Weak patriline effects are present in the cuticular hydrocarbon profiles of isolated Formica exsecta ants but they disappear in the colony environment

Discrimination of conspecifics from heterospecifics in a hybrid zone: Behavioral and chemical cues in ants

Species and nestmate recognition in social insects occurs mostly through cuticular hydrocarbons acting as chemical cues. These compounds generate a colony-specific odor profile depending on genetic and environmental factors. Species and nestmate recognition results in specific behavioral responses, regulating the level of aggression toward other individuals during an interaction. Although species discrimination and recognition cues have been poorly studied in the context of interspecific hybridization, such systems offer an opportunity to further investigate the influence of heritable and environmental factors on recognition. We explored the strength of discrimination in a hybrid zone between two ant species-Tetramorium immigrans and T. caespitum-by comparing cuticular hydrocarbon profiles and measuring intra- and interspecific worker aggression in both areas of sympatry and areas of allopatry among species. Species cuticular hydrocarbon profiles were well-differentiated and interspecific aggression was high, revealing highly discriminating species recognition cues. Hybrids' cuticular hydrocarbon profiles consisted of a mixture of the parental bouquets, but also exhibited hybrid-specific patterns. Behavioral assays showed that T. immigrans is as aggressive toward hybrids as toward heterospecifics. Finally, aggression between heterospecific workers was lower when interacting individuals came from areas of sympatry among species than from areas of allopatry. Taken as a whole, these findings paint a particularly complex picture of the recognition system in T. immigrans, T. caespitum, and their hybrids, and highlight that hybrid zones afford a still underexplored opportunity for investigating recognition mechanisms and discrimination between species.

Divide and conquer: Multicolonial structure, nestmate recognition, and antagonistic behaviors in dense populations of the invasive ant Brachymyrmex patagonicus

The ecological success of ants has made them abundant in most environments, yet inter- and intraspecific competition usually limit nest density for a given population. Most invasive ant populations circumvent this limitation through a supercolonial structure, eliminating intraspecific competition through a loss of nestmate recognition and lack of aggression toward non-nestmates. Native to South America, Brachymyrmex patagonicus has recently invaded many locations worldwide, with invasive populations described as extremely large and dense. Yet, in contrast with most invasive ants, this species exhibits a multicolonial structure, whereby each colony occupies a single nest. Here, we investigated the interplay between genetic diversity, chemical recognition, and aggressive behaviors in an invasive population of B. patagonicus. We found that, in its invasive range, this species reaches a high nest density with individual colonies located every 2.5 m and that colony boundaries are maintained through aggression toward non-nestmates. This recognition and antagonism toward non-nestmates is mediated by chemical differentiation between colonies, as different colonies exhibit distinct chemical profiles. We highlighted that the level of aggression between colonies is correlated with their degree of genetic difference, but not their overall chemical differentiation. This may suggest that only a few chemical compounds influence nestmate recognition in this species or that weak chemical differences are sufficient to elicit aggression. Overall, this study demonstrates that invasive ant populations can reach high densities despite a multicolonial structure with strong aggression between colonies, raising questions about the factors underlying their ecological success and mitigating negative consequences of competitive interactions.

Ant cuticular hydrocarbons are heritable and associated with variation in colony productivity

In social insects, cuticular hydrocarbons function in nest-mate recognition and also provide a waxy barrier against desiccation, but basic evolutionary features, including the heritability of hydrocarbon profiles and how they are shaped by natural selection are largely unknown. We used a new pharaoh ant (Monomorium pharaonis) laboratory mapping population to estimate the heritability of individual cuticular hydrocarbons, genetic correlations between hydrocarbons, and fitness consequences of phenotypic variation in the hydrocarbons. Individual hydrocarbons had low to moderate estimated heritability, indicating that some compounds provide more information about genetic relatedness and can also better respond to natural selection. Strong genetic correlations between compounds are likely to constrain independent evolutionary trajectories, which is expected, given that many hydrocarbons share biosynthetic pathways. Variation in cuticular hydrocarbons was associated with variation in colony productivity, with some hydrocarbons experiencing strong directional selection. Altogether, this study builds on our knowledge of the genetic architecture of the social insect hydrocarbon profile and indicates that hydrocarbon variation is shaped by natural selection.

Phenotypic Plasticity of Nest-Mate Recognition Cues in Formica exsecta Ants

It is well established that many ant species have evolved qualitatively distinct species-specific chemical profile that are stable over large geographical distances. Within these species profiles quantitative variations in the chemical profile allows distinct colony-specific odours to arise (chemotypes) that are shared by all colony members. This help maintains social cohesion, including defence of their colonies against all intruders, including con-specifics. How these colony -level chemotypes are maintained among nest-mates has long been debated. The two main theories are; each ant is able to biochemically adjust its chemical profile to ‘match’ that of its nest-mates and or the queen, or all nest-mates share their individually generated chemical profile via trophollaxis resulting in an average nest-mate profile. This ‘mixing’ idea is better known as the Gestalt model. Unfortunately, it has been very difficult to experimentally test these two ideas in a single experimental design. However, it is now possible using the ant Formica exsecta because the compounds used in nest-mate recognition compounds are known. We demonstrate that workers adjust their profile to ‘match’ the dominant chemical profile within that colony, hence maintaining the colony-specific chemotype and indicates that a ‘gestalt’ mechanism, i.e. profile mixing, plays no or only a minor role.

A comparative study of egg recognition signature mixtures in Formica ants

Processing of information from the environment, such as assessing group membership in social contexts, is a major determinant of inclusive fitness. For social insects, recognizing brood origin is crucial for inclusive fitness in many contexts, such as social parasitism and kin conflicts within colonies. Whether a recognition signature is informative in kin conflicts depends on the extent of a genetic contribution into the cues. We investigated colony- and matriline-specific variation in egg surface hydrocarbons in seven species of Formica ants. We show that chemical variance is distributed similarly to genetic variation, suggesting a significant genetic contribution to eggs odors in the genus. Significant among matriline components, and significant correlations between chemical and genetic similarity among individuals also indicate kin informative egg odors in several species. We suggest that egg odor surface variation could play a large role in within colony conflicts, and that a comparative method can reveal novel insight into communication of identity.

Sources of variation in cuticular hydrocarbons in the ant Formica exsecta

Phenotypic variation arises from interactions between genotype and environment, although how variation is produced and then maintained remains unclear. The discovery of the nest-mate recognition system in Formica exsecta ants has allowed phenotypic variation in chemical profiles to be quantified across a natural population of 83 colonies. We investigated if this variation was correlated or not with intrinsic (genetic relatedness), extrinsic (location, light, temperature), or social (queen number) factors. (Z)-9-Alkenes and n-alkanes showed different patterns of variance: island (location) explained only 0.2 % of the variation in (Z)-9-alkenes, but 21-29 % in n-alkanes, whereas colony of origin explained 96 % and 45-49 % of the variation in (Z)-9-alkenes and n-alkanes, respectively. By contrast, within-colony variance of (Z)-9-alkenes was 4 %, and 23-34 % in n-alkanes, supporting the function of the former as recognition cues. (Z)-9-Alkene and n-alkane profiles were correlated with the genetic distance between colonies. Only n-alkane profiles diverged with increasing spatial distance. Sampling year explained a small (5 %), but significant, amount of the variation in the (Z)-9-alkenes, but there was no consistent directional trend. Polygynous colonies and populous monogynous colonies were dominated by a rich C23:1 profile. We found no associations between worker size, mound exposure, or humidity, although effect sizes for the latter two factors were considerable. The results support the conjecture that genetic factors are the most likely source of between-colony variation in cuticular hydrocarbons.

Transcriptome characterisation of the ant Formica exsecta with new insights into the evolution of desaturase genes in social hymenoptera

BACKGROUND

Despite the recent sequencing of seven ant genomes, no genomic data are available for the genus Formica, an important group for the study of eusocial traits. We sequenced the transcriptome of the ant Formica exsecta with the 454 FLX Titanium technology from a pooled sample of workers from 70 Finnish colonies.

RESULTS

About 1,000,000 reads were obtained from a normalised cDNA library. We compared the assemblers MIRA3.0 and Newbler2.6 and showed that the latter performed better on this dataset due to a new option which is dedicated to improve contig formation in low depth portions of the assemblies. The 29,579 contigs represent 27 Mb. 50% showed similarity with known proteins and 25% could be assigned a category of gene ontology. We found more than 13,000 high-quality single nucleotide polymorphisms. The Δ9 desaturase gene family is an important multigene family involved in chemical communication in insects. We found six Δ9 desaturases in this Formica exsecta transcriptome dataset that were used to reconstruct a maximum-likelihood phylogeny of insect desaturases and to test for signatures of positive selection in this multigene family in ant lineages. We found differences with previous phylogenies of this gene family in ants, and found two clades potentially under positive selection.

CONCLUSION

This first transcriptome reference sequence of Formica exsecta provided sequence and polymorphism data that will allow researchers working on Formica ants to develop studies to tackle the genetic basis of eusocial phenotypes. In addition, this study provided some general guidelines for de novo transcriptome assembly that should be useful for future transcriptome sequencing projects. Finally, we found potential signatures of positive selection in some clades of the Δ9 desaturase gene family in ants, which suggest the potential role of sequence divergence and adaptive evolution in shaping the large diversity of chemical cues in social insects.