Abstract
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.
Highlights
In Aotearoa New Zealand, deliberate introduction of plants and animals, mostly during European colonisation (Thomson 1922), has resulted in extinctions and population decline across the native biota (Paulay 1994; Spatz et al 2017; Wodzicki and Wright 1984; Woinarski et al 2015)
The success of some biological invasions has been attributed to advantages conferred by high genetic diversity resulting from intraspecific hybridisation (Benvenuto et al 2012; Rius and Darling 2014), and this might be relevant given the origin of brushtail possums in New Zealand
Despite the rapid spread of possums across New Zealand, genetic structure within this 2004 sample is readily apparent with most population samples dominated by a single inferred genotype assignment (Fig. 1)
Summary
In Aotearoa New Zealand, deliberate introduction of plants and animals, mostly during European colonisation (Thomson 1922), has resulted in extinctions and population decline across the native biota (Paulay 1994; Spatz et al 2017; Wodzicki and Wright 1984; Woinarski et al 2015). Successful establishment followed by deliberate regional translocation resulted in a high level of genetic and phenotypic polymorphism across the country (Taylor et al 2004; Sarre et al 2014). This diversity is the obverse of the situation where natural range expansion is usually associated with low genetic diversity, leading to the so-called genetic paradox of invasion (Estoup et al 2016). The success of some biological invasions has been attributed to advantages conferred by high genetic diversity resulting from intraspecific hybridisation (Benvenuto et al 2012; Rius and Darling 2014), and this might be relevant given the origin of brushtail possums in New Zealand. In addition to enhancing the likelihood of successful invasion of novel habitats by an introduced species, genetic heterogeneity resulting from lineage mixing is expected to present challenges to pest control measures (Adams et al 2014; Prior et al 2018)
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