We are interested in the evolution of genetic mechanisms that control the astounding diversity of morphology in plants. Our lab is currently pursuing projects related to the evolution and development of floral and inflorescence morphology in the grass family. We use a combination of functional and comparative approaches to:
- Identify genetic pathways regulating plant morphology
- Investigate how these pathways have evolved to generate morphological diversity.
While these approaches could be used on any number of interesting plant groups we focus primarily on the grasses (Poaceae), although we have recently become interested in Gilia (Polemoniaceae) as well. The grass family has a number of characteristics that make it ideal for functional and comparative studies of plant development, including a large number of species and substantial morphological variation. Many grass morphological characters are highly derived relative to other angiosperms, making it possible to investigate the evolution of novel morphologies. Several grass model genetic species are fully sequenced, including rice (Oryza sativa), maize (Zea mays), and Brachypodium distachyon, which together represent the majority of grass phylogenetic diversity. Additionally, all the cereal crops (rice, maize, barley, wheat, rye, sorghum, etc.) are grasses and insights gained from developmental genetic studies often have important agronomic implications.
The Evolution of Bract Suppression
When growing vegetatively plant development is dominated by leaves. However, the transition to reproductive development (flowering) is accompanied by dramatic changes to plant morphology including leaf growth. The leaves that are produced after the transition to flowering are called bracts, and while bract growth is seen in many plants, it is also frequently suppressed. The ability to suppress bracts has evolved independently in distinct angiosperm families such as the Brassicaceae (the mustards) and Poaceae (the grasses). Since both grasses and Brassicaceae have well-developed model species, this presents an excellent opportunity to investigate how convergent morphologies evolve at a molecular level. Are the same pathways co-opted repeatedly, or have unique pathways been recruited to a novel bract suppression function in some lineages? We are taking a forward genetics approach to identify genes controlling bract suppression in maize. At least five mutant loci have been identified that no longer suppress bracts. These mutants are called tassel sheath (tsh) because the tassels tend to be ensheathed by prominent bract growth that is normally suppressed. We have cloned tsh mutants (tsh1 and tsh4) and interestingly they encode genes that have no direct bract suppression role in Arabidopsis. Furthermore, genes known to regulate bract suppression in Arabidopsis, such as LEAFY have no such function in maize. This suggests that distinct bract suppression mechanisms are involved in these two groups. We plan to further investigate the maize bract suppression pathway by cloning and characterizing other tsh loci.
Genetic Regulation of Tiller Growth in Maize
Tillers are vegetative branches that grow at ground level in many grass species. During the domestication of maize, tiller growth was selected against resulting in a main central stalk with few to no tillers. The teosinte branched1 gene is known to regulate tiller growth and was a major target of selection for reduced tiller number in maize. We have identified another gene, grassy tillers1, that also regulates tiller growth and shows evidence of selection during maize domestication. Interestingly, both grassy tillers1 and teosinte branched1 act in a pathway to regulate tiller growth in response to shade signals.In order to identify additional factors regulating tillering we have begun a genetic characterization of additional tiller mutants, including a screen for enhancers and suppressors of the grassy tillers1 and teosinte branched1 mutants. Ultimately we hope to work out the network regulating tiller growth, as well as components of the network that were selected to decrease tiller number in maize. In collaboration with colleagues and Oklahoma State University, UC Berkeley, and West Virginia University we hope to compare the tillering network in maize with other closely related domesticated cereal crops including sorghum and setaria.
Morphological Evolution in Gilia
Gilia is a genus of annual flowering plants, common in the American west that facilitated classic studies on plant speciation by Verne Grant. In collaboration with Leigh Johnson, we have begun taking a closer look at two closely related species of leafy-stemmed gilia: G. capitata and G. yorkii. G. capitata is a widespread species found from Baja California to British Columbia with a tight capitate inflorescence, while G. yorkii is endemic to the monarch wilderness of the Sierras and has a much more open inflorescence. Despite their distinctive morphologies, these species form a fertile hybrid, and F2 mapping populations can be used to identify genetic factors controlling the distinct morphologies of these species. We have generated transcriptomes for both species and are currently producing a genetic map. We are generating advanced populations (e.g. back-cross and recombinant inbred) in order to begin genetic characterization of the morphological differences in these species.
Grass Flower Development
Grasses have flowers with highly derived floral organs. We are interested in how these flowers are patterned. Which aspects of floral development are conserved with other flowering plants like Arabidopsis? Which genetic mechanisms regulate the novel grass-specific floral morphologies? We are taking a forward genetic approach to identify novel maize floral mutants. We expect that many of these mutants will encode for genes known to regulate floral organ identity such as MADS-box genes. Comparing the mutant phenotype and expression pattern with orthologous genes in Arabidopsis will shed light on conserved mechanisms as well as unique functions of the grass genes. In addition we hope to identify genes regulating morphological traits specific to grass flowers.