Polyphyletic domestication and inter-lineage hybridization magnified genetic diversity of cultivated melon, Cucumis melo L.

Melon accessions with diverse geographical origins were classified into large and small seed-types by length of seed at the boundary of 9 mm, and into five populations based on polymorphisms in the nuclear genome. They were further divided into three maternal lineages, Ia, Ib, and Ic, by polymorphisms in the chloroplast genome. By combining these three classifications, the Europe/US subsp. melo and the East Asian subsp. agrestis were characterized as [large seed, Ib, PopA1 or A2] and [small seed, Ia, PopB1 or B2], respectively, indicating nearly perfect divergence. In South Asia, in addition to the Europe/US and East Asian types, recombinant types between the two types were detected and accounted for 34.8% of South Asian melon. The finding of such an intermixed structure of genetic variation supported the Indian origin of Ia and Ib types. As to Momordica popular in South Asia, seed length was intermediate between the large and small seed-types, and chloroplast type was a mixture of Ia and Ib, suggesting its origin from the recombinant type. In Africa, three lineages of melon were distributed allopatrically and showed distinct divergence. Subsp. agrestis of the Ic type proved to be endemic to Africa, indicating its African origin.