Chloroplast DNA variation in the genus Glycine subgenus Soja

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Abstract
Chloroplast DNAs from 26 soybean (Glycine max) cultivars or plant introductions (Pls) and from five Pls each of G. soja and G. gracilis have been compared by using the restriction endonucleases Ava l, BamH l, Cla 1, EcoR l, and Xho l. Only the enzymes AVA l, CLA l, and ECOR I distinguished restriction fragment-length polymorphisms. Five unique plastome types have been identified, including three types within max and three within gracilis. Despite the apparent diversity within the soybean germplasm collection, no variation was observed among the chloroplast DNAs of the predominant maternal ancestors of the currently grown soybean cultivars. Hybridization studies demonstrated that the restriction fragment-length polymorphisms were not the result of changes that occurred in the small single-copy region, the rRNA genes, or the psb A border region of the chloroplast genomes. By inference, we can postulate that the changes probably occurred in the large single-copy region. Considering the wide phenotypic overlap, the ease of interspecific crossing, and the existence of common plastome types, we believe that it might be appropriate to consider the subgenus Soja to be comprised of two highly variable species (G. max and G. soja) rather than to attempt to delimit a third intermediate species (G. gracilis).