Many mammal species that are endangered on mainland Australia exist in stable island populations.These island populations have the potential to act as an "ark" in the case of mainland population extinctions. Genetic diversity at fitness-associated loci is integral to the viability of populations. It is thus important to investigate fitness related genetic diversity of island populations before they are considered suitable as reservoirs of their species. This was performed with regard to the black-footed rock-wallaby (Petrogale lateralis lateralis), which exists in small fragmented mainland populations in Western Australia and on Barrow Island. The Major Histocompatibility Complex class II locus DAB is associated with fitness, through its role in antigen presentation. Diversity at DAB was assessed using single strand conformation polymorphism (SSCP) and sequencing. The mainland populations
displayed moderate levels of allelic diversity (4-7 alleles) despite being small in size and isolated from one another, and contain at least two DAB loci. However, the Barrow Island population displayed low allelic diversity (2 alleles) and high monomorphism in comparison to mainland populations, and probably possess only one DAB locus. Previous studies using microsatellites have also suggested that genetic variation in island marsupial populations is low. This study is one of the first to provide explicit evidence that fitness related genetic diversity in these populations is also low. If this situation is similar across other island populations, it would be far better to concentrate resources upon preserving mainland populations than to rely upon island populations as a conservation resource.