Genetic Analysis of the Unique Epicuticular Wax Profile of ‘Odourless Greenleaf’ Onion

Abstract

The amounts and types of epicuticular waxes on onion (Allium cepa) leaves affect the severity of feeding damage by onion thrips (Thrips tabaci), a serious insect pest of onion. Onion plants with light green leaves are referred to as “glossy” and accumulate less epicuticular wax relative to the blue–green (“waxy”) foliage of wild-type onion. The onion cultivar Odourless Greenleaf (OGL) has visually glossy foliage, shows resistance to thrips feeding damage, and has the unique profile of accumulating waxes with 28 or fewer carbons. Plants of glossy OGL were crossed with the glossy inbred B9885 and waxy inbred lines DH2107, DH066619, and B8667. Hybrid progenies from glossy OGL by waxy plants had waxy foliage, indicating recessiveness of the glossy OGL phenotype relative to the waxy phenotype. Hybrids from the cross of glossy OGL with glossy B9885 were also waxy, revealing different genetic bases for the glossy phenotype in these two onions. Hybrid plants were self-pollinated and segregations in F2 families from OGL × waxy crosses fit the expected 3:1 ratio for the single locus at which the homozygous recessive genotype conditions glossy foliage. Segregations in F2 families from crosses of glossy 9885 × glossy OGL fit the 9:7 ratio, supporting two independently segregating loci, where the recessive genotype at either locus conditions the glossy phenotype. Amounts and types of epicuticular waxes on leaves of F2 progenies from crosses of OGL × waxy B8667 and glossy B9885 × OGL were determined using gas chromatography-mass spectrometry. Single-nucleotide polymorphisms were genotyped and genetic maps were constructed. The visually glossy phenotype from OGL and its unique profile of epicuticular waxes were conditioned by one locus on chromosome 6, for which we propose the name glogl. Onion populations such as OGL with unique epicuticular wax profiles will be important germplasms for the development of onion cultivars that suffer less feeding damage from onion thrips compared with waxy onion.