Making the best of a bad situation: genetic rescue in the absence of an ideal source population

In this issue’s feature article, Kronenberger et al. (2017) experimentally compare the advantages of alternate genetic rescue strategies using laboratory populations of Trinidadian guppies Poecilia reticulata. Guppies are a classic model system of evolutionary ecology (Magurran, 2005), and have been identified as an ideal species for exploring conservation questions (van Oosterhout et al., 2007). In their native range, guppy populations at downstream sites are co-resident with one of their important predators, the pike cichlid (Crenicichla spp.), experiencing higher predation pressure than upstream guppy populations (Magurran, 2005). Migration upstream is prevented by waterfalls, and consequently, upstream populations are more isolated, smaller, and show lower genetic diversity relative to populations at downstream sites (Suk & Neff, 2009; Willing et al., 2010). Thus, connected tributaries within rivers are home to populations that are expected to be more closely related relative to populations from other rivers, although populations within rivers exhibit varying levels of genetic diversity and ecological pressures. These patterns are replicated across multiple rivers in Trinidad, providing natural systems that differ in bottleneck intensity ideal for addressing important questions in conservation genetics. Genetic rescue occurs when unrelated immigrants are translocated into bottlenecked populations in order to alleviate inbreeding and increase genetic diversity and fitness, although important risks of such translocations include disease spread and outbreeding depression (Tallmon, Luikart & Waples, 2004; Frankham et al., 2011). Published guidelines (Frankham et al., 2011) can be used to evaluate the suitability of putative source and recipient populations; when these are followed, the results are almost always positive for recipient populations (Frankham, 2015). These guidelines include assessments based on the degree of differentiation between populations, time since the most recent gene flow and environmental similarity (for full details, see Frankham et al., 2011). But what if there are no suitable populations from which to translocate? This is not an uncommon problem, and alternate strategies may be required depending on the circumstance. For example, when no outbred source populations remain, reciprocal translocation of inbred populations can still provide genetic rescue benefits (e.g. Heber et al., 2013). However, when no optimal source population exists that meets all requirements for genetic rescue, conservation managers must consider how trade-offs among multiple risks may impact population viability of the recipient population. It is in this context that the authors of the feature article consider the trade-off between genetic and adaptive differentiation (Kronenberger et al., 2017). They found that greater gains were achieved through the use of immigrants from a more similar environment, despite possessing greater genetic differentiation, compared to the reciprocal. It is also important to consider that the decision to not undertake any genetic rescue activities constitutes a management decision in itself, and the risks of doing nothing should also be weighed in decision-making frameworks (for further discussion of impediments to decision-making, see Meek et al., 2015). In Kronenberger et al. (2017), the consequences of ‘doing nothing’ can be seen in their control populations: three replicates with no immigration showed an overall decline in population size over the course of the study. In this system therefore, a lack of immigration would appear to have been an inferior option to selecting immigrants from either source population. Although beyond the scope of the original study, further work to track the genetic diversity of each population involved in the experiment (recipients and potential donors) would provide valuable information about any positive or negative impacts of immigration, or lack thereof in the control populations, over time. Not only were the immigrants from the more successful translocation from an ecologically similar site, they were also from the more bottlenecked of the two possible source populations (Kronenberger et al., 2017). The more successful source population therefore likely had lower genetic diversity than the alternate, based on previous reports for Trinidadian guppies [populations from low predation sites typically have lower diversity than high predation sites (e.g. Willing et al., 2010)]. As the authors point out, bottlenecked