Measures of inbreeding and heterozygosity-fitness correlations in koalas (Phascolarctos cinereus) from south-east Queensland, Australia

Abstract Inbreeding threatens many species of conservation concern. Inbreeding decreases heterozygosity (increases homozygosity) and can drive up a population’s genetic load as deleterious mutations increase in frequency. Understanding how declining heterozygosity translates into declining fitness is of high importance for conservation practitioners. In this study, we investigated the potential effects of inbreeding on fitness traits in captive and wild populations of the iconic koala (Phascolarctos cinereus) from south-east Queensland, Australia. Using 5,757 ddRAD SNPs, we found that our sampled koalas were separated into three genetic groups: a captive group (N = 36) and two genetically distinct wild groups that were divided into northern (N = 41) and southern (N = 50) populations in the Gold Coast area. Inbreeding coefficients (FIS) were slightly lower in the captive population (0.003) compared to the wild northern (0.088). While the FIS value of the southern population (-0.006) suggests minimal inbreeding within the population. Heterozygosity-fitness correlations between four fitness traits (reproductive fitness, body mass, body condition score and disease expression) and SNP heterozygosity in each of the wild groups were non-significant. This lack of signal may have been due to small effect sizes (lower power), the greater influence of environmental contributors to measured traits, and/or possibly insufficient variation in inbreeding within the populations.