Grantee Research Project Results
2004 Progress Report: Hybridization Between an Invasive Exotic and a Declining Native Amphibian: Molecular Characterization, Ecological Dynamics, and Genetic Remediation
EPA Grant Number: R828896Title: Hybridization Between an Invasive Exotic and a Declining Native Amphibian: Molecular Characterization, Ecological Dynamics, and Genetic Remediation
Investigators: Shaffer, Howard B. , Koenig, Walter D. , Voss, S. Randal , Fitzpatrick, Benjamin
Institution: University of Kentucky , University of California - Berkeley , University of California - Davis
Current Institution: University of California - Davis , University of California - Berkeley , University of Kentucky
EPA Project Officer: Packard, Benjamin H
Project Period: August 20, 2001 through August 19, 2004 (Extended to August 31, 2005)
Project Period Covered by this Report: August 20, 2003 through August 19, 2004
Project Amount: $433,708
RFA: Exploratory Research to Anticipate Future Environmental Issues (2000) RFA Text | Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , Water , Aquatic Ecosystems
Objective:
The overall objective of this research project is to characterize the interactions between an introduced exotic salamander (the tiger salamander Ambystoma tigrinum) and a native, declining, endemic species (A. californiense) in the central California landscape. Our preliminary work demonstrates that:
- the two species interbreed in the region of cooccurrence, resulting in a severe case of genetic “biopollution”;
- and interbreeding is more complete in disturbed, compared to natural, pond habitats, suggesting that habitat restoration may be an effective, proactive remediation.
This research aims to quantify the extent of genetic invasion of an exotic species, develop a full set of informative molecular markers, and conduct detailed genetic and demographic analyses to identify the extent of genetic biopollution and reduce introduced exotic genes. This work provides the first complete assessment of the risk involved for this species as well as a novel strategy for reversing the widespread problem of genetic biopollution in endangered species.
Our research approach involves :
- developing a panel of 10-20 mapped nuclear markers to quantify a fine-scale hybrid index;
- using these markers to determine the geographic extent of hybrid introgression and the concordance of introgression across genes;
- collecting key demographic information in the field and laboratory to quantify differences among pure and hybrid salamanders;
- testing multiple ponds and genes to investigate the correlation of hybrid inviability with ecological pond characteristics;
- and conducting a preliminary field experiment to investigate the efficacy of one strategy to reduce or eliminate introduced exotic genotypes.
Progress Summary:
We have developed nine fully functional molecular markers with diagnostic restriction enzyme sites, allowing us to determine whether an individual is homozygous native, heterozygous, or homozygous nonnative at a particular marker. A tenth marker (COL1a1) turned out to be misleading because the restriction enzyme site was polymorphic within the native population. We implemented a strict quality assurance protocol requiring genotyping of 30 pure A. californiense and pure A. tigrinum each, systematically sampled from throughout their native ranges prior to declaring a molecular marker diagnostic.
Our analyses of geography and life history are still in the data analysis stage and we have nothing new to report at this time. Our analysis of habitat-dependent patterns of variation has been published.
The primary activities conducted under the grant during the last year were:
- an analysis of swimming performance in various hybrid genotypes
- and a cohort analysis comparing survival rates of different hybrid genotypes in the wild.
Performance . By statistically comparing fast-start performance with multilocus genotype, we tested whether there is systematic variation among genotypes in their vulnerability to predation. We used high-speed digital cinematography to record escape responses of more than 100 young larvae collected as eggs from the wild (hence, we were blind as to their genotypes until after the experiment ended). We used frame-by-frame digitization of these video sequences to estimate the acceleration and velocity attained by each animal. Then, we used our molecular markers to estimate an ancestry index representing the proportion of each individual’s genome that was derived from the introduced A. tigrinum (e.g., pure natives are 0.0, pure nonnatives are 1.0, and F1 hybrids are 0.5). We found that larvae with more intermediate hybrid genotypes had lower velocity than larvae more closely resembling pure native or pure nonnative genotypes (see Figure 1). This suggested hybrid dysfunction in swimming performance, and we suggested that predators may be an important source of natural selection against hybrids. A manuscript describing this study will be submitted for publication soon.
Figure 1. Burst Speed Velocities of Hybrid Tiger Salamander Larvae (Residuals After Statistically Removing the Effect of Different Source Ponds) Versus Their Ancestry Index Scores. The regression line has a significantly positive quadratic term (P < 0.05). |
Cohort Analysis. We studied a subset of the breeding sites described in an earlier paper (Fitzpatrick and Shaffer, 2004). Pond F and CVP are vernal pools and Pond G is a seasonal cattle pond. JCL2 and BW1 are perennial ponds that normally hold water through the summer; however, both ponds dried completely during the summer prior to this study.
We tested for evidence of natural selection by comparing samples of hatchlings collected as eggs from the wild to samples of larvae captured later in the same ponds. This cohort analysis design gives us a before-selection sample and an after-selection sample from each wild population. Significant differences in the relative abundance of genotypes between the two samples must be attributable to nonrandom variation in survival.
There was no consistent advantage for native or introduced alleles, and therefore no change in allele frequencies between hatchlings and larvae. There was evidence of selection, however, on heterozygote (that is, individuals that carry a copy of a gene from both species) frequencies. The GNAT1 marker, which consistently started with a deficit of heterozygotes in hatchlings, showed further reduction in heterozygote frequencies among larvae. HOXB13 heterozygotes also tended to decrease in frequency during larval development. Frequencies of heterozygotes at the other seven markers tended to increase. Pairwise linkage disequilibria (LD) tended to decrease between the hatchling and larval stages. None of the pairwise tests for change in LD were statistically significant. Average disequilbria decreased, however, in all five ponds, a pattern more consistent with overall hybrid advantage than with hybrid dysfunction.
A more organismal analysis of changes in allele frequency, heterozygosity, and LD was accomplished by analysis of the ancestry index (the estimated proportion of an individual’s genome that is derived from the introduced A. tigrinum). Larvae with intermediate hybrid genotypes had higher survival rates than pure native or introduced larvae in all five of our study ponds. This result is quite the opposite of what we expected based on the burst-speed performance results, and suggests that either burst-speed performance is not important in the field, or that other phenotypic differences lead to an overall increase in heterozygote fitness. Future work will be concerned with reconciling this intriguing problem. A more pressing question is: what is the impact of hybrid vigor on the dynamics of this biological invasion?
In Figure 2, the lines connect point estimates of relative survival values via polynomial interpolation. These survival values are ratios of the estimated frequencies of a class of genotypes after selection versus before selection (if a class is more common in larvae than in hatchlings the ratio is large, if it is less common in larvae the ratio is small). These ratios then were standardized by dividing by the maximum; so the maximum relative survival value in each pond is always 1.0. Regression models fitted to these data had statistically significant (negative) quadratic terms, indicating strong, repeated support for hybrid vigor.
Future Activities:
We will complete our characterization of the demographic, genetic, and geographic scope of this biological invasion, and determine the ecological conditions most favorable for successful remediation. We also will complete a quantitative assessment of one strategy for removing introduced genes from the landscape.
Figure 2. Changes in the Frequency Distributions of the Ancestry Index Between Samples of Hatchlings and Larvae |
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 13 publications | 5 publications in selected types | All 5 journal articles |
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Type | Citation | ||
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Fitzpatrick BM, Shaffer HB. Environment-dependent admixture dynamics in a tiger salamander hybrid zone. Evolution 2004;58(6):1282-1293. |
R828896 (2003) R828896 (2004) |
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Supplemental Keywords:
ecological effects, sensitive populations, genetic polymorphisms, aquatic ecosystems, restoration, conservation, ecology, genetics, surveys, California, CA, geographic area, health, biology, ecological effects - environmental exposure and risk, ecological indicators, ecological risk assessment, ecology and ecosystems, amphibian, animal models, biodiversity, biopollution, demographic analyses, ecological dynamics, endangered species, environmental hazard exposures, exotic genotypes, exotic species, extinction risk, hybridization, interbreeding, invasive species,, RFA, Scientific Discipline, Health, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Genetics, Ecosystem/Assessment/Indicators, exploratory research environmental biology, State, Environmental Microbiology, Ecological Effects - Environmental Exposure & Risk, Susceptibility/Sensitive Population/Genetic Susceptibility, Monitoring/Modeling, Ecological Risk Assessment, genetic susceptability, Biology, Futures, Exp. Research/future, Ecological Indicators, extinction risk, ecological effects, biodiversity, endangered species, biopollution, molecular characterization, genetic predisposition, conservation, amphibian, animal models, exotic genotypes, genetic remediation, invasive species, ecological dynamics, demographic analyses, California (CA), hybridization, futures research, interbreeding, exotic speciesProgress 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.