Data from: 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, and it now needs to be investigated whether these seeds are colocynths, watermelon or a hybrid. To help improve Citrullus taxonomy and nomenclature, we allocate some 50 scientific names to seven biological species, tabulate chromosome counts, and provide notes on misidentified germplasms traceable by accession numbers. More wild-collected herbarium-verified material from northeast Africa is urgently needed, especially given the discovery of Russian breeders that populations of wild “cordophanus” from Sudan are closer to the cultivated watermelon than are any other wild populations so far known.

西瓜是全球最重要的蔬菜作物之一，但西瓜属（Citrullus）分类学上的诸多问题阻碍了定向育种工作的开展。本研究厘清了西瓜属的命名规则、物种间亲缘关系、染色体数目，以及西瓜驯化的潜在地理起源区域。本研究纠正了近期文献中一处错误的染色体计数结果，为一种研究不足的物种提供了染色体计数数据，并补充了西瓜属全属的染色体数目资料。本研究还利用核-质体基因座系统发育分析，总结了西瓜属的繁育系统、卷须退化情况、生活史特征及地理分布范围相关数据。新测序结果得到的关键认知如下：其一，1882年施魏因富特（Schweinfurth）在埃及采集的‘普通西瓜变种"colocynthoides"’材料实为药西瓜（C. colocynthis）；其二，香橼西瓜或称苦西瓜（C. amarus）确实为独立物种，而非鲜食西瓜的亚种。施魏因富特提出的这个变种名称属于裸名（nomen nudum），已被广泛用于距今5400年的古种子标本的相关研究中，当前亟待明确这些古种子究竟属于药西瓜、普通西瓜还是二者的杂交种。为完善西瓜属的分类学与命名体系，本研究将约50个科学名称归属于7个生物学物种，整理了西瓜属的染色体计数表格，并针对可通过种质入库编号追溯的误鉴定种质资源提供了说明。目前亟需采集更多东北非洲地区的野生采集且经标本馆验证的材料，尤其是有研究发现俄罗斯育种者已证实，苏丹地区的野生"cordophanus"类群比目前已知的其他野生种群更接近栽培西瓜。