John Stanton-Geddes
Evolutionary Ecology Research
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Current Research
Genetic basis of plant - microbe interactions
Mutualisms are one of the more remarkable aspects of ecological communities because two completely unrelated individuals cooperate for each other’s benefit. However, considerable variation has been found in many mutualisms, with some individuals providing large benefits to their partner while others provide little or no benefit. Understanding the genetic basis of this phenotypic variation is a fundamental goal in legume-rhizobia research. This work is of applied importance because legumes are agriculturally important for their ability to acquire nitrogen from rhizobia
In my postdoctoral research, I am working with Drs. Peter Tiffin and Nevin Young to address this at the molecular level. I am using whole-genome resequencing data to uncover the genes that mediate the mutualism between the legume Medicago truncatula and its symbiotic rhizobia. Phenotypic data on the mutualism was collected from greenhouse experiments using 250 plant lines and two different rhizobia strains, and we are analyzing the data using association genetics methods. Our results will reveal the proportion of phenotypc variation that can be explained by sequence variants, and their frequency and distribution across the genome. See the project website for more information.
Mutualisms and species' range expansion
From my PhD work (see below), I found that the availability of suitable mutualists may be limited for the legume Chamaecrista fasciculata. I am currently exploring the extent to which this mutualism has evolved among species in the genus Chamaecrista and the reciprocal influence the symbiosis may have had on each species distribution.
Past Research
Evolution and ecology of species' range limits
Species range limits are determined by a combination of historical, ecological and genetic processes. Comprehensively understanding the relative role of these processes in limiting any single species’ range has been elusive. Paleoecological research has shown that range shifts have been a primary response of species to past climate change, but whether they can track current rates of climate change in fragmented environments is unknown. Thus, understanding the potential of species to shift their ranges in response to contemporary climate change is of fundamental importance. For my doctoral research, I examined the ecological and genetic factors that limit species range expansion.
For this work, I used the widespread native annual legume Chamaecrista fasciculata. Both abiotic and biotic factors are hypothesized to reduce individual fitness below replacement beyond the range edge, but few studies have performed the transplant experiments necessary to demonstrate that species range edges are limited by these factors. Further, the relative importance of abiotic and biotic factors is likely to differ across different range edges. To examine the role of these factors in setting range limits, I established transplant experiments beyond C. fasciculata's northern and western range edges and manipulated the presence of neighbors. This experiment revealed that fitness was reduced below replacement beyond both range edges, even in the absence of competition (Stanton-Geddes et al, 2012). Further, I tested for the both the abundance and effect of rhizobia (soil-dwelling microbes that fix nitrogen in exchange for carbon) within, at and beyond the range edge. I found that compatible rhizobia are nearly absent beyond the range edge, and that plant fitness is reduced without rhizobia present (Stanton-Geddes & Anderson, 2011). To my knowledge, this was the first demonstration that the disruption of a mutualism could potentially limit species’ range expansion in a continuous environment.
The ecological factors associated with a decrease in fitness beyond the range edge are proximate explanations. The ultimate causes of range limits are the genetic factors that prevent adaptation to conditions beyond the range edge. A commonly cited hypothesis is that gene flow from interior population consistently introduces alleles that are maladaptive at the range edge. I tested this hypothesis by sequencing nine loci in 68 individuals from four populations at different geographic range locations. My results show that populations at different range locations are highly diverged, with little indication of high levels of gene flow between populations. Overall, this work indicates that the range of C. fasciculata is not limited by dispersal or gene flow, but by the joint effects of other ecological (e.g. mutualists) and genetic factors.
Some pictures of my work
Chamaecrista seedling in one of my common gardens
Having fun searching for seedlings in the field
