Genetic mapping reveals a candidate gene for egusi seed in watermelon

Citrullus mucosospermus (egusi watermelon) and Citrullus lanatus (dessert watermelon) are closely related subspecies. Variation in their seed coats underlies a key to understand the major differences in seeds between these two subspecies. Although the genetic model is understood, the candidate genes controlling the egusi seed trait in watermelon are unknown. In this study, a locus that controls the egusi seed trait, WEG6.1, which is located on chromosome 6 of the watermelon genome, was identified. A total of 195 polymorphic primers and 325 F2 plants were used to construct a primary mapping population and linkage maps. A large population of 758 F2 plants was used for fine mapping of the eg gene. The candidate gene was mapped to a 130.7 kb region on chromosome 6. Eleven genes were present within the corresponding region of the reference genome. All eleven candidate genes were sequenced, revealing that the CDS of the Cla007520 gene had a 3 bp deletion that resulted in the omission of one amino acid (serine) in C. mucosospermus. In addition, expression of the Cla007520 gene was highest at the early stage of seed formation, exhibiting a downward trend thereafter. The predicted protein of the Cla007520 gene belonged to the CPP protein family, and its amino acid constituents were similar to those of AtTSO1, which is associated with promoting integument cell proliferation in ovule development. This study revealed genes that facilitate the formation of egusi seed and might lead to a breakthrough in seed development and cultivated watermelon domestication.