Abstract
Clonality is a common characteristic of successful invasive species, but general principles underpinning the success of clonal invaders are not established. A number of mechanisms could contribute to invasion success including clones with broad tolerances and preferences, specialist clones and adaptation in situ. The majority of studies to date have been of plants and some invertebrate parthenogens, particularly aphids, and have not necessarily caught invasion at very early stages. Here we describe the early stages of an invasion by a Northern Hemisphere Hymenopteran model in three different land masses in the Southern Hemisphere. Nematus oligospilus Förster (Hymenoptera: Tenthredinidae), a sawfly feeding on willows (Salix spp.), was recently introduced to the Southern Hemisphere where it has become invasive and is strictly parthenogenetic. In this study, the number of N. oligospilus clones, their distribution in the landscape and on different willow hosts in South Africa, New Zealand and Australia were assessed using 25 microsatellite markers. Evidence is presented for the presence of two very common and widespread multilocus genotypes (MLGs) or ‘superclones’ dominating in the three countries. Rarer MLGs were closely related to the most widespread superclone; it is plausible that all N. oligospilus individuals were derived from a single clone. A few initial introductions to Australia and New Zealand seemed to have occurred. Our results point towards a separate introduction in Western Australia, potentially from South Africa. Rarer clones that were dominant locally putatively arose in situ, and might be locally favoured, or simply have not yet had time to spread. Data presented represent rare baseline data early in the invasion process for insights into the mechanisms that underlie the success of a global invader, and develop Nematus oligospilus as a valuable model to understand invasion genetics of clonal pests.
Highlights
One of the major paradoxes of invasion biology is how species with very low genetic variation can still be successful invaders [1,2,3,4]
Seven multilocus genotypes (MLGs) were found in New Zealand, five of which were unique, while in South Africa, one of three clones was unique (Table 1, Fig. 1)
The second most widespread MLG, the ‘North clone’ comprised 24.9% of individuals. It encompassed the northern part of southeastern Australia as well as most of Western Australia, it was present in low proportion at several other locations (Fig. 3)
Summary
One of the major paradoxes of invasion biology is how species with very low genetic variation (and with expected low evolutionary potential) can still be successful invaders [1,2,3,4]. As invaders usually arrive in very low numbers, genetic bottlenecks should reduce genetic diversity in the invasive range; for example: [5,6,7]. Genetic diversity in the invasive range may increase owing to admixture between individuals from multiple sources [1,9,10]. The unique clone in Western Australia (‘Aus1’) is more closely allied to a South African MLG than it is to Australian clones ( the relationships are all extremely close). Aus may represent an independent introduction into Australia, quite possibly from South Africa, a more thorough sampling of the African continent is required. Many unwanted introductions to Western Australia have occurred from South Africa, including many plants and invertebrates such as the African black beetle, considered
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