Phylogenetic relationships of wild and cultivated species of Allium section Cepa inferred by nuclear rDNA ITS sequence analysis

Allium section Cepa consists of 12 species most of which are used by humans as condiment, vegetable or medicinal plants. Common onion (Allium cepa) and bunching onion (A. fistulosum) are cultivated species while all others are locally collected from the wild. Although common onion is the most important crop within Allium, its wild progenitor and origin are still not clear. We analyzed the phylogeny of Allium section Cepa using sequences of the nuclear ribosomal DNA internal transcribed spacer (ITS) region of 36 accessions representing eleven species of section Cepa, together with eight outgroup species with phenetic, cladistic, and model-based algorithms. These analyses confirmed section Cepa to be monophyletic and revealed three species groups within the section. These are (i) A. altaicum/A. fistulosum, (ii) A. farctum/A. roylei/A. asarense/A. cepa/A. vavilovii, and (iii) A. galanthum/A. oschaninii/A. praemixtum/A. pskemense. While the first two groups were statistically well supported for the last group support was low, although it resulted in all phylogenetic analyses conducted. Tree and network-based analyses grouped A. cepa within A. vavilovii, indicating the latter to be progenitor of the common onion. However, also an origin of A. cepa through hybridization of A. vavilovii with A. galanthum or A. fistulosum seems possible. We argue that a subdivision of section Cepa in subsections Cepa and Phyllodolon, although possible from our data, as well as the formal description of alliances do not seem reasonable in a small group of species.