The conservation of endangered species often means understanding and addressing the genetic problems that can arise in small populations. We recently interviewed, Philip Hedrick and Richard Fredrickson, leaders in the field of conservation genetics. They had just published an article in the journal Conservation Genetics in which they outlined 10 guiding principles for genetic rescue - a practice that has been used to improve the condition of certain highly endangered species.
For isolated populations with low numbers, genetic defects can become fixed in the population by chance - this negative effect on the fitness of the population is termed the genetic load and is distinct from problems of inbreeding as Dr. Hedrick describes below.
As an example of genetic load, the highly endangered Florida panther population has suffered from kinked tails, undescended testicles, and low sperm count. Genetic rescue involves the introduction of individuals from other populations to reduce the occurrence of these defects. However, the practice does not come without its risk of unintended negative consequences.
Therefore, Hedrick and Fredrickson's guidelines on genetic rescue merit close attention from anyone working on issues of genetic conservation in wildlife populations.
CM: What is genetic load and what are some examples of how it can potentially harm a population?
PH: Small, isolated populations may decline in fitness over time because of the fixation of detrimental alleles by chance. The amount of reduction in fitness, compared to a population that is not small or isolated, is called the genetic load of the population. An often cited example of genetic load was in the Florida panther population before cats were introduced from Texas.
CM: How does genetic load differ from inbreeding depression?
PH: Inbreeding depression is defined as the reduction in fitness (or fitness-related traits, such as viability or litter size) for progeny from matings between relatives, as compared to that in progeny from matings between unrelated individuals. Although both genetic load and inbreeding depression appear to be caused by homozygosity of detrimental alleles, populations with genetic load may not have inbreeding depression. This can occur when all individuals may be fixed, or have high frequencies, for detrimental alleles so that matings between close relatives do not produce individuals with lower fitness than matings between unrelated individuals. Conversely, populations with inbreeding depression may not have genetic load because large populations may have little fixation of detrimental variants due to genetic drift but still be segregating at low frequency for many detrimental variants that result in inbreeding depression.
CM: What is genetic rescue and how does it address the issue of genetic load?
PH: Given that a population has a significant genetic load, the introduction of unrelated individuals from another population should result in the reduction of the high frequency of detrimental variants, a subsequent increase in fitness, and a reduction in genetic load.
CM: Can you tell us about your background and how you first got interested in this topic?
PH: I have been studying genetic issues in small populations for many decades and have served on a number of panels involved with genetic issues on such endangered species as Florida panthers, Mexican wolves, winter run Chinook salmon, desert bighorn sheep, and Gila topminnows. These endangered species and many others now exist only in small, isolated populations and genetic load and genetic rescue have become important conservation and management issues in their recovery.
RF: I have been interested in small populations for many years, from both scientific and management perspectives. I was introduced to the topic of genetic rescue by Phil as a graduate student.
CM: Can you give a brief example in which genetic rescue was implemented with some success? And can you tell us how we know that it worked?
PH: Because of the small population size of Florida panthers in the early 1980s, less than 30, and the high frequency of several detrimental or unusual traits, such as low sperm quality, undescended testicles, and kinked tails, consistent with high genetic load, it was recommended that cats from Texas be introduced to genetically rescue this population. Eight animals were introduced in 1995 and since then the population size has increased to around 100, the frequency of undescended testicles has dropped from 49% to 0% in males, and the frequency of kinked tails has dropped from 77% to 7%.
CM: You outline a number of potential negative consequences that could arise from genetic rescue? Can you tell us about 1 issue that you think is particularly serious and how do managers avoid it?
PH: The animals introduced in a genetic rescue effort, and their descendants, may be so successful in surviving and reproducing that the resulting population may descend primarily from this small numbers of animals. Because of the generally small number of animals introduced in genetic rescue management, subsequent genetic drift and inbreeding in their descendants may become a problem after a few generations. To avoid this, the number of animals introduced should not be too small and their individual contributions should be monitored and managed so that a few animals and their descendants do not dominate the subsequent population.
RF: For endangered populations that are considered unique there may be a desire to increase the ancestry of the endangered population in the rescued population following initial genetic rescue (production of F1 individuals), particularly if large fitness increases were evident among the F1 individuals. Creating backcross individuals from the endangered population, however, runs the risk of regenerating the genetic load that contributed to endangerment. A means of assessing this risk, particularly among captive populations, is to conduct experimental backcrosses and examine the production and fitness of the progeny. For wild populations that cannot be bred and examined in a controlled setting, efforts should be made to monitor the success of backcross pairings and their progeny for multiple generations.
CM: What are some ongoing projects where genetic rescue is being implemented and what are some prospective projects that could happen in the future?
PH: Recovery efforts where genetic rescue has been implemented are for Mexican wolves, Florida panthers, and greater prairie chickens. In addition, naturally occurring genetic rescue from new immigrants has been important for wolves in Sweden. Many endangered species now exist in small, isolated populations so it is likely that genetic rescue will be part of the recovery efforts of many more species in the future.
CM: You present 10 guidelines for genetic rescue. To what extent do you think managers working on these types of projects are aware of these guidelines and following them?
PH: Some of these guidelines are commonly part of management planning and have been incorporated into projects as a result. For example, using a closely related donor population has been of high consideration. However, many of these guidelines have not been formally considered. It is our hope that by explicitly giving these guidelines, and the rationale behind them, that managers will use these guidelines in designing their recovery efforts.
RF: Genetic rescue as a management practice is new, and comes at a time of growing awareness among managers of the potential for genetic problems to limit small populations. Some of the potential negative genetic consequences of genetic rescue, however, may be counter-intuitive. As Phil notes, by presenting these guidelines we hope that the potential benefits and limitations of genetic rescue will be better understood and that this will aid managers in their recovery efforts.
Hedrick, P., & Fredrickson, R. (2009). Genetic rescue guidelines with examples from Mexican wolves and Florida panthers Conservation Genetics DOI: 10.1007/s10592-009-9999-5