Roberta J. Mason-Gamer
Molecular Systematics of Plants

Department of Biological Sciences, University of Illinois at Chicago



I am an evolutionary biologist studying the phylogenetic history of plants. I am especially interested in the complex reticulate patterns that arise when distinct lineages undergo gene exchange. The grass family includes many examples of reticulate evolution, involving hybridization and allopolyploidy. 

The wheat tribe, Triticeae, includes many examples of hybridization and allopolyploidy. The St genome of Pseudoroegneria seems especially inclined to join other genomes in a variety of allopolyploid combinations. 

Allotetraploid Elymus

Elymus is an entirely allopolyploid genus in the wheat tribe, Triticeae. Elymus comprises several different combinations of parental genome donors, but they all share at least one set of Pseudoroegneria (bluebunch wheatgrass) genomes, designated St

Native North American Elymus species are allotetraploid, combining the Pseudoroegneria genome with that of Hordeum (H; wild barley) in an StStHH configuration. Phylogenetic analyses of North American Elymus based on DNA sequence data from the chloroplast genome, and from nuclear genes encoding starch synthase, phosphoenolpyruvate carboxylase, and beta-amylase, confirm the StStHH genome complement. They further reveal introgression from and Asian Pseudoroegneria species into North American Pseudoroegneria and Elymus.

We have added a selection of Eurasion StStHH Elymus species to determine whether the St and H genomes of the Eurasion species are distinct from those of the North American species, which would indicate separate origins of StStHH tetraploids. The phylogenetic data to date, based on phosphoenolpyruvate carboxylase, beta-amylase, and starch synthase gene trees suggest a single, North American origin for the the Eurasian and North American StStHH tetraploids.

The Y genome of the Asian StStYY tetraploid species is derived from an unknown donor (which may or may not be extant). Phylogenetic analyses of the phosphoenolpyruvate carboxylase, beta-amylase, and starch synthase gene sequences confirm that the St and Y genomes represent distinct evolutionary lineages, and further show that St genes have been acquired from multiple Pseudoroegneria species.

Allohexaploid Elymus repens

This native of Europe and Asia is now widespread throughout the United States. Its phylogenetic affinities within the Triticeae are poorly understood, but earlier cytogenetic studies suggest that it is derived from Pseudoroegneria (St genome) and Hordeum (H genome), resulting in an StStStStHH hexaploid. I have been using data from the chloroplast genome and three nuclear genes (granule-bound starch synthase I, beta-amylase and phosphoenolpyruvate carboxylase) to clarify its relationships to the diploid members of the tribe. Together, the results indicate a complex origin of this enigmatic allohexaploid weed.

Chloroplast DNA - a Pseudoroegneria donor. Results from this maternally-inherited genome suggests that Pseudoroegneria was one of the genome donors to E. repens. (Most St-containing polyploids examined to date have a Pseudoroegneria-like chloroplast genome.) cpDNA tree.

Nuclear genes - three genome donors. The placement of E. repens sequences on the nuclear genes trees is partially consistent with the cytogenetic studies; they show that Pseudoroegneria and Hordeum were genome donors. In addition, however, they all show that a third distinct donor, currently unidentified, was involved in the origin of the hexaploid. Nuclear trees.

Introgression involving the starch synthase gene. In addition to the Pseudoroegneria, Hordeum, and unknown-donor gene copies seen on all three trees, Elymus repens has two additional starch synthase copies. One is nearly identical to another introduced weedy species, Taeniatherum caput-medusae, and the other is derived from the Poeae-Aveneae clade outside of the Triticeae. Because these are unique to one gene tree, they are interpreted as products of introgression. GBSSI trees.

Reticulation and speciation in Coreopsis

Coreopsis section Coreopsis falls within the eastern North American lineage of Coreopsis (Asteraceae), and includes two hypothesized cases of incipient speciation. Coreopsis grandiflora is widespread and morphologically variable, and includes several recognized infrageneric varieties, which might have been derived through a process of gradual, allopatric speciation. Coreopsis nuecensoides and C. nuecensis represent a hypothesized progenotor-derivative species pair, which appears to have arisen through rapid speciation associated with changes in chromospome number and barriers to interbreeding.

Introgression of the chloroplast genome - C. grandiflora. My previous analyses of chloroplast DNA diversity in C. grandiflora reveal that the species is split by a deep polymorphism, representing either lineage sorting or introgression. The polymorphism transcends not only the boundaries of the species, but those of section Coreopsis as well. The distribution of the polymorphism corresponds better to geographic distribution than to varietal identity. However, because the pattern of cpDNA diversity is strongly dominated by variation that predates the species's origin, additional data from nuclear genes are needed to address whether the C. grandiflora is undergoing allopatric speciation.

Rapid speciation - C. nucensoides and C. nuecensis. These species are narrowly distributed in adjacent ranges. They are intersterile and differ in chromosome number, and C. nuecensis is hypothesized to have airsen from C. nuecensoides through rapid speciation associated with the sterility barrier. Chloroplast DNA diversity are consistent with a progenitor-derivative relationship between the species, but they are inconsistent with a reduction in population size during the founding of the derivative.

Ongoing work. (1) I am broadening the phylogenetic extent of the cpDNA data set in order to fully characterize the extent and depth of the ancestral polymorphism that splits C. grandiflora and section Coreopsis. (2) I am sequencing several nuclear markers to determine whether they show comparable ancient polymorphisms. (3) Assuming the nuclear DNA data are not overwhelmed by ancestral variation, they will be used to further examine speciation in C. grandiflora, C. nuecensoides, and C. nuecensis.

For more information on ongoing projects, visit the other People in the Lab.

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My Plants

Elymus - a large allopolyploid genus in the wheat tribe

Elymus canadensis at the Brookfield Zoo

On either side the river lie 
Long fields of barley and of rye,

That clothe the wold and meet the sky
-Alfred Lord Tennyson, 1842. "The Lady of Shalott"

Ceres, most bounteous lady, thy rich leas
Thy turfy mountains, where live nibbling sheep,
And flat meads thatch'd with stover, them to keep.

-William Shakespeare, 16??  The Tempest, Act 4, Scene 1

You'll remember me when the west wind moves
Upon the fields of barley
You'll forget the sun in his jealous sky
As we walk in fields of gold

-Sting, 1993.  "Fields of Gold"

Coreopsis - the focus of many evolutionary studies
Coreopsis grandifloraCoreopsis nuecensis
Photos of C. grandiflora (by James Manhart) and C. nuecensis (by Hugh Wilson) are from the Texas A&M Image Gallery, accessed via the Texas A&M Flowering Plant Gateway.

James Thurber, 1941.  "The Secret Life of Walter Mitty

Elymus research: supported by the National Science Foundation DEB-9974181 and DEB-0426194.
research: supported by the National Science Foundation BSR-9105167.


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