An international team of geneticists has completed the genomic sequence of watermelon, Citrullus lanatus.

Watermelon is one of the most important fruits that contributes to food and economic security in addition to human nutrition. It is favored for the sweet, low calorie, high fiber, nutrient rich characters. Evidence from lots of studies suggests that watermelon is also a useful crop species for genetic research because of its small genome size, and the diverse genetic mutants and variants.

The study, published in the journal Nature Genetics, presents a genome sequence of an East-Asia watermelon cultivar 97103 and resequencing of 20 watermelon accessions spanning the genetic diversity of the species.

Many of the watermelon cultivars have narrow genetic diversity and are susceptible to a large number of diseases and pests. In the study, the team resequenced 20 watermelon accessions representing three different C. lanatus subspecies. As expected, wild watermelon contains greater genetic diversity than the cultivars.

The watermelon crop suffers significant losses from numerous diseases. It is urgent for researchers to investigate the molecular basis for better improving the pathogen resistance of this important crop. The study results showed that many resistance genes were located on chromosomes in clusters, indicating tandem duplications may serve as the evolutionary basis of resistance genes in watermelon genome. Moreover, evidence from the study supported the previous hypothesis that a large portion of disease resistance genes have been lost during watermelon domestication.

The integrative genomic and transcriptomic analysis yielded important insights into aspects of phloem-based vascular that held both in watermelon and cucumber. It is noteworthy that the watermelon phloem contained 118 transcription factors (TFs), whereas in cucumber only 46 TFs were identified and 32 TFs exit in both. Moreover, the team identified several genes associated with the valuable fruit quality traits, including sugar accumulation and citrulline metabolism.

“The high-quality genomic sequence opens a new way for the further studies of watermelon. The data resources could serve as a robust tool for better exploring the mechanisms underlying significant economic traits and regulatory networks and further for breeding improvement. It will also promote the evolutionary research of cucurbit crops and other basic biological studies such as sugar metabolisms,” said study co-author Dr Jianguo Zhang of the Beijing Genomics Institute in Shenzhen, China.

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Bibliographic information: Shaogui Guo et al. The draft genome of watermelon (Citrullus lanatus) and resequencing of 20 diverse accessions. Nature Genetics, published online 25 November 2012; doi: 10.1038/ng.2470