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.