Complementarity of statistical treatments to reconstruct worldwide routes of invasion: the case of the Asian ladybird Harmonia axyridis

Abstract

Inferences about introduction histories of invasive species remain challenging because of the stochastic demographic processes involved. Approximate Bayesian computation (ABC) can help to overcome these problems, but such method requires a prior understanding of population structure over the study area, necessitating the use of alternative methods and an intense sampling design. In this study, we made inferences about the worldwide invasion history of the ladybird Harmonia axyridis by various population genetics statistical methods, using a large set of sampling sites distributed over most of the species' native and invaded areas. We evaluated the complementarity of the statistical methods and the consequences of using different sets of site samples for ABC inferences. We found that the H. axyridis invasion has involved two bridgehead invasive populations in North America, which have served as the source populations for at least six independent introductions into other continents. We also identified several situations of genetic admixture between differentiated sources. Our results highlight the importance of coupling ABC methods with more traditional statistical approaches. We found that the choice of site samples could affect the conclusions of ABC analyses comparing possible scenarios. Approaches involving independent ABC analyses on several sample sets constitute a sensible solution, complementary to standard quality controls based on the analysis of pseudo-observed data sets, to minimize erroneous conclusions. This study provides biologists without expertise in this area with detailed methodological and conceptual guidelines for making inferences about invasion routes when dealing with a large number of sampling sites and complex population genetic structures.

Characterization of a newly established aggregation of the invasive ladybeetle Harmonia axyridis and current status of the invader in Spain

“…that the elytral color similarity in European populations is linked to limited points of origin in Europe, as Lombaert et al (2014) has recently stated. Finally, all the reported mentions of H. axyridis in the present paper come from sites at less than 150 km from the French border.…”

“…Logically, the Almeria hypothesis should be discarded, since the release was considered unestablished (Jacas et al, 2006;Brown et al, 2011a), and no individual has been reported from the south of Spain since this time. Considering the European hypothesis, Lombaert et al (2010Lombaert et al ( , 2014 reported three routes of invasion of the European populations of the beetle, one coming from its native area released for biocontrol 1982 in France (named, European Biocontrol Strain), and another two, each one coming from eastern or western USA. In absence of genetic analyses, it is impossible to determine without any doubt the origin of the Spanish populations.…”

The genomic basis of colour pattern polymorphism in the harlequin ladybird

“…450 bp were prepared using either the Illumina Truseq DNA sample prep kit (n=2) or the Nextera DNA Library Preparation kit (n=12) following manufacturer protocols using 14 DNA pools (each pool including the heador the leg for some pools -from n = 40 to n = 100 individuals) collected in eight populations representative of the world-wide genetic diversity 10 and the four main colour pattern forms of the species (Extended Data Table 2). Illumina sequencing, processing and mapping of reads to the HaxR assembly was performed as described above for individual data (see Supplementary Table 4 for further details).…”

“…Genetic crosses have demonstrated that the majority of H. axyridis melanic forms result from variation of multiple alleles segregating at a single, uncharacterised, autosomal locus 9,10 , hereafter referred to as the colour pattern locus.…”

“…Possibilities include ancient mutational events or even across species introgression events 10,21 . .…”