Journal Articles

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Origins and diversity of invasive brushtail possums (Trichosurus vulpecula) in New Zealand surveyed with mtDNA haplotype and nuclear microsatellite data
(Taylor and Francis Group on behalf of the Royal Society of New Zealand, 2025-05-07) Pattabiraman N; Morgan-Richards M; Trewick SA; Su J
The brushtail possum Trichosurus vulpecula is native to Australia where six subspecies exist in distinct regions. A composite invasive population is established in Aotearoa New Zealand, which has since been subject to localised bouts of culling. We surveyed population genetic structure across New Zealand to identify the scale of genetic diversity introduced to New Zealand and the resulting pattern of admixture. Australian brushtail possums have high mitochondrial diversity (17%) and prominent spatial structure. Thirty-eight haplotypes among 25 New Zealand population samples (n = 465) were closely related to 45 haplotypes sampled from Victoria and Tasmania in Australia (n = 120), but just one was shared. High haplotype diversity is consistent with multiple successful introductions and rapid population expansion in New Zealand. Nuclear diversity of microsatellite loci screened in 18 New Zealand population samples (n = 374) comprised five genotypic clusters (K = 5), but these groups did not correlate with geography. An overall signal of genetic partitioning within the invasive population suggests limited mixing but ongoing management towards eradication will influence patterns of population recovery, migration and evolution of traits including toxin resistance. We consider the implications of mixed ancestry of the invasive population in terms of variation in toxin tolerance detected in New Zealand.
Genotypic detection of barriers to rat dispersal: Rattus rattus behind a peninsula predator-proof fence.
(Springer Nature, 2023-02-06) Yarita S; Morgan-Richards M; Trewick SA
Clear delimitation of management units is essential for effective management of invasive species. Analysis of population genetic structure of target species can improve identification and interpretation of natural and artificial barriers to dispersal. In Aotearoa New Zealand where the introduced ship rat (Rattus rattus) is a major threat to native biodiversity, effective suppression of pest numbers requires removal and limitation of reinvasion from outside the managed population. We contrasted population genetic structure in rat populations over a wide scale without known barriers, with structure over a fine scale with potential barriers to dispersal. MtDNA D-loop sequences and microsatellite genotypes resolved little genetic structure in southern North Island population samples of ship rat 100 km apart. In contrast, samples from major islands differed significantly for both mtDNA and nuclear markers. We also compared ship rats collected within a small peninsula reserve bounded by sea, suburbs and, more recently, a predator fence with rats in the surrounding forest. Here, mtDNA did not differ but genotypes from 14 nuclear loci were sufficient to distinguish the fenced population. This suggests that natural (sea) and artificial barriers (town, fence) are effectively limiting gene flow among ship rat populations over the short distance (~ 500 m) between the peninsula reserve and surrounding forest. The effectiveness of the fence alone is not clear given it is a recent feature and no historical samples exist; resampling population genetic diversity over time will improve understanding. Nonetheless, the current genetic isolation of the fenced rat population suggests that rat eradication is a sensible management option given that reinvasion appears to be limited and could probably be managed with a biosecurity programme.
Unrestricted gene flow between two subspecies of translocated brushtail possums (Trichosurus vulpecula) in Aotearoa New Zealand
(Springer Nature Switzerland AG, 2022-01) Pattabiraman N; Morgan-Richards M; Powlesland R; Trewick SA
Two lineages of brushtail possums (Trichosurus vulpecula) were historically introduced to Aotearoa New Zealand, and these two subspecies have different phenotypic forms. Despite over 100 years of potential interbreeding, they appear to retain morphological differences, which may indicate reproductive isolation. We examined this using population samples from a confined landscape and scored each specimen for phenotype using a number of fur colour traits. This resulted in a bimodal trait distribution expected for segregated grey and black lineages. We also sought evidence for genetic partitioning based on spatial and temporal effects. Genetic structure and rates of genetic mixing were determined using seven neutral, species-specific nuclear microsatellite markers and mitochondrial DNA control region sequence. Genotype analyses indicated high levels of variation and mtDNA sequences formed two major haplogroups. Pairwise tests for population differentiation of these markers found no evidence of subdivision, indicating that these brushtail possums behave as a single randomly mating unit. Despite maintenance of two main colour phenotypes with relatively few intermediates, previous inference of assortative mating and anecdotes of distinct races, our data indicate that New Zealand brushtail possums can freely interbreed, and that in some locations they have formed completely mixed populations where neutral genetic markers are unrelated to phenotype. This has implications for effective pest management towards eradication.

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