Diversity and Transmission of Gut Bacteria in Atta and Acromyrmex Leaf-Cutting Ants during Development

Abstract

The social Hymenoptera have distinct larval and adult stages separated by metamorphosis, which implies striking remodeling of external and internal body structures during the pupal stage. This imposes challenges to gut symbionts as existing cultures are lost and may or may not need to be replaced. To elucidate the extent to which metamorphosis interrupts associations between bacteria and hosts, we analyzed changes in gut microbiota during development and traced the transmission routes of dominant symbionts from the egg to adult stage in the leaf-cutting ants Acromyrmex echinatior and Atta cephalotes, which are both important functional herbivores in the New World tropics. Bacterial density remained similar across the developmental stages of Acromyrmex, but Atta brood had very low bacterial prevalences suggesting that bacterial gut symbionts are not actively maintained. We found that Wolbachia was the absolute dominant bacterial species across developmental stages in Acromyrmex and we confirmed that Atta lacks Wolbachia also in the immature stages, and had mostly Mollicutes bacteria in the adult worker guts. Wolbachia in Acromyrmex appeared to be transovarially transmitted similar to transmission in solitary insects. In contrast, Mollicutes were socially transmitted from old workers to newly emerged callows. We found that larval and pupal guts of both ant species contained Pseudomonas and Enterobacter bacteria that are also found in fungus gardens, but hardly or not in adult workers, suggesting they are beneficial only for larval growth and development. Our results reveal that transmission pathways for bacterial symbionts may be very different both between developmental stages and between sister genera and that identifying the mechanisms of bacterial acquisition and loss will be important to clarify their putative mutualistic functions.

Abdominal microbial communities in ants depend on colony membership rather than caste and are linked to colony productivity

“…Although their age and physiology must greatly differ from workers such as foragers, T. nylanderi callows did not have less diverse communities than older workers. A similar result was found for fire ants (Ishak et al, ) and leafcutter ants (Zhukova et al, ). Hongoh et al () observed greater similarities between gut communities of same‐aged termites than between colony members.…”

“…Within worker caste, differences in gut communities seem to be especially pronounced between newly eclosed and older social insect workers: At eclosion from the pupal stage, honey bee workers are generally devoid of gut bacteria (Martinson, Moy, & Moran, ), but through frequent social interactions with older bees and exposure to hive materials young workers acquire different species of symbionts within several days after emergence (Powell et al, ). Likewise, newly eclosed individuals (callows) and late stage pupae of cephalotine (Russell et al, ) and attine ants (Zhukova, Sapountzis, Schiøtt, & Boomsma, ), respectively, lack bacteria in their guts. Holometabolous insects, such as the Hymenoptera, shed the entire inner larval gut epithelium including their gut content during metamorphosis (Hakim, Baldwin, & Smagghe, ), which can lead to the clearance of symbionts from the gut.…”

The evolution of abdominal microbiomes in fungus‐growing ants

“…These previously reported γ-Proteobacteria are mostly Enterobacteriaceae with abilities to fix nitrogen in laboratory fungus gardens of Atta leaf-cutting ants (Pinto-Tomás et al, 2009). They not only occur abundantly in fungus gardens (Aylward et al, 2014) but also have a substantial presence in the abdominal microbiomes of leaf-cutting ant larvae who only ingest fungal food, suggesting they may have important metabolic roles during development (Zhukova, Sapountzis, Schiøtt, & Boomsma, 2017).…”

“…These previously reported γ‐Proteobacteria are mostly Enterobacteriaceae with abilities to fix nitrogen in laboratory fungus gardens of Atta leaf‐cutting ants (Pinto‐Tomás et al, ). They not only occur abundantly in fungus gardens (Aylward et al, ) but also have a substantial presence in the abdominal microbiomes of leaf‐cutting ant larvae who only ingest fungal food, suggesting they may have important metabolic roles during development (Zhukova, Sapountzis, Schiøtt, & Boomsma, ). However, the scattered pieces of evidence for nitrogen supplementation by bacterial symbionts, suggested for the hindgut of Acromyrmex workers (Sapountzis et al, ) and shown for Atta fungus garden (Pinto‐Tomás et al, ), are as yet insufficient to understand the bigger picture of nitrogen turnover in attine ants (Shik et al, ).…”

Bacteria contribute to plant secondary compound degradation in a generalist herbivore system

“…and a diverse and abundant community of bacteria. In contrast, the internal gut of leaf-cutter ants has a reduced bacterial community, with adult worker guts containing primarily Wolbachia or Mollicutes 28,29 . Culturing, scanning electron microscopy 30 (Figure 1C), and metagenomics of fungus gardens demonstrate a consistent presence of garden bacteria and have established a core bacterial community that mostly consists of Proteobacteria, the majority being in the class Gammaproteobacteria 31–36 .…”