Chromosome numbers, Sudanese wild forms, and classification of the watermelon genus Citrullus, with 50 names allocated to seven biological species

Watermelons are among the most important vegetable crops worldwide, but targeted breeding is hindered by problems with Citrullus taxonomy. Here we clarify nomenclature and species relationships in Citrullus, its chromosome numbers, and the likely geographic region of watermelon domestication. We correct an erroneous chromosome count in recent literature, provide a count for an understudied species, and data on chromosome numbers for the entire genus. We also use a nuclear/plastid locus phylogeny to summarize data on Citrullus sexual systems, loss of tendrils, life history, and geographic ranges. Key insights from new sequences are that (i) material of C. vulgaris var. “colocynthoides”, collected by Schweinfurth in Egypt in 1882 represents the colocynth, C. colocynthis, and (ii) the citron or bitter watermelon, C. amarus, is indeed a separate species, not a subspecies of the dessert watermelon. Schweinfurth’s varietal name, a nomen nudum, has been widely used for seeds as old as 5400 BP,...