Submitted to: American Journal of Potato Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 15, 2012
Publication Date: July 13, 2012
Citation: Mccord, P.H., Zhang, L., Brown, C.R. 2012. The incidence and effect on total tuber carotenoids of a recessive zeaxanthin epoxidase allele in yellow fleshed potatoes. American Journal of Potato Research. 89:262-268. DOI 10.1007/s12230-012-9250-7. Interpretive Summary: Yellow fleshed potatoes derive their color from a class of compounds known as carotenoids. In addition to providing visual appeal, carotenoids may provide important health benefits, including protecting the eyes from cataracts and macular degeneration. Earlier research suggested that a gene called zep is necessary for high levels of carotenoids, and must be the only form of the gene present for high carotenoids to appear. This is very difficult to achieve in potato breeding, as potato has four possible copies of each gene. We used a molecular marker for zep to determine how many copies of the high-carotenoid form of the gene were present in a large number of different potato varieties. This allowed us to test the effect of copy number on the level of carotenoids, and gain an idea of how common the high-carotenoid form of zep is. Our results suggest that zep must indeed be present as four copies, and that this form of the gene is rare in commercial varieties of potatoes. Since our marker allows the determination of how many copies of the desirable form of the gene are present, it will useful in selecting parents for breeding that result in healthful, high-carotenoid potatoes. These kinds of potatoes, though rarely found in the market worldwide, may someday provide a nutraceutical food for eye health.
Technical Abstract: Zeaxanthin epoxidase (Zep) is one of at least two genes important for the incidence and amount of carotenoids in yellow-fleshed potato. The recessive allele of the gene encoding zeaxanthin epoxidase (Zep) has previously been shown to to inhibit the conversion of the xanthophyll zeaxanthin to other, less polar carotenoids. We examined the effect of the dosage of the recessive allele, Zep, on total carotenoids in yellow-fleshed tetraploid potato germplasm. The dosage of heterozygous individuals was determined using high-resolution DNA melting. We also surveyed a wide range of germplasm available in the USDA-ARS potato breeding program in Washington for the presence and dosage of recessive Zep. Genotypes with zero, one, or two copies of recessive Zep had statistically similar levels of total tuber carotenoids. Triplex individuals showed a small but significant increase in total carotenoids over the previous three classes. In turn, individuals that were homozygous for recessive Zep had much higher levels of total carotenoids than all other dosage classes. This suggests that a slight dosage effect is present, but that the effect of Zep is indeed largely recessive. A significant amount of variation in total carotenoids was observed within all Zep dosage classes, reinforcing the evidence that additional loci are important for high total carotenoids in potato. Our survey of breeding germplasm supported earlier research that recessive Zep is uncommon in tetraploid potato germplasm. The use of high-resolution DNA melting again allowed us to identify the dosage of recessive Zep in heterozygous individuals. This is important for breeding high-carotenoid potatoes at the tetraploid level, since recovery of homozygous recessive Zep individuals is very rare. Identification of agronomically desirable parents with two or more copies of recessive Zep would allow for higher recovery of homozygous recessive Zep progeny, permitting additional selection for other desirable traits.