Fine Mapping of Lycopene Content and Flesh Color Related Gene and Development of Molecular Marker-Assisted Selection for Flesh Color in Watermelon (Citrullus lanatus).

Chaonan Wang,Peng Gao,Feishi Luan,Lei Sun,Aohan Qiao,Shi Liu,Xufeng Fang,Angela R Davis

doi:10.3389/fpls.2019.01240

Abstract

Lycopene content and flesh color are important traits determined by a network of carotenoid metabolic pathways in watermelon. Based on our previous study of genetic inheritance and initial mapping using F2 populations of LSW-177 (red flesh) × cream of Saskatchewan (pale yellow flesh), red flesh color was controlled by one recessive gene regulating red and pale yellow pigmentation, and a candidate region related to lycopene content was detected spanning a 392,077-bp region on chromosome 4. To obtain a more precise result for further study, three genetic populations and a natural panel of 81 watermelon accessions with different flesh colors were used in this research. Herein, we narrowed the preliminary mapping region to 41,233 bp with the linkage map generated from F2 populations of LSW-177 (red flesh) × cream of Saskatchewan (pale yellow flesh) with 1,202 individuals. Two candidate genes, Cla005011 and Cla005012, were found in the fine mapping region; therein Cla005011 was a key locus annotated as a lycopene β-cyclase gene. Phylogenetic tree analysis showed that Cla005011 was the closest relative gene in gourd. LSW-177 × PI 186490 (white flesh) and another BC1 population derived from garden female (red flesh) × PI 186490 were generated to verify the accuracy of the red flesh candidate gene region. By analyzing the expression levels of candidate genes in different developmental stages of different color watermelon varieties, Cla005011 for the expression differences was not the main reason for the flesh color variation between COS and LSW-177. This indicated that the LCYB gene might regulate fruit color changes at the protein level. A new marker-assisted selection system to identify red and yellow flesh colors in watermelon was developed with flesh color–specific CAPS markers and tested in 81 watermelon accessions.

Highlights

The watermelon (Citrullus lanatus) is one of the most important cucurbitaceous crops in the world and occupies approximately 6% of the cultivated area used for all types of vegetables
Most of the mixed pale and canary yellow fruits had flesh color >50% canary or pale yellow by crosssectional area, so the two mixed-color plants could be classified as canary yellow and pale yellow. According to these classification criteria, 199 (173 + 26), 66 (48 + 18), and 87 plants were judged to have canary yellow, pale yellow and red flesh color in Pop. 1, fitting a genetic segregation ratio of 9:3:4 (χ2 = 0.02 and 1.12, P = 0.99 and 0.57 for the year of 2013 and 2014, respectively), which indicated that flesh color was affected by two major genes
Canary yellow and pale yellow could be classified into red and nonred groups by visual observation. The segregation of these two groups yielded a ratio that did not differ significantly from a 3:1 ratio by statistical analysis. These results indicated that a single major recessive gene determined red and nonred color in watermelon based on the genetic background of Pop. 1

Summary

Introduction

The watermelon (Citrullus lanatus) is one of the most important cucurbitaceous crops in the world and occupies approximately 6% of the cultivated area used for all types of vegetables. The main pigment that causes red flesh color in watermelon is lycopene, which is considered one of the most important natural carotenoids in fruits. Lycopene has been a research focus in many areas, including health care products, cosmetics, and nutrition, and has been shown to serve physiological functions in the human body. Lycopene is the precursor in some physiological, and biochemical processes in plants. The lycopene in watermelon can be absorbed by the human body directly, but for tomato, another fruit rich in lycopene, dietary lycopene is better absorbed from cooked foods. Lycopene is the precursor of β-carotene, violaxanthin, and neoxanthin, which participate in different physiological events in plants, including photosynthesis, antenna assembly, and photoprotection (Young, 1991; Latowski et al, 2011)

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