As part of ongoing investigations of the origins of the octoploid tuber crop oca (Oxalis tuberosa Molina), sequences of a region of chloroplast-active nuclear encoded glutamine synthetase (ncpGS) that contains four introns were determined for cultivated oca and wild Oxalis species from Peru and Bolivia. Results of phylogenetic analysis of sequences of wild Oxalis species were generally congruent with previous ITS results, especially in their support of the x = 8 group as the closest relatives of oca. Relationships among some of the species of this group were better resolved with ncpGS than with ITS, due in part to greater divergence (both substitutions and indels) among the sequences of these species. Sequence heterogeneity was observed within individual plants, most notably in cultivated oca and in wild tuber-bearing plants. Molecular cloning of ncpGS sequences from several cultivated oca accessions revealed multiple sequences within individuals. Apparent artifacts such as Taq error and PCR recombination complicate interpretation of these variants as either homeologous loci or as normal allelic polymorphism. However, current sampling shows fixed heterozygosity of at least three sequence classes, supporting allopolyploid origins of cultivated oca. These different sequence classes from oca belong to different clades of the ncpGS gene tree, linking the putative homeologous genomes of oca to different wild taxa. Wild tuber-bearing populations share some, but not all, of the sequence classes found in domesticated O. tuberosa.

Key words: crop evolution, domestication, glutamine synthetase, Oxalidaceae, Oxalis tuberosa, polyploidy