Origin and emergence of the sweet dessert watermelon, Citrullus lanatus

Abstract

Background and aims: Watermelons, Citrullus species (Cucurbitaceae), are native to Africa and have been cultivated since ancient times. The fruit flesh of wild watermelons is watery, but typically hard-textured, pale-coloured and bland or bitter. The familiar sweet dessert watermelons, C. lanatus, featuring non-bitter, tender, well-coloured flesh, have a narrow genetic base, suggesting that they originated from a series of selection events in a single ancestral population. The objective of the present investigation was to determine where dessert watermelons originated and the time frame during which sweet dessert watermelons emerged.

Key findings: Archaeological remains of watermelons, mostly seeds, that date from 5000 years ago have been found in northeastern Africa. An image of a large, striped, oblong fruit on a tray has been found in an Egyptian tomb that dates to at least 4000 years ago. The Greek word pepon, Latin pepo and Hebrew avattiah of the first centuries CE were used for the same large, thick-rinded, wet fruit which, evidently, was the watermelon. Hebrew literature from the end of the second century CE and Latin literature from the beginning of the sixth century CE present watermelons together with three sweet fruits: figs, table grapes and pomegranates. Wild and primitive watermelons have been observed repeatedly in Sudan and neighbouring countries of northeastern Africa.

Conclusions: The diverse evidence, combined, indicates that northeastern Africa is the centre of origin of the dessert watermelon, that watermelons were domesticated for water and food there over 4000 years ago, and that sweet dessert watermelons emerged in Mediterranean lands by approximately 2000 years ago. Next-generation ancient-DNA sequencing and state-of-the-art genomic analysis offer opportunities to rigorously assess the relationships among ancient and living wild and primitive watermelons from northeastern Africa, modern sweet dessert watermelons and other Citrullus taxa.

Keywords: Archaeobotany; Citrullus lanatus; Cucurbitaceae; citron watermelon; colocynth; crop history; dessert watermelon; egusi watermelon; evolution under domestication; fruit sweetness; talmudic cucurbits.

Fig. 1.

Fruits of wild and primitive Citrullus. (Top three rows) Small, spherical, broadly dark-striped mature fruits with bitter, hard, greenish white or white flesh. (First row, left to right) Whole, equatorially cut and longitudinally cut fruits of C. amarus, citron watermelon, PI 296341 (South Africa), 14 cm diameter. (Second row) Whole, equatorially cut and longitudinally cut fruits of C. mucosospermus, egusi watermelon, PI 457916 (Liberia), 14 cm diameter. (Third row) Whole, equatorially cut and longitudinally cut fruits of C. colocynthis, colocynth, Paqqu‘a 16 (Israel), 6–8 cm diameter. Photographs by the author. (Bottom row) Small, spherical mature fruits, 8–13 cm in diameter with white flesh, of accessions from northeastern Africa, probably C. lanatus, dessert watermelon. (Left to right) PI 193963, Agaruen Hills, Ogaden, Ethiopia, collected 26 October 1950, http://www.ars-grin.gov/cgi-bin/npgs/html/acchtml.pl?1164951; PI 481871, Wadi Noori, 26 km S of Jebel Marra, Darfur, Sudan, from non-irrigated sorghum field, collected 15 November 1981, http://www.ars-grin.gov/cgi-bin/npgs/acc/display.pl?1376807; PI 254622, Bol El Homar, Kordofan, Sudan, collected 18 December 1958, http://www.ars-grin.gov/cgi-bin/npgs/html/acchtml.pl?1195486; PI 525084, Hemidat, Qena, Egypt collected 1985, http://www.ars-grin.gov/cgi-bin/npgs/html/acchtml.pl?1420020. Photographs by A. Davis, USDA/ARS.

Fig. 2.

Ancient images of Citrullus from Egyptian tombs. (Top) A spherical, striped fruit with an adjacent section of foliage (Keimer, 1924). (Centre) Fruits and vegetables, including a basket of small, round, striped Citrullus fruits, the two large, long fruits being snake melons, Cucumis melo Flexuosus Group (Feliks, 2005). (Bottom) An oblong, striped fruit on a tray (Manniche, 1989).

Fig. 3.

Schematic timeline with highlights of the evidence, in chronological order from bottom to top, for the origin and emergence of the sweet dessert watermelon. Column at left shows a gradient of watermelon fruit-flesh colour from greenish white or white of wild and primitive watermelons to intense red of modern watermelons. On this gradient are indicated the existence of wild dessert watermelons, Citrullus lanatus, in northeastern Africa prior to 3000 bce, and the timing of domestication, diffusion to lands outside of northeastern Africa, and selection for fruit flesh sweetness. The timing of the intermediate colours in the gradient is speculative, the only basis being the orange fruit flesh illustrated in a floor mosaic from southern Israel dating to ∼425 ce (Avital and Paris, 2014) and the red fruit flesh illustrated in a Tacuinum Sanitatis manuscript from northern Italy dating to ∼1395 (Paris et al., 2009). Column at right indicates the timing of the highlights of archaeological, iconographic and literary evidence in accordance with the corresponding speculated intermediate fruit flesh colours.