Plastid phylogenetics of Oceania yams (Dioscorea spp., Dioscoreaceae) reveals natural interspecific hybridization of the greater yam (D.alata)

Phylogenetic relationships of Oceanian staple yams (species of Dioscorea section Enantiophyllum) were investigated using plastid trnL-F and rpl32-trnL((UAG)) sequences and nine nuclear co-dominant microsatellites. Analysis of herbarium specimens, used as taxonomic references, allowed the comparison with samples collected in the field. It appears that D.alata, D.transversa and D.hastifolia are closely related species. This study does not support a direct ancestry from D.nummularia to D.alata as previously hypothesized. The dichotomy in D.nummularia previously described by farmers in semi-perennial and annual types was reflected by molecular markers, but the genetic structure of D.nummularia appears more complex. Dioscorea nummularia displayed two haplotypes, each corresponding to a different genetic group. One, including a D.nummularia voucher from New Guinea, is closer to D.tranversa, D.alata and D.hastifolia and encompasses only semi-perennial types. The second group is composed of semi-perennial and annual yams. However, some of these annual yams also displayed D.alata haplotypes. Nuclear markers revealed that some annual yams shared alleles with D.alata and semi-perennial D.nummularia, suggesting a hybrid origin, which may explain their intermediate morphotypes and the difficulty met in classifying them.