RESEARCH
Outline of research activities of the “Center for Environmental
Stress Research” (ACES) at the University of Aarhus, headed by
Professor Volker Loeschcke and funded by a center grant from the Danish
Natural Sciences Research Council:
Evolutionary Implications of Environmental Stress: an Ecological
and Evolutionary Functional Genomics Approach
Summary
We aim at studying the evolution of resistance to environmental stress
and its implications at different levels of biological organization.
Stress influences cellular processes, an individual's physiology, genetic
variation at the population level, and the process of natural selection.
To investigate these different, though highly connected levels of stress
effects, the stress group intends to integrate approaches from ecology,
evolution, ecophysiology, molecular biology and genetics. For investigating
the mechanisms of stress resistance, how resistance evolves and what
conflicting factors constrain its evolution, the center will use well-defined
model systems. Experiments will be conducted with Drosophila melanogaster,
an organism well studied both in genetics and evolutionary biology and
with thermal stress as a stress model.
We will identify quantitative trait loci for thermal stress resistance,
and investigate DNA-sequence variation in candidate genes and its relation
to variation in stress resistance. Population studies will be undertaken
to study genetic differentiation as a response to environmental stress,
rates of evolution, and synergistic effects of different stress factors.
The interactions between longevity and stress resistance will be investigated
together with the effects of repeated mild heat shocks on aging processes.
The kinetics of cellular responses to stress and their alteration by
artificial selection will be studied by expression profiling. Complementary
to the cellular physiology, we will investigate the kinetics of physiological
and biochemical changes in whole organism occurring during acclimation
to thermal stress. With this highly integrated multidisciplinary approach
we will be able to test novel hypotheses on the role of heat shock proteins
and membranes for stress resistance, the role of genetic variation in
stress response mechanisms for evolution and on the interaction between
stress resistance and longevity.
Research plan
Introduction
There is a growing awareness that environmental stress has played
and still plays a significant role in the evolution of biological systems,
from the level of the gene to that of the ecosystem. In this context
environmental stress is regarded as an «environmental factor causing
a change in a biological system, which is potentially injurious»
(Hoffman and Parsons, 1991, Evolutionary genetics and environmental
stress, Oxford Sci. Publ., Oxford). Recent developments in molecular
genetics have stimulated even further interest in stress responses,
while detailed study of stress responses has revealed that most organisms
have evolved sophisticated mechanisms to cope with different environmental
stresses. Examples of such mechanisms include heat shock proteins to
counteract thermal and other stresses, mixed function oxidases to degrade
xenobiotics, DNA repair systems to protect the genome, and the major
histocompatibility complex to fight biotic attacks.
In the last century, increasing levels of environmental stress have
been inflicted on the biosphere at a global scale by the human population.
This has caused and will increasingly cause major environmental changes,
such as climatic shifts, chemical pollution, and habitat destruction.
The size and rate of these changes continue to threaten life on this
planet. Habitat destruction has caused an accelerating rate of species
extinction, global warming may exert thermal and desiccation stress,
and the consequences of chemical pollution are yet unforeseeable. Understanding
the nature and consequences of environmental stress at a global level
from an ecological and evolutionary perspective is of paramount importance
for the development and evaluation of countermeasures. To gain this
understanding, in depth investigations of the mechanisms that allow
organisms to cope with environmental stress are essential.
In order to investigate stress response mechanisms, collaboration between
evolutionary biologists, ecophysiologists and molecular geneticists
is required. These disciplines are concerned with the same questions,
asking how organisms have adapted and acclimatize to environmental stress.
Traditionally these disciplines are not combined and therefore there
is a considerable potential for major advances in the understanding
of stress adaptation by bringing these approaches together.
Thermal stress is used as the model stress system, as most organisms
are exposed to varying temperatures in time and space and occasionally
even to thermal environments that induce a stress response. In addition,
it appears that stress responses are similar for very different forms
of stress and thus results obtained for a thermal stress model system
can be extrapolated to other environmental stresses. Using Drosophila
melanogaster as a model organism for this multidisciplinary investigation
bears the advantage that we can draw on a wealth of investigations that
make such an approach possible. The Drosophila genome project has just
finished and provides the full genome sequence along with annotations
to a large number of genes (Adams et al., 2000, Science 287:2185-2196).
We will study the evolutionary genetics of stress acclimation and adaptation,
as well as the physiological consequences of environmental stress. We
hypothesize that besides protein stability and functioning, maintenance
of membrane fluidity will be important for flies to withstand temperature
and desiccation stress. Also, thermal stress might cause a failure of
translational termination in protein synthesis causing C-terminal protein
extension, which can substantially affect protein properties. Study
of gene expression pattern of the stress response will provide an overview
of the primary cellular responses to stress. By comparing the immediate
and the evolutionary genomic response we will not only be able to corroborate
the physiological investigations and identify yet unknown mechanisms,
but also be able to judge which physiological processes do respond both
to artificial evolution and natural selection. One of our working hypotheses
is that the well known stress response genes, i.e., the heat shock genes
will, besides maintaining the proteome in working order, initiate the
expression of genes that in turn provide resistance in a more direct
way. By investigating which parts of the genome contribute most strongly
to stress adaptation in artificial selection we will be able to tell
which of the stress response genes will evolutionarily respond and which
may underlie genetic or functional constraints that in turn will constrain
the evolutionary trajectory. Genetic variation in the stress resistance
genes is necessary if the response is to evolve adaptively by natural
selection. However, as has been shown by Rutherford and Lindquist (1998,
Nature 396:336-342), that existing genetic variation may only manifest
itself in the phenotypes under environmental stress. Therefore, we will
investigate the influence of stress on evolutionary rates to test the
hypothesis that stress promotes or constrains adaptive evolutionary
processes.
This brief account of our research plan, with its high degree of subproject
integration, defines the newly emerging field of ecological and evolutionary
functional genomics (EEFG). EEFG is a multidisciplinary approach to
study mechanisms and constraints of evolution. It reunites genetics,
molecular biology, biophysics, biochemistry, physiology, evolutionary
biology, and ecology and will bring ecology and evolution into the genomics
and post-genomics era. The quest of EEFG is to investigate which genes
are accountable for acclimation and adaptation on both an ecological
and evolutionary time scale.
To achieve our research goals, we will investigate molecular damages
incurred by stress (sections 2 and 6), map quantitative trait loci responsible
for stress resistance (sections 1 and 2), perform life-history trait
analyses (section 3), investigate the cellular physiology of whole flies
(section 4), identify biochemical changes that contribute to resistance
(section 5) and study candidate gene expression (sections 2, 4, 5 and
6) to investigate the evolution of stress resistance. Thus, we will
contribute to the body of knowledge of relationships between genes,
functions of their protein products, interactions of proteins in biochemical
pathways and cellular structures, and the intracellular interactions
contributing to the fitness of an individual in its specific environment.
To obtain a high ecological and evolutionary significance of our investigations
we plan to carry out most studies with whole flies and to investigate
the genetic and physiological kinetics of the response aspects that
have been largely neglected in the field. Using natural and artificially
selected genetic variants as well as genetically engineered flies we
want to establish gene to function relations for genes involved in stress
resistance.
Outline
The stress group in Aarhus focuses on six main projects to study the
implications of environmental stress on different levels of biological
organization using a multidisciplinary approach. We aim at contributing
to a broad understanding of the evolution of stress response mechanisms
and their role for evolution of other traits.
- Identification and analysis of quantitative trait loci
for thermal stress resistance and their possible impact on longevity
in Drosophila melanogaster
Objective: identify QTLs for heat resistance and, additionally,
to test if these QTLs affecting heat tolerance also affect traits such
as cold tolerance, desiccation tolerance or longevity.
- DNA-sequence variation in candidate genes and its
relation to variation in stress resistance
Objective: elucidate the significance of genetic variation
at specific loci for stress resistance.
- Population studies on quantitative genetic variation
in thermal resistance, rates of evolution and multiple stresses.
Objective: assess rates of evolution under environmental stress,
intra-specific variation in thermal resistance traits and the possible
non-additive effects of different stress factors.
- Functional Genomics
- Genomic responses during acclimation to stress
Objective: obtain a comprehensive overview of changes in gene
expression in D. melanogaster to heat, cold, desiccation and
oxidative stress with special emphasis on resolving the temporal patterns
of gene expression
Proteomic responses during acclimation to stress
Objective: investigate the protein changes involved in providing
the immediate stress response and the development of resistance.
- Physiological responses during acclimation to climatic
stress
Objective: investigation of the physiological changes in insects
during acclimation to drought, cold and heat stress.
- Modulating aging and longevity in Drosophila
by repeated mild heat stress
Objective: determine the effects of single and repeated mild
and severe heat shock on the survival, reproduction, aging and longevity
of Drosophila.
