The work in the Baucom lab integrates across the fields of ecology, evolution, and genetics to understand the mechanisms that underlie the success and persistence of noxious agricultural weeds as well as the evolution of important plant functional traits. While we work on a variety of topics, a major focus in the lab currently is on the problem of herbicide resistance. Our research asks: What is the genetic basis of herbicide resistance, and is it the same across populations? Are there constraints or “brakes” on herbicide resistance evolution? How does the plant mating system influence the evolution of resistance?
We primarily use the common morning glory, Ipomoea purpurea, a noxious agricultural weed of corn, cotton and soy crops in the United States to examine these questions. This species is resistant to glyphosate, the active ingredient in the herbicide RoundUp, which, due to the widespread adoption of RoundUp Ready crops, is the most utilized herbicide in agriculture worldwide. We use a combination of manipulative field experiments, genomic sequencing and bioinformatics applications to address our questions. Field experiments allow us to understand the broad evolutionary patterns that influence the maintenance of RoundUp resistance in nature, while genome sequencing and bioinformatics tools allow us to examine its genetic basis. Below, we describe a portion of the ongoing work in the lab.
What is the genetic basis of herbicide resistance, and is it the same across populations? Because glyphosate is the most predominant herbicide used worldwide, many acres are being exposed to it on a yearly basis. Thus, a very large natural experiment in evolutionary biology is currently being performed with replicated populations of the same weeds exposed to the same herbicide. We can use these natural replicates to address central questions in evolutionary biology and genetics. For example, does the same genetic basis underlie resistance across these geographically separate populations? Is the herbicide selecting to increase the frequency of novel mutations, or are we selecting on ancestral and similar genetic variation?
We have recently identified striking variation for glyphosate resistance among populations of I. purpurea at the level of the landscape—while some populations exhibit high resistance, others still show high susceptibility.
This work has also shown that gene exchange across the landscape occurred prior to the development and widespread use of this herbicide, suggesting that resistance is evolving independently in a mosaic of resistance “hotspots.” We are currently funded by the USDA to uncover the genetic basis of resistance among these replicated resistant populations and will determine if the same or a different genetic basis underlies resistance across the landscape. This work will provide crucial applied information—if novel and different genetic mechanisms underlie resistance across the landscape, a high priority should be placed on preventing gene flow between populations.
Are there brakes on resistance evolution?
Critical prerequisites for the evolution of herbicide resistance in a weed population are the presence of genetic variation underlying resistance and positive net selection acting on resistance. Consequently, a lack of genetic variation could act as a constraint, or “brake” on resistance evolution. Our previous research has identified additive genetic variation—the fraction of the variation that can respond to selection—underlying glyphosate resistance within populations of I. purpurea and evidence for positive selection on resistance in the field. The presence of genetic variation underlying resistance, however, does not guarantee that it will evolve to a higher level in a population. Lower fitness of resistant compared to susceptible plants in the absence of the herbicide (known as “fitness costs”) can also act as a brake on continued increases in the level of resistance.
We have recently identified a fitness penalty associated with glyphosate resistance in I. purpurea: in the absence of herbicide, populations that are highly resistant exhibit very low seed germination and they are also smaller in size, which may influence competitive interactions.
How does the plant mating system influence herbicide resistance evolution?
Unlike most animal species, plants exhibit remarkable variation in their mating system—defined as the relative frequency of cross-fertilization to self-fertilization. Approximately 32% of plant weeds are hermaphroditic, meaning that they have both male and female reproductive organs, and can produce progeny by outcrossing and by selfing. Although the mating system is a key life history trait that influences the quantity and quality of offspring, as well as patterns of gene flow and population structure, the influence of the mating system on the evolution of herbicide resistance within and among populations is virtually unexamined.
Recent work in the lab has identified a negative correlation between I. purpurea‘s outcrossing rate and herbicide resistance: populations that are highly resistant to glyphosate produce seeds from selfing more often than susceptible populations. We hypothesize that this pattern has evolved to preferentially reduce gene flow from non-adapted, susceptible individuals. Other hypotheses, however, could explain this relationship and we are currently developing follow-up work to examine the possibilities.
Genetic basis of ecologically relevant traits in the sweet potato
Sweet potato, Ipomoea batatas, ranks 7th in global food production, with 1.3 and 106 million metric tons produced annually in the US and globally, respectively. This crop is of extreme importance worldwide: it can flourish in many environments, produces high carbohydrate content, and is a vital source of vitamin A and C in developing countries. Unfortunately, genetic improvement in sweet potato has been stymied by the lack of genomic resources—the hexaploid I. batatas has 90 chromosomes and a complicated evolutionary history, making it recalcitrant to genomic investigation. Further, we understand very little about the environmental conditions that influence sweet potato to re-direct resources from the developing leaf canopy to its storage roots. As a result, there are large gaps in our understanding of the genes and the genotype-environment interactions underlying yield in this important orphan crop.
We are developing a multi-faceted research program that will examine resource allocation trade-offs between roots, flowers and the leaf canopy in sweet potato. We will be identifying eco-physiological, phenotypic, and associated transcriptomic differences underlying resource allocation in accessions of this crop across replicated common gardens. We are developing the diploid I. trifida, a close relative of the sweet potato, as a model to understand the genetic basis of leaf number and leaf shape in the sweet potato. This highly collaborative work will identify the conditions that influence a plant to alter its allocation from vegetative growth to reproduction, which can have manifold consequences on sweet potato yield while concomitantly producing genomic resources vital for sweet potato improvement.