The evolution of abdominal microbiomes in fungus-growing ants.

Panagiotis Sapountzis,Morten Schiøtt,Jacobus J Boomsma,David R Nash

doi:10.1111/mec.14931

Abstract

The attine ants are a monophyletic lineage that switched to fungus farming ca. 55–60 MYA. They have become a model for the study of complex symbioses after additional fungal and bacterial symbionts were discovered, but their abdominal endosymbiotic bacteria remain largely unknown. Here, we present a comparative microbiome analysis of endosymbiotic bacteria spanning the entire phylogenetic tree. We show that, across 17 representative sympatric species from eight genera sampled in Panama, abdominal microbiomes are dominated by Mollicutes, α‐ and γ‐Proteobacteria, and Actinobacteria. Bacterial abundances increase from basal to crown branches in the phylogeny reflecting a shift towards putative specialized and abundant abdominal microbiota after the ants domesticated gongylidia‐bearing cultivars, but before the origin of industrial‐scale farming based on leaf‐cutting herbivory. This transition coincided with the ancestral single colonization event of Central/North America ca. 20 MYA, documented in a recent phylogenomic study showing that almost the entire crown group of the higher attine ants, including the leaf‐cutting ants, evolved there and not in South America. Several bacterial species are located in gut tissues or abdominal organs of the evolutionarily derived, but not the basal attine ants. The composition of abdominal microbiomes appears to be affected by the presence/absence of defensive antibiotic‐producing actinobacterial biofilms on the worker ants' cuticle, but the significance of this association remains unclear. The patterns of diversity, abundance and sensitivity of the abdominal microbiomes that we obtained explore novel territory in the comparative analysis of attine fungus farming symbioses and raise new questions for further in‐depth research.

Highlights

Several insect lineages have evolved farming symbioses with fungi
Across 17 representative sympatric species from eight genera sam‐ pled in Panama, abdominal microbiomes are dominated by Mollicutes, α‐ and γ‐ Proteobacteria, and Actinobacteria
After the ancestor of these ants specialized on fungus farming ca. 55–60 MYA, the symbiosis was elaborated in two steps—first by fully domesticating a sin‐ gle lineage of crop fungi which allowed the ants to abandon less specific subsistence farming, and second by developing indus‐ trial‐scale leaf‐cutting farming in tighter co‐evolution with in‐ creasingly multinucleate and polyploid crop fungi that gradually adapted to decompose more nutritious fresh plant material (De Fine Licht et al, 2013; Kooij et al, 2015; Nygaard et al, 2016; Schultz & Brady, 2008; Shik et al, 2016)

Summary

| INTRODUCTION

Several insect lineages have evolved farming symbioses with fungi. The oldest farmers are the platyponine ambrosia beetles (Smith, Kent, Boomsma, & Stow, 2018), which domesticated fungi ca. Workers of the evolutionarily de‐ rived Acromyrmex leaf‐cutting ants appear to have microbiomes in their guts and associated abdominal organs of comparable simplicity to those of fungus‐growing termite royal pairs, consistent with all colony members having a specialized fungal diet (Sapountzis et al, 2015). It was discovered that ant fungus farms have suffered from a single specialized genus of Escovopsis mycopathogens throughout their evolutionary history (Currie, 2001; Currie et al, 2006), which arose shortly after the attine ants started to maintain their crop fungi in dense garden aggre‐ gations (de Man et al, 2016; Gerardo, Jacobs, Currie, & Mueller, 2006; Yek, Poulsen, & Boomsma, 2012) This specific disease pres‐ sure is relevant because the attine ants evolved cuticular cultures of antibiotic‐producing Actinobacteria to suppress Escovopsis and possibly other pathogens (Barke et al, 2010; Currie et al, 2006; Currie, Scott, Summerbell, & Malloch, 1999; Haeder, Wirth, Herz, & Spiteller, 2009; Mattoso, Moreira, & Samuels, 2012; Seipke et al, 2011). Our present study was essen‐ tially completed before Branstetter et al (2017) was published, but we explicitly analyse, interpret and discuss our results in the light of this new biogeographical frame of reference

| METHODS

| DISCUSSION

Findings

CONFLICT OF INTEREST