Southern expansion of the invasive ant Wasmannia auropunctata within its native range and its relation with clonality and human activity

Abstract

The little fire ant Wasmannia auropunctata, native to the Neotropics, has become a serious pest worldwide over the past 100 years. It was originally distributed from Mexico to northern Argentina and new evidence suggests a recent southern range expansion during the last 60 years reaching central Argentina. This supercolonial ant species has a polymorphic reproductive system. Some populations, mostly found in undisturbed natural environments, are characterised by a classical sexual haplodiploid reproductive system. In other populations, which mainly occur in human-modified habitats, diploid queens and haploid males are produced clonally while workers are produced sexually. Here we studied the association between the recent southern range expansion of W. auropunctata in relation to human activity and clonality. We carried out an extensive survey within the southern limit of the species' native distribution and characterised the type of habitat where populations were found. Moreover, we genetically determined the type of reproductive system in 35 populations by genotyping at 12 microsatellite loci a total of 191 reproductive individuals (i.e. queens and/or males). Clonality was the most common reproductive system, occurring in 31 out of 35 populations analysed. All the populations found in the recently colonised area in central Argentina were clonal and established in human-modified habitats, suggesting that clonality together with human activity might have facilitated the southwards expansion of W. auropunctata.

Fig 1. Data on the presence and reproductive system of Wasmannia auropunctata.

The circles and stars represent natural and human-modified habitats, respectively. Symbol colour represents the type of population’s reproductive system (red: clonal, green: sexual, grey: not tested because no reproductive individuals were detected). The area below the dotted line represents the region colonised by W. auropunctata over the last 60 years. The sites mentioned in the main text are identified by the code given in S1 Table. Underlined sites specify clonal populations with more than one queen genotype.

Fig 2. Neighbour-joining dendrogram of microsatellite distances.

Each dotted line box represents a clonal lineage. The colour of each location corresponds to the type of reproductive system: green for sexual, red for clonal headed by only one queen genotype, and pink for clonal with more than one queen genotype.