Population and Ecological Genetics of the Brush-Tailed Possum (Trichosurus Vulpecula) in New Zealand

Abstract

This thesis examines the population and ecological genetics of the brush-tailed possum in New Zealand. Nine polymorphic and 36 monomorphic allozyme loci were resolved from possum tissues using starch-gel electrophoresis. The genetic basis of polymorphism was supported by analysis of the genotypes of mother - offspring combinations. Four sites in southeast Australia were sampled to determine the amount and distribution of variation in the areas from which possums were exported to New Zealand. The level of genetic divergence between the four sites was only moderate (Nei's D = 0.004-0.017). The level of variation, as measured by P, na, and H, appeared to decrease with latitude in southeast Australia. Two morphological types were introduced to New Zealand; smaller, grey possums from mainland Australia and larger, black possums from Tasmania. The two morphs are not randomly distributed in New Zealand. Larger, black possums predominate in cold, wet areas, whereas smaller, grey possums are more common in warm, dry areas. Analysis of genetic variation and genetic relationships in New Zealand indicated that predominantly black New Zealand populations are genetically similar to the Tasmanian population, whereas predominanatly grey populations are genetically closer to Victoria and New South Wales possums. This finding has important implications for pest control, as Tasmanian possums have a greater resistance to the commonly used poison 1080 (sodium monofluoroacetate) than mainland Australian possums. In most mainland New Zealand populations, loss of variation has been minimal. Rare alleles have been lost from many populations, but heterozygosity has decreased only slightly or increased over expected values. Two populations, Wanganui and Taupo, have lost alleles and decreased in heterozygosity, possibly as a result of a genetic bottleneck. Effective founder sizes of mainland populations, as calculated from loss of alleles, extent of differentiation, and historical records, range from about 20 to 100. Two isolated island populations appear to have gone through a severe bottleneck. A third of the original number of alleles have been lost from these populations, indicating a bottleneck size of about 5-10 individuals on each island. Genetic drift in these two small, isolated populations has resulted in strong genetic divergence between mainland and island populations. However, heterozygosity has not decreased substantially on either island, suggesting that selection has played a role in maintaining heterozygosity levels. The genetic structure of a local possum population was analysed using F-statistics. Significant geographic structuring occured over distances of several kilometres; samples separated by 2 km were essentially panmictic. Effective population size was very approximately estimated by genetic and ecological methods to be of the order of 103-104. Thus, possum populations are much less structured than the populations of many social mammal species, which consist of discrete demes with effective sizes in the range 101-102. The significant level of inbreeding found within local populations was consistent with consanguineous mating, presumably resulting from the high proportion of young (particularly female) possums that do not disperse. No relationship was found between level of inbreeding and population density, suggesting that the mating system does not change with density. In contrast to other loci, Icd-2 had a negative inbreeding coefficient in almost all populations and a high heterozygosity in populations which had been through a bottleneck, suggesting that selection for heterozygosity occurs at Icd-2. Selection was also the likely cause of the significant increase in overall heterozygosity between immature and adult males. Immature males with a high heterozygosity were significantly heavier than those with a low heterozygosity.