Evolution of an Allotetraploid Genus: Relationships among North American Species of Elymus (Poaceae:Triticeae)

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9974181Mason-Gamer The process of speciation is usually characterized as a splitting, during which populations within a species diverge from one another, eventually accumulating enough differences to be classified as separate species. However, new species can also arise when two divergent lineages come together, or reticulate, creating a hybrid lineage. Such reticulation of distinct lineages appears to be far more common in plants than in animals. The most common mechanism of hybridization in plants, polyploidy, involves additive combinations of entire sets of genomes (of chromosomes) from two parents. The resulting polyploid hybrids therefore have more chromosomes than either parent. Polyploidy is rare in some groups of plants, but extremely common in many others. In the grass family Poaceae, for example, which includes the species examined in this research by Dr. Mason-Gamer at the University of Idaho, about 70% of all species appear to be polyploid. Furthermore, polyploidy is involved in the origin of many important crops in the Poaceae, including the wheats, oats, and sugarcane. Because it is so common in many groups of plants, polyploidy deserves increased study to clarify patterns of polyploid origin and diversification. The grass genus Elymus, a member of the wheat tribe Triticeae, serves as a useful system for the study of polyploidy. First, it is an extremely widespread and diverse group of plants, and so illustrates well the evolutionary potential of polyploid species. Second, the genomes of its presumed progenitors are well characterized, particularly for the North American taxa in the diploid genera Critesion and Pseudoroegneria; therefore, the genomes of Elymus are likely to have recognizable origins. Studies of nuclear and chloroplast DNA markers from the North American species of Elymus will further understanding of several questions. Do all North American species of Elymus have the same or similar combinations of genomes? The answers will help to establish the taxonomic and evolutionary boundaries of the genus. Have similar polyploid genome combinations arisen more than once? In other words, can a single species or genus have multiple origins? The idea that one species may have multiple origins adds yet another level of complexity to our views of how species originate, persist, and diversify. While the specific features of polyploidy in Elymus may not necessarily apply to all plants, understanding the processes that have shaped this complex genus will advance knowledge of plant evolution and diversity.