Submitted to: Plant Systematics and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 2, 2011
Publication Date: May 18, 2011
Citation: Culumber, C.M., Larson, S.R., Jensen, K.B., Jones, T.A. 2011. Genetic differentiation and admixture among Eurasian and North American Leymus (Triticeae) wildryes detected using chloroplast DNA sequences and AFLP profiles. Plant Systematics and Evolution. 294:207-225. Interpretive Summary: The genus Leymus is a genetically defined group of perennial wildrye grass species that show homologous chromosome pairing when hybridized and contain two similar sets of chromosomes. Recent studies have showed that Leymus wildryes from North American and Eurasia contain different chloroplast DNA, which is maternally inherited via the seed parent. Studies of single-copy nuclear genes, which are inherited from both parents, indicate that Leymus wildryes from North American and Eurasian all contain two distinct and similar sets of genes. However, previous studies have not been able to conclusively distinguish species or determine species relationships using DNA markers. Our objective was to compare nine Eurasian and seven North American Leymus wildrye species using both chloroplast and multi-locus nuclear DNA markers. Results indicate that approximately 74.9% of the chloroplast DNA polymorphism was apportioned between North American and Eurasian clades but only 19.3% among taxa. Conversely, only 9.4% of the overall AFLP variation was apportioned between North American and Eurasian samples and 36.5% among taxa. The multilocus nuclear DNA markers distinguished all species, except for North American L. ambiguus, which appears to be a hybrid of Great Basin wildrye (L. cinereus) and Salina wildrye (L. salinus). These findings support the hypothesis that Leymus wildryes evolved by hybridization of distinct North American and Eurasian ancestors. The maternally inherited chloroplast genomes maintain distinct ancestral North American and Eurasian genotypes. Biparentally inherited nuclear DNA markers show less difference, perhaps as a result of hybridization and blending of genes from these two continental regions.
Technical Abstract: Leymus is a genomically defined group of allopolyploid Triticeae taxa that show homologous chromosome pairing when hybridized and contain the Ns and Xm subgenomes. Recent investigations showed that Leymus chloroplast DNA sequences are polyphyletic with most Eurasian taxa similar to the Psathyrostachys Ns-genome ancestor, whereas North American taxa are more similar to other Triticeae genera and possibly carry the Xm chloroplast genome. Comparisons of ribosomal ITS and single-copy nuclear genes show that North American and Eurasian taxa both contain distinct Ns and Xm sequences, but genetic relationships among Leymus taxa have not been fully resolved. Our objective was to compare nine Eurasian and seven North American Leymus taxa using both chloroplast and multi-locus nuclear AFLP markers. Bayesian and UPGMA cluster analyses separated most AFLP genotypes by taxa and continental region, except that North American L. innovatus grouped with Eurasian samples and L. ambiguus is a hybrid genotype of North American L. cinereus and L. salinus alleles. Parsimony analysis also separated most North American and Eurasian cpDNA sequences, except that L. mollis from the Primorye Kray region of Russian grouped with the North American clade. Approximately 74.9% of the chloroplast DNA polymorphism was apportioned between North American and Eurasian clades but only 19.3% among taxa. Conversely, only 9.4% of the overall AFLP variation was apportioned between North American and Eurasian samples and 36.5% among taxa. Circumpolar L. mollis and high-latitude L. innovatus taxa showed mixed ancestry and shared alleles that bridge allopatric Leymus wildryes of the North America and Eurasia.