Just about everyone knows what a watermelon (Citrullus lanatus) looks like. They come in various sizes and shapes, and the edible interior portion in various colors (white, red, yellow, etc.), but, how many are familiar with the ancestors of the watermelon - the primitive, wild forms?
The genus Citrullus is small in terms of the number of recognized species – containing only 7 or so. All the members of the genus are native to xerophytic (containing little liquid water) environments and the distribution ranges of individual species includes portions of the African continent, the Middle East and South and Central Asia. Each Citrullus species possesses one or more unique characteristics that allow it to survive, and thrive, in a hostile environment.
A great example of the diversity within the genus Citrullus is the gemsbok cucumber (it’s not really a cucumber at all!). The fruit of this desert-loving plant (scientific name Citrullus naudinianus) are, in fact, eaten by gemsbok. However, the fruit are also favored by mole-rats, jackals and honey badgers. Squirrels, porcupines, crickets and other insects use the water that gathers in the skin of old fruit after rains.
The fruit of the gemsbok cucumber are bitter. This is likely due to the presence of tetracyclic terpines common in the fruit of some other members of the genus. Nonetheless, the cooked fruit are edible. The bushmen of the Kalahari eat the fruit after they have been roasted in a fire or boiled. The cooking renders the terpines harmless. The fleshy fruits are also known to serve as a source of water and have even been used to make pickles. In addition to its fleshy fruits, the gemsbok cucumber produces large underground storage roots.
We report here a chromosome-length genome sequence of the gemsbok cucumber (Citrullus naudinianus), a plant bearing small (6-12cm in length) oval-shaped fruits with rudimentary spines. This plant is native to southern Africa including Botswana, Namibia, Mozambique, Zambia, Zimbabwe and South Africa. It represents the basal branch in the taxonomic tree of Citrullus and the Citrullus species most distantly related to the common watermelon. Unlike all other members of the genus Citrullus, the gemsbok cucumber is dioecious having separate male and female plants. The genetic mechanism accounting for the conversion from dioecy (gemsbok cucumber) to monoecy (all other Citrullus species), has yet to be determined.
The genome sequence of the gemsbok cucumber serves to provide an evolutionary anchor point for a pan-genus study on genome evolution in the genus Citrullus. It also facilitates an examination of the evolution of the gemsbok cucumber’s many unique adaptive traits that allow it to survive in an environment where few other plants can.
Check the interactive map below and explore the Hi-C contacts across the 11 chromosomes of the gemsbok cucumber, and don't forget to visit the assembly page for more details on this HiFi+Hi-C genome assembly!
References:
1. Chomicki, G. and S. Renner. 2015. Watermelon origin solved with molecular phylogenetics including Linnaean material: another example of museomics. New Phytologist (2015) 205: 526–532 doi: 10.1111/nph.13163.
2. Jeffrey, C. 1978. Cucurbitaceae. Fl. Zambes. 4:414-499. Flora Zambesiaca Managing Committee, Glasgow.
3. Retief, E. and N.L. Meyer. 2017. Cucurbitaceae. In: E. Retief and N.L. Meyer (Eds.) Plants of the Free State: Inventory and identification guide. Strelitzia 38:484-489. South African National Biodiversity Institute, Pretoria.
4. Schaefer, H. and S.S. Renner. 2011. Cucurbitaceae. Cucurbitaceae Durande (1782), nom. cons. In: K. Kubitzk (ed.) Flowering Plants. Eudicots: Sapindales, Cucurbitales, Myrtaceae. Springer. pp. 112-174.
5. Tropical Plants Database, Ken Fern. tropical.theferns.info. 2021-12-26. <tropical.theferns.info/viewtropical.php?id=Acanthosicyos+naudinianus>
6. WFO. 2021: Acanthosicyos naudinianus (Sond.) C. Jeffrey. Published on the internet; http://www.worldfloraonline.org/taxon/wfo-000051376.