Absence of inbreeding depression
Inbreeding depression can be defined as heterozygosity-fitness correlations and, using this approach, we found no evidence of inbreeding depression in disease (a key fitness trait) in this koala population. Inbreeding depression is difficult to investigate in wild populations, not least because obtaining necessary datasets (including pedigrees and/or genome-wide markers and fitness measures) is both costly and labor intensive. However, given the potential consequences of inbreeding depression on population growth and survival , identifying the risk it poses to populations is critical to implementing informed management. Interestingly, empirical evidence of an association between disease and inbreeding in wild populations is mixed, with effects varying both between and within species , and with pathogen type . In koalas, evidence to date for inbreeding depression is also mixed. For instance, inbreeding depression was found when comparing cryptorchidism levels in island to mainland populations , but the relationship was not evident at the individual level , and did not correspond to decreased fitness, as the most inbred populations were also experiencing exponential population growth .
The effects of inbreeding on individual or population fitness can be dependent on environment, age, sex and the genetic constitution of populations . Interestingly, it has been suggested that koalas may have a reduced susceptibility to inbreeding depression, due to low historical effective population sizes and the elimination of recessive deleterious alleles . Although we did not detect an age-dependent effect of inbreeding in our analyses, there may have been other context dependent effects of inbreeding that we did not measure. As such, although the lack of inbreeding depression could reduce concerns for this population, our results provide no guarantee of future resilience to chlamydial disease, especially as the context in which koalas live rapidly changes under continued environmental degradation.