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Low genetic diversity and inbreeding depression in Queensland koalas 

Worthington-Wilmer, JM, Melzer, A, Carrick, F, & Moritz, C, 1993, Wildlife Research, vol. 20, pp. 177-188.

  Analysis of mitochondrial DNA (mtDNA) of two Queensland koala populations reveals that genetic variation within and between these population is extremely low, though the variation appears to be structured geographically.  Furthermore, the analysis also shows inbreeding levels in a captive colony in one of these populations to be moderate and high, but koalas within this colony do not show strong evidence of inbreeding depression apart from the male-biased sex ratio.

  Low genetic diversity in a population can lead to a loss in evolutionary flexibility, making the population susceptible to changes in the environment.  Moreover, it can also cause inbreeding depression leading to decreased fecundity and viability of offspring.  In this study, the authors suspected koala populations in Queensland may suffer some degree of inbreeding. Thus they investigated the amount and distribution of genetic variation in mtDNA in two natural populations. The variation in mtDNA of these populations was found to be exceeding low. The difference observed between two populations separated by 600km was similar to that typical of an island population of other mammalian species.  Despite having high levels of inbreeding, the koalas of the captive colony do not express any severe effects of inbreeding depression such as decreased juvenile survivorship or growth rate, though the authors emphasise that it will be premature to conclude that koalas are or are not susceptible to inbreeding depression based on the limited data acquired in this study.

  The authors argue that this low level of diversity may be reflective of natural variation, due to similar results reported in a DNA fingerprinting study of free-ranging Queensland koalas. Koala populations in Queensland are subjected to extensive clearing for human settlement and logging industry. The result is repeated contraction and expansion of these populations as they adapted to these changes, leading to a widely distributed but small meta-population that have shared gene ancestry and low effective population sizes.

  It is essential to investigate the natural genetic diversity of koalas in Queensland which have received little human intervention historically in comparison to the Victorian and South Australian populations, understanding that natural variation will let us know when manipulation will be required to avoid further loss of genetic diversity and maintain genetically sustainable and viable populations.


Summarised by Cherie Chan


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