Fine-mapping reveals that the bHLH gene Cla011508 regulates the bitterness of watermelon fruit

The bitterness of the fruit seriously affects the taste of watermelon. Therefore, genetic research on bitterness is of great significance to improve watermelon fruit quality. However, there is no research on fine mapping of watermelon fruit bitterness. In the present study, an F2 population was developed, and two inbred lines, '9904' (bitter flesh) and 'Handel' (non-bitter flesh) as parental materials. Genetic analysis indicated that the bitterness of watermelon was a dominant trait. The initial mapping revealed a 6.16 Mb region on chromosome 1. SNP and CAPS markers were used to narrowed the candidate region to 116.7 kb with four candidate genes (Cla011507, Cla011508, Cla011509, and Cla011510). Notably, Cla011508, and Cla011510 have high homology with gene Csa5G156220 and Csa5G157230, which regulate the bitterness trait in cucumber, and the mutation base at the locus 3,168,882 on chromosome 1 of Cla011508 led to the premature termination of the translation process; at the same time, a non-synonymous mutation caused the amino acid sequence change of Cla011510. The results of qRT-PCR showed that Cla011508 was highly expressed in watermelon flesh of '9904', while no such significant difference was found in Cla011510. Furthermore, the analysis of 16 watermelon accessions showed that 8 bitterness accessions with base C at the locus 3,168,882 had higher gene expression level of Cla011508, which was significantly different from that of the 8 non-bitter accessions with base T. In conclusion, the bitterness of watermelon fruit may be controlled by Cla011508, and the important mutation loucs in this gene provide molecular insights for marker-assisted breeding of target trait.