National Science Foundation, DEB 0343735: Phylogenetic relationships and character evolution in tribe Lycieae (Solanaceae)

This National Science Foundation funded project (with co-PI Dr. Rachel A. Levin) examined evolutionary relationships among three closely-related genera (Grabowskia, Lycium, Phrodus) in tribe Lycieae in the tomato family (Solanaceae).  We traveled widely with students to document distributions and collect species in this group. 

National Science Foundation, DEB 03437352: Architectural effects and the analysis of sexual size dimorphism

This National Science Foundation award examines the importance of architectural effects to the expression of floral sexual size dimorphism.  Unisexual flowers have evolved repeatedly among angiosperms, and male and female flowers frequently show sexual size dimorphism, with one gender having larger flowers.

An important component of flower size variation is floral position within inflorescences and inflorescence position within the overall architecture of the plant.  Despite this positional variation, the role of architecture and its potential importance in explaining sexual size dimorphism has not been explored. The overall goal of this research is to explore the hypothesis that positional effects can contribute to the expression of sexual dimorphism.

This project is in collaboration with Dr. Pam Diggle in the Department of Ecology and Evolutionary Biology at the University of Colorado, Boulder.

Molecular evolutionary genetics of mating system genes and mate choice in plants

In the Solanaceae, gametophytic self-incompatibility (GSI) is controlled by two tightly linked genes.  The first gene (S-RNase) is expressed in the style, whereas the second gene (SLF) is expressed in the haploid pollen grain. When the haploid specificity of the pollen grain matches that of either of the two S-RNases expressed in the pistil of the maternal parent, pollen tube growth is terminated.  Thus, under GSI, rare alleles at the S-RNase locus have a selective advantage and negative frequency-dependent selection leads to high levels of allelic diversity in natural populations.

Our lab studies the population genetics and molecular evolution of mating system genes in natural populations of Lycium and in the model system Solanum.  In particular, we are interested in using such data to make inferences about the maintenance (or loss) of self-incompatibility.  These genes also have useful properties that make them appropriate for making historical inferences regarding patterns of plant dispersal and ancestral mating systems.

Reproductive ecology of Lobelia (Lobeliaceae)

This field project (June-September) investigated the reproductive biology of two species of Lobelia native to Massachusetts.  Lobelia spicata and L. siphilitica have gynodioecious populations, in which two mating types are present: hermaphroditic or female plants. 

More information about this system is available here.

Evolutionary diversification of andromonoecy in Solanum section Lasiocarpa (Solanaceae)

In collaboration with Dr. Pamela K. Diggle at the University of Colorado, I investigated the origin and diversification of andromonoecy, a sexual system in which plants possess both hermaphroditic and staminate flowers. 

Additional information is here.

Evolution of gender dimorphism (separate sexes) in Lycium (Solanaceae)

My dissertation research in the Department of Ecology and Evolutionary Biology at the University of Arizona focused on evolutionary transitions between hermaphroditic and dimorphic sexual systems. 

Plants of North American Lycium californicum, L. exsertum, and L. fremontii are either male-sterile or morphologically hermaphroditic, and populations are functionally dioecious.  Flowers on hermaphrodites are larger and have broader calyces and corollas than those on females (Miller & Venable 2003), and controlled pollinations and allozyme estimates of mating systems in the cosexual relatives of dimorphic species indicate that gender dimorphism has evolved on a background of self-incompatibility.  In contrast, studies of pollen tube growth indicate that hermaphrodites in the dimorphic species are self-compatible (Miller & Venable 2002).

Within the genus, gender dimorphism is strongly associated with the presence of polyploidy (Miller & Venable 2000).  Indeed, all the dimorphic taxa within Lycium are polyploids, whereas cosexual species are diploids.  We suggested a novel hypothesis for the evolution of separate sexes involving polyploidy.  In this scenario, polyploidy disrupts self-incompatibility leading to inbreeding depression.  Subsequently, male-sterile mutants invade and increase because they are fully outcrossed.

Most recently, our lab has documented polymorphism among populations of Lycium californicum; some populations are polyploid and dimorphic, whereas others are diploid and hermaphroditic (Yeung, Miller et al. 2005).  Despite this variation, phylogenetic analyses including eleven diploid and seven polyploid accessions have found strong support for the monophyly of this species (Yeung et al. 2005; Levin et al. 2007).  Such intraspecific variation in both sexual system and ploidy provides a rare opportunity to distinguish factors important in shaping reproductive traits.

Link to publications from this work here.

National Science Foundation, DEB 0843364: Fine scale phylogenetic relationships in Lycieae (Solanaceae): a multilocus approach for understanding biogeography, polyploidy, and reproductive evolution

The field of phylogenetic systematics has been transformed recently by the use of multilocus and genomic data, and this National Science Foundation funded project (with co-PI Dr. Rachel A. Levin) will continue to develop genomic resources and test newly developed methods for phylogenetic inference. The plant genus Lycium has emerged as an excellent model system for understanding the evolution of hybridization, polyploidy and reproductive strategies, and a robust understanding of relationships will facilitate the study of these characteristics. Read more here.

Browse research projects in the lab...

   Broadly, my research interests are in the ecology and evolution of plant reproductive systems.  In particular, my most recent research has considered two features that promote outbreeding in plants: the evolution of separate (as opposed to combined) sexes and the evolution of physiological mechanisms that prevent self-fertilization.  I am interested in the evolutionary and population genetic histories of plant populations, especially as these topics relate to the evolution of plant mating systems. 

   In addition, I am interested in the inference and interpretation of phylogenetic histories, the impact of hybridization on plant speciation (and mating systems), and comparative studies of features that accompany transitions in sexual strategies, such as the evolution of floral sexual dimorphism or the temporal/spatial segregation of gender function in flowers.  I also maintain an interest in the development of floral morphologies and the role of plant architecture in molding these features.

   My research has focused in large part on the plant genus Lycium (Solanaceae), which has proven a useful system in which to study mate choice in plants.  Members of this genus vary both in the deployment of sexual function (i.e., some species and populations are hermaphroditic, whereas others have separate sexes), and in the presence of genetically controlled self-incompatibility systems.  This group is also interesting from a molecular systematic perspective given its cosmopolitan distribution, species richness, patterns of hybridization (coupled with variation in ploidy levels), and diverse reproductive systems.