Tracing the invasion of the mediterranean land snail Cornu aspersum aspersum becoming an agricultural and garden pest in areas recently introduced

Abstract

This study is the first on the genetics of invasive populations of one of the most widely spread land mollusc species known in the world, the "Brown Snail" Cornu aspersum aspersum. Deliberately or accidentally imported, the species has become recently a notorious pest outside its native Mediterranean range. We compared the spatial structure and genetic variability of invasive (America, Oceania, South Africa) versus native populations using five microsatellite loci and mitochondrial (Cyt b and 16S rRNA) genes as a first step towards (i) the detection of potential source populations, and (ii) a better understanding of mechanisms governing evolutionary changes involved in the invasion process. Results based on multivariate analysis (Discriminant Analysis of Principal Components), Bayesian statistical inference (Clustering, Approximate Bayesian Computation) and demographic tests allowed a construction of the introduction pathways of the species over recent centuries. While emigrants originated from only one of the two native lineages, the West one, the most likely scenario involved several introduction events and "source switching" comprising (i) an early stage (around 1660) of simultaneous introductions from Europe (France, Spain) towards Oceania (New Zealand) and California, (ii) from the early 18(th) century, a second colonization wave from bridgehead populations successfully established in California, (iii) genetic admixture in invasive areas where highly divergent populations came into contact as in New Zealand. Although these man-made pathways are consistent with historical data, introduction time estimates suggest that the two putative waves of invasion would have occurred long before the first field observations recorded, both in America and in Oceania. A prolonged lag period as the use of an incorrect generation time could explain such 100-150 years discrepancy. Lastly, the contrasting patterns of neutral genetic signal left in invasive populations are discussed in light of possible ways of facing novel environments (standing genetic variation versus new mutation).

Figure 1. Worldwide native and invasive ranges of C. a. aspersum and likely scenario of invasion.

A: Sampling locations of the 14 invasive populations analyzed and introduction pathways (routes and periods) from the Mediterranean native area inferred from Diy ABC analysis (population codes and numbers are given in the Materials and Method section and in Table S1). B: Schematic representation of the two hypothetical scenarios tested using Diy ABC to infer population introduction especially in New Zealand (NZ₂), and graph of logistic regression showing the posterior probabilities of both scenarios tested (see Materials and Methods section for description of scenario 1 and 2).

Figure 2. Median-joining network for the 16S rRNA mtDNA haplotypes of C. a. aspersum.

Each circle represents a haplotype, and circle size is proportional to haplotype frequency. Colors indicate native (grey) versus invasive (red) status of haplotypes. Branch lengths are approximately equal to inferred mutational steps. A: Phylogenetic relationships of 390 individual sequences of C. a. aspersum and schematic geographic location of the main haplogroups defined (East versus West lineages are represented by pink and green color respectively, the Kabylia putative hybrid zone is in blue). C. a. maximum is used as outgroup. B: Focus on the relationships inside the West lineage represented by the W1 and W2 sub-lineages. Dashed lines represent superflous links deleted using the Network’s MP option. Haplotype codes according to those in Table S1.

Figure 3. Median-joining network for the cyt b mtDNA haplotypes of C. a. aspersum.

A: Phylogenetic relationships of 406 individual sequences of C. a. aspersum. B: Focus on the relationships inside the West lineage represented by the W1 and W2 sub-lineages (see legend of Figure 2 for details).

Figure 4. Results of the first DAPC performed with 10 native and 10 invasive populations as prior.

A: Scatterplots of the first two principal components (first axis: 39.3% of total variance; second axis: 21.2% of the total variance) of the DAPC of 424 individuals genotyped at 5 microsatellite loci. Dots shown by different colours represent individuals of each population. Native and invasive populations are represented by green and red labels respectively. B: Percentage of miss-assignment per population with information on origin of native (black) and/or invasive (red) population(s) to which miss-assigned individuals would be clustered with the highest probability.

Figure 5. Population structure from Bayesian STRUCTURE analyses using 5 microsatellite loci for different values of k.

Each distinct cluster is represented by a particular color. Each vertical bar represents an individual. A: Population clustering for k = 2 and model 1 (admixture and allele frequencies correlated). B: Population clustering for k = 2 and model 2 (admixture, allele frequencies correlated, sampling location of populations as prior information). C: Idem as A with k = 14. D: Idem as B with k = 14 (see Materials and Methods section for models description).

Figure 6. Mismatch distributions in native versus invasive subdivisions for cyt b sequences and 16S rRNA sequences.

The continuous and interrupted (connecting circles) lines indicate the expected and observed distributions of pairwise differences obtained by fitting a model of sudden population expansion. Cyt b graphs based on (a) all invasive excluding NZ₂, (b) North American, (c) Californian, (d) Chilean sequences. 16S rRNA graphs based on (e) all invasive, (f) all invasive excluding NZ₂, (g) North American, (h) Californian sequences.