The evolution and domestication of multiple crop species in the watermelon genus Citrullus

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Watermelons are important vegetable crops worldwide, but targeted breeding is hindered by insufficient understanding of Citrullus phylogenetics and geographic ranges. Linnaeus named the crop watermelon based on cultivated plants from southern Italy and Sicily, and a taxonomic misjudgment in the 1930s resulted in his species becoming confused with a South African species collected and named by one of his students. This confusion, combined with geographically insufficient sampling, has resulted in a poor understanding of the domestication of the watermelon as well as three other cultivated species of Citrullus, viz. the Egusi melon, the preserving melon, and the colocynth. The objective of this project is to use genome skimming of herbarium specimens and freshly collected material from Namibia, the center of Citrullus species diversity, Egypt, and Sudan, the countries with the oldest archeobotanical records, to resolve species boundaries and divergence times in this genus of just seven species of which two pairs involving a crop and its wild relative. Until now, the domestication of Citrullus spp. has been inferred from the linear measurements of seeds in archaeological sites. We will use 3D geometric morphometrics using computer tomography (CT) of the fossil and recent seeds of all species to test the reliability of seed identification and to obtain additional information about past species ranges and watermelon domestication. The project combines field-collecting, morphological analyses, phylogeographic, and phylogenetic approaches.