RESEARCH

Developmental plasticity and flower morphology

Genetic mechanisms of adaptation to crowding

Circadian rhythm and fitness in heterogeneous environments

Molecular evolutionary genetics of crop and weed responses to crowding

Domestication of wild species is characterized by changes in diverse phenotypic traits, including inflorescence architecture, fruit and seed morphology, infructescence shattering, and the timing of developmental events. These changes result from human selection to increase the harvest index and harvest efficiency. Selection for appropriate environmental responses is also important in this regard. For instance, higher planting densities increase yield in many crop species, but densities are typically limited by developmental responses to crowding, in which time to flowering decreases and relative allocation to structural organs increases at the cost of agronomically desirable traits such as leaf, root, and fruit production. Regardless of the trait selected in a crop species, agricultural weeds exhibit dramatic and rapid compensatory evolution that increases weed fitness at the cost of crop productivity. In general, agricultural practices impose strong and predictable directional selection across many generations, and therefore comprise an ideal experimental setting to address many basic evolutionary questions. Using comparative genetic and genomics tools, we (my collaborator Julin Maloof and I) are investigating the level at which selection acts (individual vs. stand), rates of evolutionary response, and the genetic basis of adaptation to crowding.

In response to crowding, plants exhibit a range of phenotypic responses, including accelerated flowering, increases in stem and petiole elongation, elevated leaf angles, and reduced branching. These phenotypic responses are collectively known as "shade-avoidance" responses and are stimulated by shifts in the ratio of red to far-red wavelengths (R:FR) associated with neighbor proximity. Specifically, light transmitted through a vegetative canopy has reduced R:FR levels relative to full sunlight, because chlorophyll selectively absorbs R. Similar changes in the R:FR ratio are observed in light reflected off of neighboring plants; a plant 12-cm tall can reduce R:FR levels 30-cm away. Thus, plants experience decreases in R:FR prior to direct shading by neighbors, and this shift in light quality provides a reliable indication of neighbor proximity and future competition for sunlight.

This avenue of my research uses both Brassica rapa and the genetic model, Arabidopsis thaliana . In addition to the morphologically diverse domesticated varieties of B . rapa , naturalized populations occur in both agricultural fields and disturbed, weedy sites. Thus, this system is ideal for investigating both the loci targeted during domestication and those underlying adaptation of weed species to agricultural settings. Moreover, the near relative, Arabidopsis thaliana, is a model for the genetic characterization of shade-avoidance responses, facilitating the identification of orthologous loci under selection in B . rapa .

Arabidopsis thaliana

crowded