Grantee Research Project Results
Final Report: How Likely is it That Fish Populations Will Successfully Adapt to Global Warming?
EPA Grant Number: R829420E02Title: How Likely is it That Fish Populations Will Successfully Adapt to Global Warming?
Investigators: Klerks, Paul L. , Leberg, Paul L.
Institution: University of Louisiana at Lafayette
EPA Project Officer: Chung, Serena
Project Period: June 10, 2002 through June 9, 2004 (Extended to June 9, 2006)
Project Amount: $121,598
RFA: EPSCoR (Experimental Program to Stimulate Competitive Research) (2001) RFA Text | Recipients Lists
Research Category: EPSCoR (The Experimental Program to Stimulate Competitive Research)
Objective:
A major factor determining the long-term ecological effects of global warming is whether organisms will be able to genetically adapt to global warming. Successful adaptation would mean that global warming does not displace species from their current habitats. Distribution shifts and extinctions would occur if the organisms fail to adapt to deleterious effects of global warming. At present there is insufficient information to predict almost any species’ evolutionary response to climate change. The issue of adaptation to increased water temperatures is of special importance to the southeastern United States, as organisms in warm waters may already be living close to their temperature tolerance limit, and because of the importance of fishery resources to the region’s economy. In this research project we investigated how likely it is that fish populations will successfully adapt to temperature changes associated with global warming.
The potential for adaptation to elevated water temperature in fish was assessed using different approaches. First, we compared heat tolerance among fish populations from sites with different temperature regimes (normal, cool-water springs, and thermal effluent) to see whether adaptation had occurred successfully in these natural populations. Second, we determined the potential of least killifish (Heterandria formosa) populations to adapt to elevated water temperatures by determining the presence of genetic variation in heat tolerance in the populations. Third, we further assessed the potential for adaptation in this species by performing selection experiments, in which replicate populations were subjected to selection for an increased heat tolerance. The heritability assessments and selection experiments were done for both normal populations and populations that had undergone a drastic reduction in population size (a population bottleneck). Microsatellite markers were used to quantify the loss of genetic variation incurred in the bottlenecking process.
Summary/Accomplishments (Outputs/Outcomes):
As microsatellite markers had been used in related species but not in H. formosa itself, we started with the development of the microsatellites in H. formosa. We identified seven primers that worked consistently for the least killifish and had considerable variation (3-7 alleles). We used the seven primers to quantify genetic diversity in the least killifish bottlenecked and normal lines. Gene diversity was significantly lower in the bottlenecked line than it was in the normal line, for two of the three pairs of lines. Gene diversity averaged 0.750 in the normal lines, whereas it averaged 0.695 for the bottlenecked lines from the three source populations. Thus, while the bottlenecking (starting populations with only two individuals) resulted in a discernable loss in genetic variation, this loss was nevertheless relatively minor.
We compared heat tolerance among populations of least killifish and among populations of the eastern mosquitofish (Gambusia holbrooki). This was done for four pairs of populations subjected to different thermal regimes, in a total of six comparisons. For two of the comparisons, thermal regime did not have a statistically significant effect on heat tolerance. For the third pair of populations, thermal tolerance was actually slightly higher in fish from the site with lower ambient temperatures. For the fourth pair of populations, thermal tolerance was higher in fish collected from the warmer site than it was in fish collected from the normal-temperature site. These two fish populations were maintained in the laboratory and again tested in the first and second generation reared under laboratory conditions. The differences in heat tolerance observed in the field-collected fish were no longer observed in the offspring born in the laboratory, indicating that the initial differences in heat tolerance did not have a genetic basis (i.e., were a result of physiological acclimation rather than genetic adaptation).
We quantified the heritability of temperature tolerance in two sets of H. formosa base populations (each set consisting of a bottlenecked and a regular population from the same site). Thermal tolerance was quantified in individual fish, and for pairs of fish and their offspring. Heritabilities were determined using resemblance among relatives, specifically parent-offspring regressions and full-sib analyses. Heritability estimates ranged from 0 to 0.462, with estimates generally averaging approximately 0.3. There was evidence for one of the source populations (but not the other one) that fish from the bottlenecked population had a lower heritability than did those from the non-bottlenecked population.
Results from the selection experiment, in which normal and bottlenecked lines of least killifish from three different sites were selected for an increased heat tolerance, showed a response (albeit relatively slow) to this selection. The regression of selection response on the number of generations selected was highly significant (p = 0.004), confirming that these laboratory populations were able to adapt to high water temperatures. After two generations of selection, the selected populations were able to withstand a temperature that was on average 0.068°C higher than the temperature that was lethal to fish from the control lines. There was some evidence that the response to selection was stronger for the lines started from the normal populations than those started from the bottlenecked populations, with responses to selection of respectively 0.08 and 0.04°C. A more definitive conclusion of this bottlenecking effect awaits a more in-depth analysis of the results in which the amounts of within-population variation and the intensities of selection are also taken into consideration.
Conclusions:
Our selection experiment showed that laboratory populations of the least killifish were able to adapt to elevated water temperatures. This is consistent with our quantification of the heritability of resistance to high temperatures in these populations, where we found that the populations contained the genetic variation that is required for the occurrence of adaptation to elevated water temperatures. These results indicate that some fish populations have the capacity to adapt to increased water temperatures. In contrast, when comparing heat tolerance among natural populations of fish from sites with different thermal regimes, no clear pattern emerged. One possibility is that other factors masked actual differences in heat tolerance, as any two natural populations differ in habitat characteristics other than thermal regime. It is also possible that the differences in thermal regime were not strong enough for water temperature to exert a selection pressure. Finally, a response to selection for increased resistance to elevated water temperatures may have been offset by natural selection opposing an increase in heat tolerance, which would occur if fitness costs are associated with the development of heat tolerance.
Our results showed that populations that have undergone a population bottleneck may have a reduced potential to respond to the selection pressures associated with increases in water temperature. In spite of the bottlenecks resulting in relatively small decreases in genetic variation, both the heritability estimates and the selection experiment indicated a lower potential for adaptation to elevated water temperature in the bottlenecked populations.
The results of this project have various implications to the mission of the U.S. Environmental Protection Agency. Our earlier research had shown that natural populations have the potential to adapt to environmental pollution, with one implication being that it affects the ecosystem composition at contaminated sites. The current project indicates that adaptation also may occur when the environmental stressor is a high water temperature, with implications analogous to those for the adaptation to pollutants. Although global warming may result in range shifts in many species, other species may be able to maintain their current ranges by virtue of their potential to genetically adapt to the heat stress. Assessments of the long-term effects of global warming thus need to include an evaluation of populations’ potential for genetic adaptation.
Journal Articles:
No journal articles submitted with this report: View all 8 publications for this projectSupplemental Keywords:
global climate, ecological effects, vulnerability, aquatic, ecology, temperature, adaptation, genetic variation,, RFA, Scientific Discipline, Air, Geographic Area, Hydrology, climate change, State, Environmental Monitoring, Atmospheric Sciences, Ecological Risk Assessment, wetlands, fish habitat, watershed, global change, Louisiana (LA), coastal ecosystems, aquatic ecology, global warming, land and water resources, climate variability, Global Climate ChangeRelevant Websites:
http://www.louisiana.edu/Departments/BIOL/klerks.html Exit
http://www.louisiana.edu/Departments/BIOL/leberg.html Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.