Unexpected Patterns of Admixture in German Populations of Aedes japonicus japonicus (Diptera: Culicidae) Underscore the Importance of Human Intervention

Abstract

The mosquito Aedes japonicus japonicus, originally restricted to temperate East Asia, is now widespread in North America and more recently has become established in Europe. To ascertain the putative number of separate introductions to Europe and examine patterns of expansion we analyzed the genetic makeup of Ae. j. japonicus populations from five cemeteries in North Rhine-Westphalia and Rhineland-Palatinate, two western German federal states, as well as of specimens from populations in Belgium, Switzerland, and Austria/Slovenia. To do so, we genotyped individual specimens at seven pre-existing polymorphic microsatellite loci and sequenced part of the nad4 mitochondrial locus. We found evidence of two different genotypic signatures associated with different nad4 mitochondrial haplotypes, indicating at least two genetically differentiated populations of Ae. j. japonicus in Europe (i.e. two distinct genotypes). Belgian, Swiss, and Austrian/Slovenian populations all share the same genotypic signature although they have become differentiated since isolation. Contrary to expectations, the German Ae. j. japonicus are not closely related to those in Belgium which are geographically nearest but are also highly inbred. German populations have a unique genotype but also evidence of mixing between the two genotypes. Also unexpectedly, the populations closest to the center of the German infestation had the highest levels of admixture indicating that separate introductions did not expand and merge but instead their expansion was driven by punctuated human-mediated transport. Critically, the resulting admixed populations have higher genetic diversity and appear invasive as indicated by their increased abundance and recent spread across western Germany.

Complex patterns of global spread in invasive insects: eco-evolutionary and management consequences

“…For example, in an extensive review of medfly (Ceratitis capitata) studies, Malacrida et al (2007) found that multiple introductions consistently contribute to the maintenance or enhancement of genetic diversity in this important global invader. Similar processes have been described in invasive populations of the Eucalyptus Bronze Bug, Thaumastocoris peregrinus (Nadel et al 2009), Harlequin Ladybird, Harmonia axyridis (Lombaert et al 2010), Red Tomato Spider Mite, Tetranychus evansi (Boubou et al 2012), Eastern Subterranean Termite, Reticulitermes flavipes (Scaduto et al 2012), mosquito Aedes japonicas japonicas (Zielke et al 2014), and a number of other insect invaders (Table S1). The changing nature of invasive alien populations caused by multiple introductions over time highlights the need to periodically reassess diversity in such populations (something that is not currently being done routinely for most invasive populations), especially in light of the implications this can have for management, as discussed below.…”

Aedes albopictus and Aedes japonicus - two invasive mosquito species with different temperature niches in Europe

“…Currently , Ae. japonicus is known from six European countries [30], including Belgium [26], The Netherlands [30], and is regarded as established in Switzerland [26], Austria and Slovenia [29]. Since 2008, Ae.…”

“…albopictus and from Schaffner et al [26]; Huber et al [27]; Huber et al [19]; Krebs et al [28]; Zielke et al [29]; Melaun et al [18] and Zielke et al [30] for Ae. japonicus .…”

Evidence that implicit assumptions of ‘no evolution’ of disease vectors in changing environments can be violated on a rapid timescale

“…Both these types of DNA loci are highly variable and provide useful and somewhat independent information on demographic history [46]. We found evidence of multiple separate introductions to the USA and to Europe that often mixed and expanded [41,42,47 …”

“…Currently, Ae. j. japonicus has multiple expansion foci in Germany, Switzerland and on both sides of the border between Austria and Slovenia [41].…”