Research in the Hileman lab

In the Hileman Lab, we take an integrative approach to investigate how the evolution of developmental patterning contributes to floral diversification.  Changes in developmental programs are fundamental to morphological evolution, but we are just beginning to understand the nature of these changes.  By integrating phylogenetic, molecular evolutionary and molecular developmental approaches, we aim to understand patterns of floral evolution, the underlying genetic mechanisms leading to innovations in floral form, and how diversification of gene families affects the evolution of developmental genetic programs.

The role of symmetry genes in establishing petal and stamen phenotypic diversity
Funding: National Science Foundation 9/1/06-8/31/09

We are studying how the developmental genetic interactions that specify bilateral flower symmetry have diversified among close relatives of the model species snapdragon (Antirrhinum majus).  Major research objectives include 1) determining the extent to which the developmental genetic program specifying bilateral symmetry is conserved, 2) determining the role of gene duplication/loss in the diversification of this developmental program, and 3) determining how regulatory and/or functional changes in symmetry genes are responsible for the evolution of floral form among close relatives of snapdragon. 

Bilaterally symmetrical flowers have evolved multiple times within the angiosperms, and evolutionary transitions from radial to bilateral symmetry are considered to have enhanced the utilization of diverse and reliable pollinators through increased specialization.  Lamiales s.l. is a lineage of flowering plants that is a focal point for evolutionary, ecological and developmental genetic research on the causes and consequences of floral symmetry.  Nearly all species of Lamiales develop bilaterally symmetrical flowers with 5 stamens.  However, Lamiales forms a large and diverse lineage, and there have been multiple evolutionary modifications to the bilaterally symmetrical floral plan.  These include dramatic reversals to radial symmetry, differences in corolla tube length, shifts in petal lobe size and symmetry, and fusion or near fusion of petal lobes.  Replacement of the 5th, dorsal stamen with a residual staminode is a common phenotype in Lamiales, but there have been multiple evolutionary reversals to a 5-stamen condition, as well as continued stamen loss through reduction of one of the two remaining stamen pairs.  These evolutionary shifts in floral morphology likely reflect selection for effective and specialized plant-pollinator interactions, and represent the floral diversity that form the foundation of this research project.

Many of the floral modifications just described are found within Veronicaceae (Lamiales), the family to which the model species, snapdragon, belongs.  Evolutionary novelties within Veronicaceae that are of specific interest for my research program include: reductions in stamen number in the lineages Mohavea, Gratiola and Veronica, shifts in the symmetry of Mohavea dorsal petals, and fusion of the dorsal petals into a single organ in Gratiola and Veronica.  Genes that specify bilateral flower symmetry (identified through developmental genetic work in snapdragon) are good candidate genes for explaining aspects of these diverse floral phenotypes.  These include the dorsal flower identity genes, CYCLOIDEA (CYC), DICHOTOMA (DICH), and RADIALIS (RAD), as well as the ventral identity gene, DIVARICATA (DIV).

Currently, we are taking a two-pronged approach to study the role of symmetry genes in the evolution of floral diversity within Veronicaceae.  First, using both expression (quantitative rt-PCR and in situ mRNA hybridization) and functional (virus-induced gene silencing, VIGS) analyses, we are determining the extent to which the genetic program establishing bilateral flower symmetry has diversified among close relatives of snapdragon including Mohavea, Veronica and Gratiola.  As described above, significant advances have recently been made towards understanding the network of genetic interactions controlling dorso-ventral symmetry in snapdragonflowers.  This includes an understanding of the genetic interactions among the dorsal identity genes, CYC, DICH and RAD, and how the dorsal identity genes interact genetically with the ventral identity gene, DIV.  However, the extent to which this network of genetic interactions is conserved remains unclear.  Second, also using expression and functional analyses, we are investigating the specific role that CYC-like genes have played in shaping dorsal petal morphology and variation in stamen number in Mohavea, Veronica and Gratiola.  Ultimately, by focusing on taxa exhibiting evolutionary innovations in floral organ number, we aim to determine how changes in the expression and/or function of genes controlling dorso-ventral flower symmetry have contributed to phenotypic novelties. 

 

In addition to the major research objectives described above, we are pursuing a number of other projects; two of these projects are centered on Mimulus guttatus, which is emerging as a model ecological and evolutionary species in the Lamiales, with the completed genome sequence expected by the end of the year.

Symmetry genes in Mimulus

We are using a combined bioinformatic and phylogenetic approach to recover all putative orthologs of the symmetry genes from the nearly complete M. guttatus genome sequence.  We have identified a number of putative orthologs of these symmetry genes that arose from gene duplication events along the lineage leading to Mimulus.  To generate an understanding of the role these genes play in Mimulus flower development, we are using detailed floral dissections and rt-PCR to assay spatial patterns of gene expression.  We will further investigate the role of these genes in Mimulus flower development using in situ mRNA hybridization and reverse-genetic functional approaches.

Maternal inheritance of trichome patterning in Mimulus

In collaboration with John Kelly at the University of Kansas, we are exploring the genetic basis for maternal inheritance of trichome patterning in lines of M. guttatus.  The general observation is that in some M. guttatus lines, leaf-wounding leads to increased trichome density on subsequent leaves, and this pattern of increased density is inherited by the progeny of the wounded parent (a maternal effect).  Using a combined bioinformatic and phylogenetic approach, we have identified M. guttatus orthologs of candidate genes involved in trichome development.  We are using quantitative rt-PCR to assay levels of expression of these candidate genes in the progeny of wounded and control plants.  Based on the results of these data, we will explore possible alternatives for the maternal inheritance of gene expression levels.

Gene duplication and the diversification of developmental networks

Gene duplication, and the subsequent evolution of duplicated genes, likely plays an important role in shaping the evolution of developmental networks, and may be key for shifts in developmental patterning. In the Hileman lab we are interested in dissecting the fate of duplicate genes using multiple approaches including studies of gene family molecular evolution, as well as studies of gene expression and protein function.

Virus-Induced Gene Silencing: a great new tool for plant evo-devo research