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CROP BREEDING, GENETICS & CYTOLOGY

The Western Wheatgrass Chloroplast Genome Originates in Pseudoroegneria

^a USDA-ARS, Forage and Range Research Lab., Utah State Univ., Logan, UT 84322-6300 USA
^b USDA-ARS, Corn and Soybean Research, OARDC, 1680 Madison Ave., Wooster, OH 44691-4096 USA
^c Utah State Univ., Logan, UT 84322-6300 USA

tomjones{at}cc.usu.edu

The octoploid Pascopyrum smithii (Rydb.) A. Löve (StHNsXm) arose from hybrization among four diploid genera, three of which are known from genome analysis, but the diploid origin of the Pascopyrum chloroplast genome remains unknown. Identification of the maternal parents in the hybridizations leading to Pascopyrum will guide efforts to reconstruct allopolyploid germplasm from ancestral taxa. We compared the DNA sequences of a 762-base pair (bp) segment of the ndhF (nicotinamide adenine dinucleotide dehydrogenase subunit F) chloroplast gene of western wheatgrass (Pascopyrum smithii; StHNsXm) with that of its putative allotetraploid, i.e., Elymus (StH) and Leymus (NsXm), and diploid, i.e., Pseudoroegneria (St), Hordeum (H), Psathyrostachys (Ns), ancestors. To ascertain the 2x to 8x chloroplast phylogeny, the gene sequences were aligned and a phylogenetic tree was constructed by the neighbor-joining method. Pascopyrum smithii differed by 0 to 2 and 6 to 9 bp from its two 4x ancestors, Elymus and Leymus, respectively. Elymus differed by 2 and 10 to 13 bp from its two 2x ancestors, Pseudoroegneria and Hordeum, respectively. Pascopyrum, Elymus, and Pseudoroegneria taxa clustered together, but separately from Leymus and Hordeum taxa. The Pascopyrum chloroplast genome appears to have originated from the diploid Pseudoroegneria through the tetraploid Elymus. De novo synthesis of Pascopyrum germplasm from its tetraploid ancestors should be conducted with cognizance of the preference for Pseudoroegneria cytoplasm found in nature. Leymus differed from its diploid ancestor, Psathyrostachys, by 14 to 15 bp, indicating that the second unknown diploid ancestor of Leymus may have contributed its chloroplast DNA.