Reduced Epicuticular Waxes Are Associated With Insect Tolerance.

Thrips are a serious insect pest of many crops, especially onion.  Thrips cause significant damage to leaves and stored bulbs, as well as transmitting diseases such as Iris Yellow Spot Virus and bacterial bulb rots. Reduced quantity and possibly altered chemistry of epicuticular waxes in onion are associated with reduced damage by thrips.  Natural variation exists for amounts of epicuticular waxes in onion and phenotypes have been classified as waxy, semi-glossy, and glossy for copious, intermediate, and low amounts of waxes respectively. The goals of this research are to measure chemical differences in these wax phenotypes using gas chromatography/mass spectrometry (GC-MS), and to determine the genetic bases of these differences.  Independent sources of glossy, semi-glossy, and waxy onions from the USDA germplasm collection and breeding program were evaluated.  Leaf samples were dipped in chloroform to dissolve the waxes. The chloroform solutions were concentrated, derivatized, and analyzed using GC-MS. Variations in the quantities of several long-chain organic molecules were significantly associated with the phenotypes.  Segregating progenies from waxy by semi-glossy plants are being used for phenotyping and genotyping of single nucleotide polymorphisms (SNPs).  Analyses revealed significant QTL for waxiness on chromosomes 5 and 8 of onion.