Abstract
Genetic rescue is increasingly considered a promising and underused conservation strategy to reduce inbreeding depression and restore genetic diversity in endangered populations, but the empirical evidence supporting its application is limited to a few generations. Here we discuss on the light of theory the role of inbreeding depression arising from partially recessive deleterious mutations and of genetic purging as main determinants of the medium to long-term success of rescue programs. This role depends on two main predictions: (1) The inbreeding load hidden in populations with a long stable demography increases with the effective population size; and (2) After a population shrinks, purging tends to remove its (partially) recessive deleterious alleles, a process that is slower but more efficient for large populations than for small ones. We also carry out computer simulations to investigate the impact of genetic purging on the medium to long term success of genetic rescue programs. For some scenarios, it is found that hybrid vigor followed by purging will lead to sustained successful rescue. However, there may be specific situations where the recipient population is so small that it cannot purge the inbreeding load introduced by migrants, which would lead to increased fitness inbreeding depression and extinction risk in the medium to long term. In such cases, the risk is expected to be higher if migrants came from a large non-purged population with high inbreeding load, particularly after the accumulation of the stochastic effects ascribed to repeated occasional migration events. Therefore, under the specific deleterious recessive mutation model considered, we conclude that additional caution should be taken in rescue programs. Unless the endangered population harbors some distinctive genetic singularity whose conservation is a main concern, restoration by continuous stable gene flow should be considered, whenever feasible, as it reduces the extinction risk compared to repeated occasional migration and can also allow recolonization events.
Highlights
Genetic rescue is the reduction of the extinction probability of endangered populations through the introduction of migrant individuals
They show that, under no rescue program, the expected fitness of threatened populations declined in the early generations and partially recovered a few generations later due to genetic purging
The impact on conservation practice of the theoretical considerations and the simulation results discussed here depend on many factors that determine the genetic architecture of the inbreeding load, as the distribution of mutational effects and dominance patterns for deleterious mutations or the complexity of demographic histories, and all of them are worthwhile to be explored
Summary
Genetic rescue is the reduction of the extinction probability of endangered populations through the introduction of migrant individuals. Concern has been raised by the extinction of the Isle Royale wolves population, where the genetic contribution of a single migrant wolf from the large mainland population quickly spread in the resident population thanks to the breeding vigor of its offspring, possibly causing an increase in inbreeding and an associated fitness decline that triggered population extirpation (Hedrick et al 2014, 2017, 2019). Despite the multiple studies supporting the practice of genetic rescue (Frankham 2015; Kolodny et al 2019), its consequences in the medium to long term remain uncertain (Hedrick and Fredrickson 2010; Hedrick and GarcíaDorado 2016; Bell et al 2019; Kyriazis et al 2020). We review the main theoretical aspects behind the impact of inbreeding depression and purging on the long-term success
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