Diversification and genetic differentiation of cultivated melon inferred from sequence polymorphism in the chloroplast genome

Molecular analysis encouraged discovery of genetic diversity and relationships of cultivated melon (Cucumis melo L.). We sequenced nine inter- and intra-genic regions of the chloroplast genome, about 5500 bp, using 60 melon accessions and six reference accessions of wild species of Cucumis to show intra-specific variation of the chloroplast genome. Sequence polymorphisms were detected among melon accessions and other Cucumis species, indicating intra-specific diversification of the chloroplast genome. Melon accessions were classified into three subclusters by cytoplasm type and then into 12 subgroups. Geographical origin and seed size also differed between the three subclusters. Subcluster Ia contained small-seed melon from Southern Africa and South and East Asia and subcluster Ib mainly consisted of large-seed melon from northern Africa, Europe and USA. Melon accessions of subcluster Ic were only found in West, Central and Southern Africa. Our results indicated that European melon groups and Asian melon groups diversified independently and shared the same maternal lineage with northern African large-seed melon and Southern African small-seed melon, respectively. Cultivated melon of subcluster Ic may have been domesticated independently in Africa. The presence of 11 cytoplasm types in Africa strongly supported African origin of cultivated melon and indicated the importance of germplasm from Africa.