Breeding of dry storage onion for the last 60 years largely focused on production of inbred lines for creation of hybrid varieties after Jones and Clarke (1943) and Jones and Davis (1944) outlined how the cytoplasmic male sterility system could be used for large-scale production of hybrid seed. Hybrids were rapidly adopted by growers as a result of their significant increase in marketable yield as well as greater uniformity for horticultural traits compared with standard open-pollinated onion varieties (Dowker and Gordon, 1983; Evoor et al., 2007; Hosfield et al., 1977; Joshi and Tandon, 1976).
Creation of superior onion hybrids depends on the development of high-quality, fecund inbred lines to use as parents. These lines must be sufficiently inbred, because commercially acceptable hybrids must be uniform for critical horticultural traits such as bulb size, bulb shape, and days to maturity. This uniformity is achieved by increasing the homozygosity of the inbred parent lines through cycles of self-pollination. However, like many cross-pollinated crops, onions suffer from severe inbreeding depression when self-pollinated for several generations (Bohanec, 2002). Inbreeding depression reduces plant vigor, bulb size, and therefore seed production. As a result, the negative impact of inbreeding depression reduces the number of cycles of inbreeding possible while maintaining lines that are sufficiently vigorous to produce hybrid seed on the scale required for commercial onion seed sales (Bohanec, 2002). Therefore, the inability to repetitively self-pollinate a line greatly reduces a breeder’s ability to create uniform highly inbred lines (Alan et al., 2004; Bohanec, 2002). Any increase in heterozygosity and heterogeneity of the inbred parent lines leads to an increase of non-uniformity of the hybrids (Alan et al., 2004) and can also impede hybrids from reaching their maximal heterotic potential (Bohanec, 2002). The biennial nature of onion and requirement for bulb vernalization before flowering push the seed-to-seed generation time of onion to two years. Therefore, typical selection practices to develop inbred lines using the single-seed descent method takes 10 to 12 years (Bohanec, 2002).
DH lines have been used to accelerate inbred line development in commercial breeding programs of various crops, including rice, maize, rapeseed, tobacco, and barley, as a result of the rapid production time and superior uniformity compared with conventionally bred inbred lines (Bong and Swaminathan, 1995; Maluszynski et al., 2003; Röber et al., 2005). DH plants can be produced in vitro by generating plantlets from gynogenic or androgenic haploid cells. These haploid plantlets either spontaneously double their chromosomes or are chemically stimulated to do so, creating DH plants that are completely homozygous (Maluszynski et al., 2003). Development of homozygous DH onion lines can be much quicker and more cost-effective than conventional breeding procedures (Alan et al., 2003, 2004; Bohanec, 2002).
A series of DH onion lines was produced from diverse, highly heterozygous material in development within the Cornell onion breeding program (Alan et al., 2003, 2004). The objectives of the current study were to evaluate these DH lines individually and to assess their use as males in hybrid combinations in terms of vegetative vigor, bulb quality, and uniformity of horticultural traits for their potential in commercial onion production.
Alan, A.R., Brants, A., Cobb, E., Goldschmied, P., Mutschler, M.A. & Earle, E.D. 2004 Fecund gynogenic lines from onion (Allium cepa L.) breeding materials Plant Sci. 167 1055 1066
Alan, A.R., Mutschler, M.A., Brants, A., Cobb, E. & Earle, E.D. 2003 Production of gynogenic plants from hybrids of Allium cepa L. and A. roylei Stearn Plant Sci. 165 1201 1211
Bohanec, B. 2002 Doubled haploid onions, p. 145–158 In: Rabinowitch, H.D. and L. Currah (eds.). Allium crop science: Recent advances. CABI Publishing, Wallingford, UK
Bong, B.B. & Swaminathan, M.S. 1995 Magnitude of hybrid vigor retained in double haploid lines of some heterotic rice hybrids Theor. Appl. Genet. 90 253 257
Boodley, J.W. & Sheldrake, R. 1972 Cornell peat-lite mixes for commercial plant growing. Cornell University Cooperative Extension Division Informational Bulletin 43.3
Dowker, D. & Gordon, G.H. 1983 Heterosis and hybrid cultivars in onions, p. 220–231. In: Frankel, R. (ed.) Heterosis. Monographs on Theoretical and Applied Genetics. Springer-Verlag
Evoor, S., Veere Gowda, R., Gangappa, E. & Krisna Monohar, R. 2007 Heterosis for yield, yield components and quality traits in onion (Allium cepa L.) Karnataka Journal of Agricultural Science 20 813 815
Hosfield, G.L., Vest, G. & Peterson, C.E. 1977 Heterosis and combining ability in a diallel cross of onions J. Amer. Soc. Hort. Sci. 102 355 360
Jones, H.A. & Clarke, A. 1943 Inheritance of male sterility in the onion and the production of hybrid seed Proceedings American Society of Horticultural Science 43 189 194
Jones, H.A. & Davis, G. 1944 Inbreeding and heterosis and their relation to the development of new varieties of onions. USDA Technical Bulletin 874
Kim, S., Yoo, K.S. & Pike, L.M. 2007 Production of doubled haploid onions (Allium cepa) and evaluation of their field performance Horticulture Environment, and Biotechnology 48 143 147
Maluszynski, M., Kasha, K.J., Forster, B.P. & Szarejko, I. 2003 Doubled haploid production in crop plants: A manual. Kluwer Academic Publ., Boston, MA
Röber, F., Gordillo, G.A. & Geiger, H.H. 2005 In vivo haploid induction in maize—Performance of new inducers and significance of doubled haploid lines in hybrid breeding Maydica 50 275 283