The epicuticular waxes on onion (Allium cepa) foliage are primarily composed of fatty alcohols, alkanes, and a ketone (Damon et al., 2014). The foliage of wild-type onion has a blue–green color, accumulates relatively high amounts of epicuticular waxes, and is referred to as “waxy.” The waxy appearance on leaves of wild-type onion is likely due to large amounts of the ketone 16-hentriacontanone (H16) relative to the other waxes that build as platelets on the leaf surface (Damon et al., 2014). The “glossy” phenotype of onion is characterized by a light green leaf color and low amounts of epicuticular waxes (Damon et al., 2014; Molenaar, 1984). The leaf color and amounts of epicuticular wax on the leaf surface affect feeding damage by onion thrips (Thrips tabaci), which are a serious insect pest of onion worldwide. Onion plants with a light green leaf color and/or different amounts and types of epicuticular waxes support fewer onion thrips per plant and suffer significantly less feeding damage relative to waxy foliage (Boateng et al., 2014; Cramer et al., 2014; Coudriet et al., 1979; Damon et al., 2014; Diaz-Montano et al., 2010; Jones et al., 1934; Munaiz et al., 2020a).
Jones et al. (1944) studied the genetics of glossy plants selected from the onion cultivars Australian Brown and White Persian. For plants selected from ‘Australian Brown’, the glossy phenotype was conditioned by the homozygous recessive genotype at one locus named gl (Jones et al., 1944). For glossy plants from ‘White Persian’, segregations did not fit a single recessive model because too few glossy progenies were observed in families (Jones et al., 1944). Molenaar (1984) completed a genetic study using glossy plants descended from the same selections from ‘White Persian’ identified by Jones et al. (1934) and observed that segregation ratios fit a single recessive model. Munaiz and Havey (2020) mapped the glossy phenotype from ‘White Persian’ to chromosome 8 and named the glwp locus at which the homozygous recessive genotype conditions glossy foliage. The glwp locus interacts with a region on chromosome 1 to increase the numbers of waxy progenies (Munaiz and Havey, 2020), consistent with the observation by Jones et al. (1944) of too few glossy progenies in segregating families.
The onion cultivar Odourless Green Leaf (OGL) has a light green leaf color, is less attractive to thrips relative to waxy onion (Cramer et al., 2014), and has a unique profile of epicuticular waxes (Munaiz et al., 2020b). During this research, we determined the genetic basis of the wax profile and glossy phenotype of OGL, and we showed that it is inherited independently from the glwp locus (Munaiz and Havey, 2020). Germplasms such as OGL with unique wax profiles may be useful for the development of onion cultivars that suffer less damage by onion thrips compared with waxy onion.
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Loci, chromosome (Chrom), position (Pos) in centiMorgans, observed (Obs) segregations, and probabilities of goodness-of-fit to the expected (Exp) segregation ratios for the F2 family from the cross of glossy ‘Odourless Greenleaf’ with waxy B8667 onion.
Loci, chromosome (Chrom), position (Pos) in centiMorgans, observed (Obs) segregations, and probabilities of goodness-of-fit to the expected 1:2:1 segregation ratios for the F2 family from the cross of glossy B9885 with glossy ‘Odourless Greenleaf’ onion.