Recent behavioural and population genetic divergence of an invasive ant in a novel environment

Ongoing Coevolution of Wolbachia and a Widespread Invasive Ant, Anoplolepis gracilipes

While Wolbachia are commonly found among arthropods, intraspecific infection rates can vary substantially across the geographic populations. Here we report nearly 100% prevalence of Wolbachia in the global populations of the yellow crazy ant, Anoplolepis gracilipes. To understand coevolutionary history between Wolbachia and A. gracilipes, we identified single nucleotide polymorphisms (SNPs) in Wolbachia from the ant across 12 geographical regions and compared the phylogeny of SNP-based Wolbachia to patterns of the ant's mitochondrial DNA (mtDNA) variation. Our results revealed a strong concordance between phylogenies of Wolbachia and host mtDNA, providing immediate evidence of co-divergence. Among eight identified SNP loci separating the genetic clusters of Wolbachia, seven loci are located in potential protein-coding genes, three of which being non-synonymous SNPs that may influence gene functions. We found a Wolbachia hypothetical protein gene with signature of positive selection. These findings jointly allow us to characterize Wolbachia-ant coevolution and also raise a question about mechanism(s) underlying maintenance of high prevalence of Wolbachia during the colonization of this invasive ant.

The complete mitochondrial genome of yellow crazy ant, Anoplolepis gracilipes (Hymenoptera: Formicidae)

The yellow crazy ant Anoplolepis gracilipes is an invasive species that threatens biodiversity in introduced ecosystems. We sequenced the A. gracilipes mitogenome using next-generation sequencing methods. The circular mitogenome of A. gracilipes was 16,943 bp included 13 protein-coding genes, two ribosomal RNA genes, 22 transfer RNAs, and a single large non-coding region of 893 bp. The base composition was AT-biased (72%). Three genomic rearrangements compared to ancestral insects were found. Phylogenetic analysis based on the concatenated nucleotide sequences of the 13 protein-coding genes supports A. gracilipes belonging to the Formicinae subfamily. We announce the A. gracilipes mitogenome as a DNA reference for further population genetic, phylogenetic, and evolutionary analyses.

Targeted research to improve invasive species management: yellow crazy ant Anoplolepis gracilipes in Samoa

Lack of biological knowledge of invasive species is recognised as a major factor contributing to eradication failure. Management needs to be informed by a site-specific understanding of the invasion system. Here, we describe targeted research designed to inform the potential eradication of the invasive yellow crazy ant Anoplolepis gracilipes on Nu’utele island, Samoa. First, we assessed the ant’s impacts on invertebrate biodiversity by comparing invertebrate communities between infested and uninfested sites. Second, we investigated the timing of production of sexuals and seasonal variation of worker abundance and nest density. Third, we investigated whether an association existed between A. gracilipes and carbohydrate sources. Within the infested area there were few other ants larger than A. gracilipes, as well as fewer spiders and crabs, indicating that A. gracilipes is indeed a significant conservation concern. The timing of male reproduction appears to be consistent with places elsewhere in the world, but queen reproduction was outside of the known reproductive period for this species in the region, indicating that the timing of treatment regimes used elsewhere are not appropriate for Samoa. Worker abundance and nest density were among the highest recorded in the world, being greater in May than in October. These abundance and nest density data form baselines for quantifying treatment efficacy and set sampling densities for post-treatment assessments. The number of plants and insects capable of providing a carbohydrate supply to ants were greatest where A. gracilipes was present, but it is not clear if this association is causal. Regardless, indirectly controlling ant abundance by controlling carbohydrate supply appears to be promising avenue for research. The type of targeted, site-specific research such as that described here should be an integral part of any eradication program for invasive species to design knowledge-based treatment protocols and determine assessment benchmarks to achieve eradication.

Quantification of supercolonial traits in the yellow crazy ant, Anoplolepis gracilipes

Supercoloniality is a social structure displayed by many invasive ant species, but there has been surprisingly little research quantifying the extent to which individual species display traits underlying such social organisation. This study quantifies three traits for the yellow crazy ant, Anoplolepis gracilipes Smith (Hymenoptera: Formicidae): little or no aggression between workers from different nests; the exchange of workers among nests; and resource exchange among nests, as well as supercolony structure arising from patterns of distribution and density of detections. Supercolonies displayed a structural continuum from being small ( < 10 ha) and "aggregated" with great continuity among detections through to being large (>10,000 ha) and "diffuse" with little continuity among detections. Smaller supercolonies had greater ant densities than larger supercolonies. In laboratory trials, no aggression was observed between workers from different nests sourced from different supercolonies, and paired nests merged within 24 hours. Workers lacked nest fidelity by rapidly populating artificial nests containing alien queens. The daily worker turnover rate per nest was estimated to be below 20%. Resources were readily moved among nests, with a resource being detected up to 13 m away from a source within 24 hours, and as far as 32 m after four days. The rate and distance of resource movement increased with increasing worker and nest density. This research has demonstrated that A. gracilipes displays supercoloniality equivalent to that of the well-studied Argentine ant Linepithema humile. Quantification of these traits is required for other supercolonial species to improve our understanding of this social strategy, especially for invasive ants to aid in understanding factors that promote invasion success and to improve management.

Genetic diversity is positively associated with fine-scale momentary abundance of an invasive ant

Many introduced species become invasive despite genetic bottlenecks that should, in theory, decrease the chances of invasion success. By contrast, population genetic bottlenecks have been hypothesized to increase the invasion success of unicolonial ants by increasing the genetic similarity between descendent populations, thus promoting co-operation. We investigated these alternate hypotheses in the unicolonial yellow crazy ant, Anoplolepis gracilipes, which has invaded Arnhem Land in Australia's Northern Territory. We used momentary abundance as a surrogate measure of invasion success, and investigated the relationship between A. gracilipes genetic diversity and its abundance, and the effect of its abundance on species diversity and community structure. We also investigated whether selected habitat characteristics contributed to differences in A. gracilipes abundance, for which we found no evidence. Our results revealed a significant positive association between A. gracilipes genetic diversity and abundance. Invaded communities were less diverse and differed in structure from uninvaded communities, and these effects were stronger as A. gracilipes abundance increased. These results contradict the hypothesis that genetic bottlenecks may promote unicoloniality. However, our A. gracilipes study population has diverged since its introduction, which may have obscured evidence of the bottleneck that would likely have occurred on arrival. The relative importance of genetic diversity to invasion success may be context dependent, and the role of genetic diversity may be more obvious in the absence of highly favorable novel ecological conditions.