression of important traits such as disease resistance. DH lines could also support the work being done to develop molecular markers and maps for onion. Two of the Cornell DH lines are already being used by programs in and Wisconsin to support work being done to create and test SSR markers (New Zealand) and generate DH x DH derived mapping populations (Wisconsin) to simplify creation of onion molecular maps.

The Cornell onion breeding program cooperated with Lisa Earle to develop and test onion doubled haploids (DH). Earle’s lab modified and developed methods to culture unpollinated onion flowers from the onion breeding program and generate large numbers of DH plantlets, as well as related haploid, mixaploids and tetraploids. The breeding program carefully grew the plants to bulbing, characterizing the bulbs, and treating them through vernalization to induce flowering. In 2006, a large number of DH lines, along with controls, mixaploids and haploids, were assessed for scape development, flower development, pollen viability, and seed set to analyze the resulting data for the effect of ploidy on bulb and reproductive characteristics. The largest bulbs and greatest numbers of fertile and fecund bulbs was found in true DH lines, rather than mixaploid or tetraploids. In addition, 2006 replicated trials of pungent and mild lines demonstrated the uniformity of these lines, and the remarkable vigor and relative lack of inbreeding depression in a number of these lines, despite the absolute inbreeding of these lines. Several of the doubled haploid lines from the mild program showed good levels of mildness, a trait for which these lines should be fully uniform. Bulbs from the 2006 trials were used in 2007 to generate large lots of self seed, to facilitate line release, as well as seed of experimental hybrids.

Public or commercial onion breeding programs could use the information generated by this project to efficently and effectively use doubled haploid production to accelerate the process of developing new onion varities. This would enable them to more rapidly produce the improved onion varities needed to support the onion industry. Similarly, the doubled haploid lines generated in this program would be useful in creating more vigourous and uniform long day onion hybrids for use in NYS and other locations in US and internationally that use long day onion varieties. The doubled haploid lines will also be useful tools for molecular work in onion genetics. Some of these lines have already been used by one colleague to test molecular markers being used to create onion genetic maps, and another to create DH x DH derived mapping populations to simplify mapping these markers.

onion breeding, doubled haploid production

Lisa Earle