Candidate gene analysis of watermelon stripe pattern locus ClSP ongoing recombination suppression

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Abstract Stripe pattern is an important commodity trait in watermelon, displaying diverse types. In this study, two segregating populations were generated for genetic mapping the single dominant locus ClSP, which was finally delimited to a 611.78 Kb interval with suppression of recombination. According to polymorphism sites detected among genotypes, four discrete haploblocks were characterized in this target region. Based on reference genomes, 81 predicted genes were annotated in the ClSP interval, including seven transcription factors namely as candidate No1-No7. Meanwhile, the ortholog gene of cucumber ist responsible for the irregular stripes was considered as candidate No8. Strikingly, gene structures of No1-No5 completely varied from their reference descriptions and subsequently re-annotated. Notably, the adjacent distribution candidates No2 and No3, as well as No4 and No5, were confirmed to derive from longer transcripts designated as No2_3 and No4_5, respectively. Sequence analysis demonstrated the third polymorphism in CDS of re-annotated No4_5 resulting in truncated proteins in non-stripe plants. Furthermore, only No4_5 was down-regulated in non-stripes relative to stripes contrast to other candidates. Transcriptome analysis identified 356 DEGs between striped and non-striped peels, with genes involved in photosynthesis and chloroplast development down-regulated in non-stripes but calcium ion binding related genes up-regulated. Additionally, 38 DEGs were annotated as transcription factors, with the majority up-regulated in non-stripes, such as ERFs and WRKYs. This study not only contributes to a better understanding of the molecular mechanisms underlying watermelon stripe development, but also provides new insight into the genomic structure of ClSP locus and valuable candidates.

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