Georeferenced phylogenetic analysis of a global collection of wild and cultivated Citrullus species

The geographical origin of watermelon (Citrullus lanatus) remains debated.
While a first hypothesis suggests the center of origin to be west Africa,
where a sister endemic species C. mucosospermus thrives, a second
hypothesis suggests north-eastern Africa where the white-fleshed Sudanese
Kordophan melon is cultivated. In this study, we infer biogeographical and
haplotype genealogy for C. lanatus, C. mucosospermus, C. amarus, and C.
colocynthis using non-coding cpDNA sequences (trnT-trnL and ndhF-rpl32
regions) from a global collection of 135 accessions. In total, we
identified 38 haplotypes in C. lanatus, C. mucosospermus, C. amarus, and
C. colocynthis; of these, 21 were found in Africa and 17 appear endemic to
the continent. The least diverse species was C. mucosospermus (5
haplotypes) and the most diverse was C. colocynthis (16 haplotypes). Some
haplotypes of C. mucosospermus were nearly exclusive to West-Africa, and
C. lanatus and C. mucosospermus shared haplotypes that were distinct from
those of both C. amarus and C. colocynthis. The results support previous
findings C. mucosospermus to be the closest relative to C. lanatus
(including subsp. cordophanus). West Africa, as a center of endemism of C.
mucosospermus, is an area of interest in the search of the origin of C.
lanatus. This calls for further historical and phylogeographical
investigations and wider collection of samples in West and North-East
Africa.