Karyotype structure and cytogenetic markers of Amoimyrmex bruchi and Amoimyrmex silvestrii: the contribution to understanding leafcutting ant relationships

Leafcutting ants are considered the most important herbivores in terrestrial environments throughout the Neotropics. <i>Amoimyrmex</i> is the sister clade of the remaining leafcutter ants from the genera <i>Atta</i> and <i>Acromyrmex</i>. <i>Amoimyrmex striatus</i> was the only species cytogenetically studied within the genus and shares the same chromosomal number with <i>Atta</i>, bearing 22 chromosomes, whereas <i>Acromyrmex</i> bears 38 chromosomes, with the exception of the social parasite <i>Acromyrmex ameliae</i> (2n=36). Our objective here was to analyze cytogenetically the species of <i>Amoimyrmex bruchi</i> and <i>Amoimyrmex silvestrii</i>, as well as to describe the karyotype of these sister species, by means of an integrative approach using classical and molecular cytogenetics. We aimed to characterize cytogenetic markers that contribute to the systematics and taxonomy of the genus. Our results showed that the karyotypes of these two species are very similar, with an identical chromosome number (2n=22), chromosome morphology (2K=20m+2sm), and location of 18S rDNA and the telomeric repeat TTAGG on the chromosomes. Yet, the microsatellite probe GA(15) showed variation across the species and populations studied. We suggest that both species diverged relatively recently and are unmistakably sisters because of the many shared characteristics, including the highly conserved karyotypes.

The scent of symbiosis: gut bacteria affect social interactions in leafcutting ants

Animal gut microbiota affect host physiology and behaviour. In eusocial Hymenoptera, where colony-level integrity is preserved via a nestmate discrimination system based on cuticular hydrocarbon mixtures, microorganismal effects may influence social dynamics. Although nestmate recognition has undergone a thorough exploration during the last four decades, few studies have investigated the putative role of gut microbes. Here we integrate metagenomic, chemical and behavioural approaches to test whether gut microbes affect nestmate recognition in Acromyrmex echinatior leaf-cutting ants. Treating workers with a sterile diet or with antibiotics resulted in a substantial alteration of their gut microbial communities. In pairwise social interactions, untreated vs. antibiotic-treated nestmates behaved more aggressively than other nestmate and non-nestmate pairs, suggesting that the suppression of microbes indirectly alters chemical social cues and triggers aggressive behaviour. Chemical analyses on treated individuals revealed a decrease in the abundance of two metapleural gland antifungal compounds, and we confirmed the correspondence between aggression levels and chemical profile differences. Feeding microbiota-remodelled ants with conspecific faecal droplets partially restored the original bacterial communities. Furthermore, non-nestmates fed with faecal droplets from different colonies were unusually aggressive compared to pairs fed with faecal droplets from the same colony. This suggests that chemicals derived from microbial strains may shape nestmate recognition, opening novel questions about the role of microorganisms in the evolution of social behaviour.