Abstract

Two albino Queensland koalas (Phascolarctos cinereus adustus) have been born at the San Diego Zoo, one in 1985 and the other in 1997. Both appear to be instances of complete oculocutaneous albinism. In other animals, this defect is most often due to a mutation in the gene for tyrosinase enzyme, which catalyzes the first steps of the melanin-synthesis pathway in melanocytes of the skin, hair follicles, and retina.^1-4 Consequently, along with unpigmented skin and fur, albinism is associated with altered development and anatomy of the retina and optic nerve leading to reduction in visual acuity. The albino koalas at the San Diego Zoo showed signs of similar visual deficits. An autosomal-recessive pattern of inheritance is observed for such mutations. Pedigree analysis has unambiguously identified an original carrier in addition to five other carriers in three succeeding generations, all of which were phenotypically normal (A. Kumamoto, unpublished data). The original carrier, imported from Lone Pine Koala Sanctuary (near Brisbane, Queensland, Australia), gave birth to the first albino individual as a result of a mother/son mating. The second albino was generated by a pairing of two less closely related animals which were, nevertheless, both descendants of the original carrier. By use of the molecularly based techniques of polymerase-chain-reaction assay (PCR), cloning, and sequencing, we have obtained genetic sequence information from the entire exon 1 region of the koala tyrosinase gene for the purpose of detecting individuals harboring the mutant gene before they are bred.

Koalas are now listed by the U.S. Fish and Wildlife Service as a threatened species (50 CFR, Part 17.11). The Australian Koala Foundation reports that this change of status will not likely slow the koala population decline in the wild, as natural-habitat areas are essentially unprotected and are rapidly being degraded by logging, fire, and development. Although captive propagation is not the solution to koala conservation needs, it will be of increasing importance in monitoring and maintaining genetic diversity as fragmentation of habitat leads to reduction of gene flow in disjunct wild populations. Besides maintaining diversity of alleles, however, the identification of specific mutations in phenotypically abnormal animals allows the development of diagnostic tests for the detection of carriers of heritable genetic problems. As the first zoo-based, full-time laboratory dedicated to such work for genetic and infectious diseases, the Molecular Diagnostics Laboratory at the San Diego Zoo is uniquely positioned for this task.

Literature Cited

1.  Chintamaneni CD, Halaban R, Kobayashi Y, Witkop CJ Jr., Kwon BS. A single base insertion in the putative transmembrane domain of the tyrosinase gene as a cause for tyrosinase-negative oculocutaneous albinism. In: Proceedings from the National Academy of Science. 1991;88:5272–5276.

2.  Ruppert S, Muller G, Kwon B, Schutz G. Multiple transcripts of the mouse tyrosinase gene are generated by alternative splicing. EMBO J. 1988;7:2715–2722.

3.  Shibahara S, Okinaga S, Tomita Y, Takeda A, Yamamoto H, Sato M, Takeuchi T. A point mutation in the tyrosinase gene of BALB/c albino mouse causing the cysteine-->serine substitution at position 85. Eur J Biochem. 1990;189:455–461.

4.  Spritz RA, Hearing VJ Jr. Genetic disorders of pigmentation. In: Harris H, Hirschhorn K, eds. Advances in Human Genetics, Vol. 22. New York, NY: Plenum Press, 1994:1–45.