Testing the effectiveness and repeatability of genetic rescue from inbreeding depression in Tribolium castaneum

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BACKGROUND: When populations become depleted or isolated, reduced genetic variation can lead to reduced population fitness through inbreeding depression. The environmental circumstances leading to this situation are only likely to increase in the future, and genetic variation is now recognised by the IUCN as a conservation priority. There is therefore a pressing requirement to address research questions in conservation genetics, with a recent high-impact review identifying priorities for understanding the primary phenotypic routes to inbreeding depression, and whether inbred populations can be 'rescued' through the introduction of new genetic variation from related individuals. Problems identified by this review were: (a) wild studies usually failed to control for potential environmental confounds known to influence fitness; (b) for many systems, the specific phenotypic routes to inbreeding depression within 'fecundity' remain unconfirmed, so a wider range of more detailed reproductive trait measures are required; and (c) the potential for outbreeding depression via disruption of local adaptation is an important consideration and therefore assessment to the F2 generation and beyond (when unlinked genes begin recombining) is required. This project aims to overcome these weaknesses using a controlled, multi-generational and replicated experimental approach. THE MODEL: The flour beetle Tribolium castaneum has been successfully employed as a laboratory system for answering questions in evolution and ecology. Our recent experience with this system through a recent NERC project mean we have detailed familiarity with measuring a range of reproductive fitness traits. We also have an established phylogeny for 50 strains at UEA, so we can objectively identify populations with increasingly-distant genetic backgrounds, and assess their relative impact on genetic rescue. Importantly, we have consistent inbreeding depression of reproductive output for 20 replicate inbred populations, by comparison with similarly-bred controls. METHODS: We have two general aims: (1) conduct detailed measurement of a number of potential reproductive fitness traits to identify which are responsible for inbreeding depression, and (2) measurement of the consistency and effectiveness of 'genetic rescue' under different but controlled conditions. (1) Most examples of inbreeding depression occur via disruption to reproduction. We plan to determine whether different reproductive traits are more susceptible to inbreeding depression using 20 inbred replicate populations and 10 non-inbred controls, across 4 increasing inbreeding coefficients. Detailed and replicated measurement of 15 established reproductive fitness traits (from sperm quality and competitiveness to female egg output and larval hatch) will allow assessment of the specific reproductive phenotypes responsible for depression. (2) Genetic rescue might be possible with even small numbers of 'immigrants'. We will assess genetic rescue in our established inbred lines (and non-inbred controls) using introgression of 4 different populations of known genetic relatedness from a previous project. Using a five-fold range of immigration, coupled with monitoring of genotypic spread measured using phenotypic markers, we will be able to establish how much introgression can rescue an inbred population, and how different genetic backgrounds play a role. Importantly, we will monitor across 3 generations, to determine the likely role of outbreeding depression through loss of local adaptation genes that might occur in the second generation. CONCLUSIONS: Our results will inform conservation management programs concerned about inbreeding by (1) allowing targeted screening of susceptible traits, and (2) whether genetic rescue should be instigated according to the choice of immigrant genes and the risk of outbreeding depression. Material from our experiments will be freely available for further genomic analyses.