Ph.D. project by Peter Foged Larsen (DIFRES / University of Aarhus)
Supervisors: Einar Eg Nielsen (DIFRES), Dorte Bekkevold (DIFRES), Peter Grønkjær (University of Aarhus) and Volker Loeschcke (University of Aarhus).
Most population genetic studies have concentrated on analysing variation in neutral markers, which provide information about neutral processes such as migration and random genetic drift. However, neutral markers do not provide direct information about population differences in traits of importance for survival, reproduction and adaptation, so-called local adaptations (Waples, 1998). Although little is known of the genetic composition of spatially structured marine fish species, a major future objective in conservation and management of marine fish is to obtain knowledge about the distribution of adaptive genetic variation within and among populations (Waples, 1998).
Many aquatic organisms can adapt to a wide range of physical environments, e.g. different salinity and temperature regimes (Ju et al., 2002). Changes in gene expression are thought to be an important component of adaptation to changes in the marine environment (Schulte, 2001). However, little is known about the molecular mechanisms underlying adaptation to various environmental conditions. In fishes, studies on two populations of killifish (Fundulus heteroclitus) along a steep thermal gradient on the East-coast of North America, showed large variation in gene expression when exposed to temperatures at the extremes of the species’ thermal range (Schulte, 2001). Assuming that much of the variation in gene expression is heritable, this may result from the fish adapting to two different environments, namely cold water in the northern population and warmer water in the southern population, leading to local adaptations to their different habitats (Schulte, 2001; Oleksiak et al., 2002). Gene expression analysis will give us a tool to measure how individual fish and fish populations respond to specific environments and to changes in their environment, e.g. due to global warming.
European flounder (Platichthys flesus) and Atlantic cod (Gadus morhua) are interesting candidates for studying local adaptations, because both species experiences highly variable environmental conditions, in particular with respect to salinity and temperature, throughout their distribution range in Europe.
In this study, flounder and cod from different populations are sampled and acclimatized under experimental conditions. Following a major “common garden” experiment including reciprocal transplantations, measurements of gene expression by microarray analysis are conducted in co-operation with Dr. Williams at the School of Biosciences, University of Birmingham. Dr. Williams and co-workers have developed a species specific microarray for European flounder (Williams et al., 2003) which can be used to screen a large number of genes previously shown to be activated by stress in flounder and cod.
The main objective of this project is to examine the expression of several possible candidate genes from populations of European flounder and Atlantic cod living under different environmental conditions. Expression levels both at rest and in response to stress are examined to describe population structure and subdivision in relation to environmental clines. Moreover, I want to quantify the magnitude of selection and the potential for local adaptations by comparing the level of gene expression among populations to the level of genetic differentiation in neutral microsatellite markers.
If successful, this project will contribute to a deeper understanding of population structure in marine fishes. In addition, gene expression experiments using stress-genes can supply a tool to expand the existing knowledge within the field of local adaptations and help us understand the importance of adaptive markers when describing genetic variation with fitness effects. On a longer timescale the study may help understand consequences of global warming, and point to which species are able to handle stress associated with warming, and for how long they will be able to survive and reproduce.
References
Ju, Z., R. Dunham & Z. Liu (2002). Differential gene expression in the brain of channel catfish (Ictalurus punctatus) in response to cold acclimation. Molecular Genetics and Genomics 268: 87-95.
Oleksiak, M.F., G.A. Churchill & D.L. Crawford (2002). Variation in gene expression within and among natural populations. Nature Genetics 32: 261-266.
Schulte, P.M. (2001). Environmental adaptations as windows on molecular evolution. Comparative Biochemistry and Physiology Part B 128: 597-611.
Waples, R.S. (1998). Separating the wheat from the chaff: Patterns of genetic differentiation in high gene flow species. Journal of Heredity 89: 438-450.
Williams, T.D., K. Gensberg, S.D. Minchin & J.K. Chipman (2003). A DNA expression array to detect toxic stress response in European flounder (Platichthys flesus). Aquatic Toxicology 65: 141-157.