Dynamics of genetic rescue in inbred Drosophila melanogaster populations

Abstract

Genetic rescue has been proposed as a management strategy to improve the fitness of genetically eroded populations by alleviating inbreeding depression. We studied the dynamics of genetic rescue in inbred populations of Drosophila. Using balancer chromosomes, we show that the force of heterosis that accompanies genetic rescue is large and allows even a recessive lethal to increase substantially in frequency in the rescued populations, particularly at stress temperatures. This indicates that deleterious alleles present in the immigrants can increase significantly in frequency in the recipient population when they are in linkage disequilibrium with genes responsible for the heterosis. In a second experiment we rescued eight inbred Drosophila populations with immigrants from two other inbred populations and observe: (i) there is a significant increase in viability both 5 and 10 generations after the rescue event, showing that the increase in fitness is not transient but persists long-term. (ii) The lower the fitness of the recipient population the larger the fitness increase. (iii) The increase in fitness depends significantly on the origin of the rescuers. The immigrants used were fixed for a conditional lethal that was mildly deleterious at 25°C but lethal at 29°C. By comparing fitness at 25°C (the temperature during the rescue experiment) and 29°C, we show that the lethal allele reached significant frequencies in most rescued populations, which upon renewed inbreeding became fixed in part of the inbred lines. In conclusion, in addition to the fitness increase genetic rescue can easily result in a substantial increase in the frequency of mildly deleterious alleles carried by the immigrants. This can endanger the rescued population greatly when it undergoes recurrent inbreeding. However, using a sufficient number of immigrants and to accompany the rescue event with the right demographic measures will overcome this problem. As such, genetic rescue still is a viable option to manage genetically eroded populations.

A Test of the ‘Genetic Rescue’ Technique Using Bottlenecked Donor Populations of Drosophila melanogaster

“…A way to investigate environmental effects in the laboratory could be to subject both inbred and hybrid individuals to changes in the environment, such as increases in temperature or salinity, or exposure to pathogens, and to test if the groups differ in their ability to respond to novel challenges (e.g. [44], [45]). …”

Conserving small and fragmented populations of large mammals: Non-invasive genetic sampling in an isolated population of Asian elephants in Nepal

“…Such a mating system will produce a pattern of heterozygosity excess. More important is the evidence for male immigration and its potential to counteract genetic erosion, which in turn reduces the risk of inbreeding depression (Beauclerc et al 2010;Bijlsma et al 2010;Couvet 2002;Ingvarsson 2001;Keller & Waller 2002;Schwartz & Mills 2005). Since the re-establishment in 1994, the population has been increasing due to local recruitment, as also evident from the large proportion of young animals (70% calves, juveniles and sub-adults).…”

Genetic rescue and the increase of litter size in the recovery breeding program of the common hamster (Cricetus cricetus) in the Netherlands. Relatedness, inbreeding and heritability of litter size in a breeding program of an endangered rodent

“…However, only the genetic contribution of the single individual of the German population increased litter size, while the two males of the Belgian population had no effect on litter size. This result can be explained by the presence of beneficial alleles in the German population and this result illustrates the complexity and uncertainties involved in genetic rescue (Hedrick and Fredrickson 2010; Bijlsma et al 2010; Tallmon et al 2004). Restoring gene flow between threatened populations will not always lead to an increase of population fitness, but in the case of the common hamster litter size is only one of the traits influencing population persistence in the wild and hopefully other beneficial traits have been introduced by the Belgian males.…”