Highlighting the article written by A. Friedel, R. J. Paxton, A. Soro (2017) in Insectes Sociaux

Written by Insectes Sociaux Editor-in-Chief, Michael Breed (michael.breed@colorado.edu)

In this issue of Insectes Sociaux, Friedel et al. (2017) report on their investigations of population structure of a eusocial sweat bee, Lasioglossum malachurum. Their fascinating study exemplifies the complex choices that dispersing animals face. An animal that moves far from its birth location encounters unknown arrays of predators, uncertainty in finding a suitable destination habitat, and the possibility that all of the good habitat space in their path is already occupied. On the other hand, staying close to home raises the likelihood of competing with close relatives as well as inheriting any diseases or parasites that beset the previous generation.

For animals living in aggregations some challenges may be amplified. In addition to high potential for competition with close relatives, disease and parasite problems are compounded, as many possible hosts live in close proximity to one another. Aggregations may form via philopatry, in which animals establish nests close to their natal nest, which creates potential questions about social evolution. If more than one individual lives in a nest, as is the case in the closely related Lasioglossum zephyrum, Then the mechanisms of social recognition that maintain high levels of familial relationship within each nest may break down. High genetic similarity among nearby colonies, as in Lasioglossum malachurum, may blur kin distinctions, setting up possible difficulties for kin-selection explanations of the evolution of a worker caste. Given these intriguing contradictions between advantages in settling near the natal location and dispersing further, this investigation (Friedel et al. 2017) of genetic structure is particularly interesting and timely.

Friedel et al. (2017) specifically test the hypothesis that when gynes (potential queens) search for a new nesting site they are likely to choose a location near their natal nest. By using microsatellite markers to investigate genetic similarity they were able to determine that bees in neighboring nests have higher degrees of genetic similarity than bees from more distant nests within the same aggregation. They found in three of four aggregations studied that bees from very close nests were more genetically similar than expected if random dispersal had taken place. In other words, very short range dispersal seems to have resulted in the formation of aggregations of individuals or nests in small pockets within larger expanses of seemingly suitable habitat. Colonies located further apart within each aggregation showed random degrees of genetic similarity. The aggregation in which no population substructure was observed is very large and perhaps older, leading the authors to suggest that founder effects account for small scale genetic similarity within aggregations and that over time immigration of unrelated bees in the aggregation dilutes these effects. This study sets the stage for assessing how population genetic substructure affects social evolution and disease-host relationships.

The halictid bee studied by Friedel et al. (2017) is typical of many eusocial insects, which establish their nests in aggregations of tens, hundreds or even thousands of individual colonies. Aggregated nests are common in many species of halictid sweat bee, in some species of Apidae, such as Apis dorsata, and in some species of the social wasps Polistes, Mischocyttarus, and Ropalidia. We can also look beyond insects to find many interesting behavioral and evolutionary analogies that can be seen in the aggregated nesting of birds, such as oropendolas, Psarocolius spp., and various swallows Hirundo spp., and in mammals such as prairie dogs, Cynomys spp.

The repeated appearance of aggregated nesting across this wide range of taxa raises many interesting evolutionary and behavioral questions. On the surface, if aggregated nests result from limited dispersal from natal sites, this could have major effects on population genetic structure, leading to high levels of genetic similarity among individuals within small sampling areas in the aggregation. As Friedel et al. (2017) point out, this may represent a transitory phase in the development of aggregations across generational time, as long-term movement of unrelated animals into the aggregation may counterbalance the short-term effects of an aggregation having been initiated by a single female and her immediate offspring.

 Reference

A. Friedel, R. J. Paxton, A. Soro (2017) Spatial patterns of relatedness within nesting aggregations of the primitively eusocial sweat bee Lasioglossum malachurum. Insectes Sociaux DOI: 10.1007/s00040-017-0559-6

 

 

 

A blog post highlighting the article by L. Valaderes and F.S. do Nascimento in Insectes Sociaux

By Lohan Valadares  (@valadareslohan, @lo.han.792)

About 30 million years ago, in the South American savannahs, ants that cultivate fungus have over evolutionary time become the leaf-cutting ants (genera Atta and Acromyrmex). These ants reside in large subterranean nests of the Neotropical region, with galleries and fungus chambers that shelter thousands to millions of individuals, where they farm the fungus Leucoagaricus gongylophorus. The fungus has never been found free-living without the ants, and it is generally accepted that both organisms have co-evolved into an obligate mutualism, which means that one organism cannot live without the other. For the ants, the fungus is priceless as it serves as a rearing site and unique nutritional substrate for brood development. In exchange, the ants protect the fungus against parasites and forage for fresh vegetation, which is used as substrate for fungal growth. Recently, chemical analyses of the fungus gardens of Acromyrmex species have revealed the presence of hydrocarbons, a class of chemical compound found commonly on the cuticle of insects (Viana et al., 2001, Richard et al., 2007). These two species are so intertwined, however, that whether it is the ants or the fungus which are the ultimate producers of these hydrocarbons is not yet clear.

Cuticular hydrocarbons (CHCs) are of particular importance for social insects, and decades of studies have demonstrated that they are important as nestmate recognition cues. Consequently, variations in these substances have been associated with caste, age, dominance hierarchy, worker subcastes, and developmental stages. Furthermore, the leaves used as the fungus substrate seem to be an important source of CHCs for leaf-cutting ants (Richard et al., 2004, Valadares et al., 2015). This leads us to hypothesise that the intermediate organism in this process – the symbiotic fungus – is actually the one promoting the transference of these substances, or may actually be involved in the synthesis of hydrocarbons itself.

To explore the resemblance between the chemical compounds of both organisms involved in this symbiotic association, we studied the resemblance between the CHC profile of fungal cultivars with those of the ants, focusing on the changes in the CHC composition during the ants’ larval-to-adult moulting in the leaf-cutting ant Atta sexdens and its associated fungus. Our main question was: how do the changes in the CHC profile associated with the moulting cycle correlate with the hydrocarbon profile of the fungus cultivated by the ant Atta sexdens? To assess it, we collected pieces of fungal mycelium, and ant larvae, pupae and adult workers from laboratory populations, and soaked them in a solvent to extract their hydrocarbons, which later on were injected into a gas chromatography–mass spectrometry (GC–MS) for separation and identification of hydrocarbons.

Our analyses demonstrated that ant and fungus shared 58% of hydrocarbons, and the rest were all ant-specific hydrocarbons that were absent from the symbiotic fungus. Our comparative analyses revealed a great similarity between the hydrocarbon profiles of larva and fungus, due to the fact that both groups shared mainly highly concentrated linear hydrocarbons. As individuals progressed through developmental stages, the chemical profiles between ant and fungus became increasingly different. As the moulting cycle progressed, the hydrocarbon profile was characterised by a great shift from the ‘less pronounced’ state of the brood’s chemical profile towards a more diverse chemical profile of adult workers comprised of highly concentrated branched hydrocarbons.

Our findings coincide with the observations made for Acromyrmex leaf-cutting ants (Vianna et al. 2001, Richard et al. 2007), strengthening the evidence for the intimate relationship between brood and fungus as a variable shaping the hydrocarbon profile of both species. However, because we don’t know in detail how the hydrocarbons are biologically synthesised and transferred between these organisms, it is difficult to interpret whether the ants or the fungus (or even both organisms) are the ones actually synthesising hydrocarbons. However, we believe that our descriptive and comparative analyses indicating such strong resemblances has suggested new exciting opportunities of research tha­t will help us to understand how this co-evolutionary relationship has shaped the chemical profile of both organisms. If we consider the optimistic scenario where the fungus is actually the one to be transferring these compounds to the ants, how does this transference impact colony odour? Do these substances play a role in nestmate recognition? Several questions remain to be answered and hopefully our work has contributed to the journey towards the understanding of the chemo-ecology between leaf-cutting ants and their fungal cultivars.

 

References

Branstetter, M.G., Jesovnik, Aa, Sosa-Calvo, J., Lloyd, M.W., Faircloth, B.G., Brady, S.G., Schultz, T.R. (2017). Dry habitats were crucibles of domestication in the evolution of agriculture. Proceedings Royal Society B. 284: 20170095.

Richard, F.J., Heftz, A., Christides, J.P., Errard, C. (2004) Food influence on colonial recognition and chemical signature between nestmates in the fungus-growing ant Acromyrmex subterraneus subterraneus. J Chem Ecol 14: 9–16

Richard, F.J., Poulsen, M., Hefetz, E.C., Nash, D.R., Boomsma, J.J. (2007). The origin of the chemical profiles of fungal symbionts and their significance for nestmate recognition in Acromyrmex leaf-cutting ants. Behav Ecol Sociobiol 61(11):1637-1649

Valadares, L., Nascimento, D., Nascimento, F.S. (2015) Foliar substrate affects cuticular hydrocarbon profiles and intraspecific aggression in the leafcutter ant Atta sexdens. Insects 6: 141-151

Viana, A.M., Frézard, A., Malosse, C., Della Lucia, T.M., Errard, C., Lenoir, A. (2001) Colonial recognition of fungus in the fungus-growing ant Acromyrmex subterraneus subterraneus (Hymenoptera: Formicidae). Chemoecol 11(1): 29-36