Expanding on previous work demonstrating that ecological differences between Daphnia habitats are probably driving evolutionary divergence of aging between ecotypes (Dudycha & Tessier 1999; Dudycha 2001, 2003, 2004), we seek to understand the genetic basis for this divergence and correlated life history traits. Like most organisms, aging in Daphnia is phenotypically plastic, with individuals experiencing reduced aging and extended longevity when they occupy a resource-limited environment. We are interested in this both from the perspective of the evolutionary ecology of life histories, and from the perspective of the genetic basis for aging.
Usefully, Daphnia harbor genetic variation of aging, and of the response of aging to resource limitation. We can combine analyses of this genetic variation with ecological knowledge of Daphnia consumer-resource interactions to better understand the evolution of aging and stress responses. We are approaching this with a combination of quantitative genetics, molecular evolution, genomics, functional genetics, epigenetics and fieldwork. Current efforts are directed at identifying genes involved in natural divergence of aging, the mechanistic basis for differences in aging, the role of sex and mutation accumulation in aging, and standing genetic variation within populations.
To better advance our understanding of human aging, our genetic work emphasizes the thousand human genes for which the only known arthropod homolog is in Daphnia. In fact, there are many characteristics that make Daphnia a useful human model with complementary advantages to existing invertebrate models. These include a greater presence of homologs of human genes, higher similarity of human genes for those that are conserved throughout Arthropoda, adult tissue regeneration, and the absense of metamorphosis. Sean Place and Rekha Patel collaborate with us in our research on aging, and make for an exciting interdisciplinary research group. Currently our research is supported by the American Federation for Aging Research and the National Institute on Aging.
Much of our past work has drawn on the notion that trade-offs associated with aquiring, assimilating and allocating resources underpin life history evolution. We thus want to understand evolutionary divergence with respect to resource regime, and how adaptation to different resource environments re-casts the framework for life history evolution. Daphnia are ideal for this. They exist in replicate populations across a habitat gradient that produces major shifts in resource base and competitive pressures on Daphnia: ponds provide rich but dynamic resources and lakes provide poor but stable resources.
All organisms confront the problem of transforming resources into offspring. But should an organism convert those resources in the most efficient manner possible, even if there is a cost to the speed of conversion? Or should an organism convert resources to offspring as rapidly as possible, despite the potential for waste? Ideally, an organism would maximize both traits, but thermodynamics tells us that the power and efficiency of energy conversions cannot simultaneously be maximized. As a consequence, power – the rate at which energy can be turned into useful work, or, in fitness terms, resources into offspring – necessarily trades off against efficiency, the proportion of resources that are converted into offspring. Though this trade-off is less familiar to biologists than the problem of allocating limited resources to competing functions, trade-offs between power and efficiency in resource exploitation are widespread. Power strategists outcompete efficiency strategists when resources are abundant and the marginal cost of acquiring more resources is low. Efficiency strategists win when resources are scarce or acquisition costs become too high. We are also pursuing the genetic basis of such a trade-off, using a combination of physiological analysis, candidate genes and gene-expression profiling. Eventually, we plan to extend this work to field-based experiments. We are collaborating with Barrie Robison’s lab at the University of Idaho on this project to identify genes whose expression is modulated by resource environment, and Rekha Patel's lab to verify functional interactions of candidate genes. Funding for this research comes from the South Carolina Freshwater Initiative and the office of the USC Vice President for Research.
What happens when an unmovable object meets an irresistable force? If that object is a geographically restricted species, and the force is an adaptive landscape shifting in the face of climate change, the result is likely to be extinction. But perhaps species are not unmoveable. It is possible that they are sufficiently plastic in their physiological tolerances at the individual scale, or have sufficient genetic variation at the population scale, to withstand climate change. In a project led by Sean Place, we are seeking to understand the potential of Antarctic fish to respond to the joint effects of temperature rise and acidification in the Southern Ocean. To do this, we are analyzing sequence and expression variation in key metabolic genes in the context of the phylogenetic radiation of Nototheniod fish. For the ecologically dominant species, we are also quantifying standing genetic variation as indicated by neutral markers. If we find that these fish, which occupy one of the most stable and durable environments on Earth, are likely to respond successfully, this bodes well for the many other organisms that are already adapted to much more variable environments.
We have the great fortune to be located near Congaree National Park, one of the largest old-growth forests remaining in the U.S. The park is a forested floodplain, with hundreds of small ponds and oxbows spread across it. Periodically, the entire system floods, and aquatic organisms may find themselves transported to an inhospitable environment. We are interested in how this intermittent connectedness influences population and community structure, and what strategies allow populations to persist in the face of such unpredictable disturbances. Our ongoing studies of demography and population genetic structure have been greatly facilitated by the Park’s research staff, particularly Dr. Theresa Thom.
Our interest in population connectedness is also linked to our interest in divergence among populations occupying different classes of habitats. This is because there is an ongoing tension between divergence on one hand, and persistent gene flow on the other. One of the most fascinating aspects of Daphnia in the pulex species complex is that they are morphologically indistinguishable, show no ecological divergence in simple traits or neutral genetic regions (e.g., allozymes, microsatellites, mtDNA), and yet the complex traits most important to their ecological interactions (e.g., resource exploitation reaction norms and integrated life histories) show strong genetic differentiation. We are interested in understanding the mechanisms that drive these functionally important differences despite ongoing genetic exchange.
Much of our work is directed at the ecological and evolutionary differentiation between populations of D. pulex (temporary ponds) and D. pulicaria (stratified lakes). However, these “species” are known to hybridize in the wild, where hybrids are obligately asexual. This means that females reproduce only asexually, perhaps due to reproductive isolation between the taxa, perhaps due to inheritance of a sex-limited meiosis suppresor. Artificial hybrids, in contrast, are sexually competent, and we have developed a multi-stage model to explain the discrepancy between the wild and the tame. Our goals are to test the assumptions and prediction multi-stage model. As part of this, we are beginning a widespread biogeographic analysis of reproductive isolation and hybrid performance in D. pulex and D. pulicaria, and in collaboration with Bob Friedman are conducting a genome-wide assessment of genetic divergence.
These projects are not the current focus of major efforts in the lab, but could quite possibly become significant if a student was particularly interested in pursuing them.
1. Evolutionary responses to long-term pollution, particularly toxic metals. This is part of a collaboration led by Joe Shaw at Indiana University.
2. Heritability & other quantitative genetic parameters in the wild. The most important consequence of this interest has been an ongoing collaboration with Debbie Roach at the University of Virginia in a long-term project with Plantago laceolata.
3. Molecular & ecological effects of spontaneous mutation accumulation. The potential exists for ongoing work with MA lines I created as a post-doc in Mike Lynch's lab, or for development of new lines.
4. Conservation genetics of the common loon, Gavia immer, in collaboration with Amy McMillan at Buffalo State.
Interested in joining us as an undergrad, grad or post-doc?
Some of our amazing ponds and lakes in South Carolina, Wisconsin and Michigan.
Photos of our favorite Daphnia.
Information on clones used by the Daphnia Genomics Consortium
Protocols for Daphnia maintenance and molecular wizardry.