Multiple Loci Controlling Heading of Iceberg Lettuce

Iceberg lettuce is a major leafy lettuce worldwide. Iceberg lettuce is believed to be bred from butterhead lettuce, which was derived from romaine lettuce. Therefore, the tight head in iceberg lettuce is most likely controlled by multiple genes. A F₂ segregating population was constructed by crossing an iceberg cultivar PI536839and a romaine cultivar PI344074. As expected, the heading trait showed a continuous distribution, varying from no head to tight head in the F₂ population. Two pools were generated, one (head pool) containing 30 individuals with extremely tight head and the other (no head pool) containing 30 individuals with no head. RNAs were extracted from each pool and sequenced using Hiseq2000 (100 bp pair end). A total of 5.2 Gb and 4.2 Gb clean data were obtained for the head pool and no head pool, respectively. Comparison of allele frequency variation at each SNP locus between the two pools suggested at least three loci controlling the heading trait. Markers were designed for each potential heading controlling locus and were used to screen the F₂ population for individuals that are heterozygous at one target locus but homozygous at the other loci. F_2:3 or F_3:4 populations were generated from these identified individuals for fine mapping of corresponding genes. Using above single-locus segregating sub-populations, three loci controlling lettuce leading were fine mapped and candidate genes were identified. The function of the candidates will be confirmed and their mechanism underlying heading will be investigated in future study.