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.
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