The demographic history of invasive fire ants
Pnina Cohen, Pnina Cohen , DeWayne Schomaker , Beth Wade , Eyal Privman
Department of Evolution and Environmental Biology, University of Haifa; Department of Evolution and Environmental Biology, University of Haifa ; USDA-ARS, Gainesville, Florida, USA ; USDA-ARS, Gainesville, Florida, USA ; Department of Evolution and Environmental Biology, University of Haifa
Two closely related fire ant species Solenopsis richteri and Solenopsis invicta were introduced to Southern USA from their native range of South America early in the previous century. Here we use population genomic approach to reconstruct the demographic history of these species Samples were collected from multiple introduced and native populations of both species. Restriction site associated DNA sequencing (RAD-seq), following by stringent filtering allowed for the genotyping of 10,272 single nucleotide polymorphic loci between the samples. The catalogue of polymorphic sites was used to infer the structure of the sampled populations. This served as a basis for building a demographic history model of the two fire ants species. The model details a timeline of demographic events in the ants’ history including speciation, divergence and introduction. Running hundreds of thousands of coalescent simulations and using approximated Bayesian computations (ABC) we estimated the effective population size of the invading S. invicta ants to be 39 (95% Bayesian credibility interval of 12-140), and the number of generations this population was bottled necked to be 2 (95% credibility interval of 0-16). Analyzing the samples genomic sequences against a closely related ant species, S. fugax, we used another demographic inference algorithm based on maximum likelihood calculations. We inferred that there had been no genetic flow between the two species in their native range, in contrast to the admixture taking place in the introduced range. We also discovered that the speciation between S. invicta and S. richteri took place 68,000±9,500 generations ago and the speciation between all three Solenopsis species 1,108,600±13,250 generations ago. Our study of the demographic history of two related species that were independently introduced into the USA provide insights into the evolutionary and demographic processes underlying the genetics of invasive populations.