The Postpharyngeal Gland: Specialized Organ for Lipid Nutrition in Leaf-Cutting Ants

Insulin signaling in the long-lived reproductive caste of ants

In most organisms, reproduction is correlated with shorter life span. However, the reproductive queen in eusocial insects exhibits a much longer life span than that of workers. In Harpegnathos ants, when the queen dies, workers can undergo an adult caste switch to reproductive pseudo-queens (gamergates), exhibiting a five-times prolonged life span. To explore the relation between reproduction and longevity, we compared gene expression during caste switching. Insulin expression is increased in the gamergate brain that correlates with increased lipid synthesis and production of vitellogenin in the fat body, both transported to the egg. This results from activation of the mitogen-activated protein kinase (MAPK) branch of the insulin signaling pathway. By contrast, the production in the gamergate developing ovary of anti-insulin Imp-L2 leads to decreased signaling of the AKT/forkhead box O (FOXO) branch in the fat body, which is consistent with their extended longevity.

Fat-Soluble Substance Flow During Symbiotic Fungus Cultivation by Leaf-Cutter Ants

Leaf-cutter ants perform a series of specialized behaviors in preparing plant substrates for their symbiotic fungus. This process may be related to contamination of workers by substances such as insecticides, leading us to hypothesize that substances are spread among workers through behaviors they perform to grow the fungus. To test this hypothesis, we analyzed the behavioral acts of workers during the processing of the pellets by using a fat-soluble tracing dye, since the active ingredient that composes toxic baits, used for control of leaf-cutter ants, is fat-soluble. The frequencies of performed behaviors were recorded and the number of dyed workers was assessed after fungus cultivation. The most frequent behavior is allogrooming and corresponds to 45.87% of the contamination process in workers, followed by holding, licking, and cutting pellets, which account for 40.22% of the process. After pellet processing, the workers had their external and internal morphological structures marked by the tracing dye-93.75% and 79.25%, respectively. These results confirm that behaviors performed during fungus cultivation contribute to dispersing substances such as insecticides, causing the contamination of workers.

Is the Salivary Gland Associated with Honey Bee Recognition Compounds in Worker Honey Bees (Apis mellifera)?

Cuticular hydrocarbons (CHCs) function as recognition compounds with the best evidence coming from social insects such as ants and honey bees. The major exocrine gland involved in hydrocarbon storage in ants is the post-pharyngeal gland (PPG) in the head. It is still not clearly understood where CHCs are stored in the honey bee. The aim of this study was to investigate the hydrocarbons and esters found in five major worker honey bee (Apis mellifera) exocrine glands, at three different developmental stages (newly emerged, nurse, and forager) using a high temperature GC analysis. We found the hypopharyngeal gland contained no hydrocarbons nor esters, and the thoracic salivary and mandibular glands only contained trace amounts of n-alkanes. However, the cephalic salivary gland (CSG) contained the greatest number and highest quantity of hydrocarbons relative to the five other glands with many of the hydrocarbons also found in the Dufour’s gland, but at much lower levels. We discovered a series of oleic acid wax esters that lay beyond the detection of standard GC columns. As a bee’s activities changed, as it ages, the types of compounds detected in the CSG also changed. For example, newly emerged bees have predominately C₁₉-C₂₃n-alkanes, alkenes and methyl-branched compounds, whereas the nurses’ CSG had predominately C_31:1 and C_33:1 alkene isomers, which are replaced by a series of oleic acid wax esters in foragers. These changes in the CSG were mirrored by corresponding changes in the adults’ CHCs profile. This indicates that the CSG may have a parallel function to the PPG found in ants acting as a major storage gland of CHCs. As the CSG duct opens into the buccal cavity the hydrocarbons can be worked into the comb wax and could help explain the role of comb wax in nestmate recognition experiments.

Microbial Communities in Different Tissues of Atta sexdens rubropilosa Leaf-cutting Ants

Bacterial endosymbionts are common in all insects, and symbiosis has played an integral role in ant evolution. Atta sexdens rubropilosa leaf-cutting ants cultivate their symbiotic fungus using fresh leaves. They need to defend themselves and their brood against diseases, but they also need to defend their obligate fungus gardens, their primary food source, from infection, parasitism, and usurpation by competitors. This study aimed to characterize the microbial communities in whole workers and different tissues of A. sexdens rubropilosa queens using Ion Torrent NGS. Our results showed that the microbial community in the midgut differs in abundance and diversity from the communities in the postpharyngeal gland of the queen and in whole workers. The main microbial orders in whole workers were Lactobacillales, Clostridiales, Enterobacteriales, Actinomycetales, Burkholderiales, and Bacillales. In the tissues of the queens, the main orders were Burkholderiales, Clostridiales, Syntrophobacterales, Lactobacillales, Bacillales, and Actinomycetales (midgut) and Entomoplasmatales, unclassified γ-proteobacteria, and Actinomycetales (postpharyngeal glands). The high abundance of Entomoplasmatales in the postpharyngeal glands (77%) of the queens was an unprecedented finding. We discuss the role of microbial communities in different tissues and castes. Bacteria are likely to play a role in nutrition and immune defense as well as helping antimicrobial defense in this ant species.

Allogrooming, Self-Grooming, and Touching Behavior: Contamination Routes of Leaf-Cutting Ant Workers Using a Fat-Soluble Tracer Dye

The aim of this study was to determine whether worker self-grooming, allogrooming, and direct contact promotes the dispersal of substances among members of the colony. For this purpose, a tracer (Sudan III dye) was applied topically to a worker ant and the social interactions between the worker with the tracer and workers without the tracer were studied. Additionally, the worker heads were dissected to visualize whether or not the post-pharyngeal gland was stained. The post-pharyngeal glands from 50% to 70% of workers were stained depending on the size of the group. With the increase in the experimental group size, the frequency of interactions between workers increased, with touching being the most frequent behavior. The tracer dye was probably passed on by direct contact between workers, followed by self-grooming and allogrooming. These behaviors are responsible for the rapid dispersal of substances among colony members as observed in our experiment. The results therefore support the hypothesis that contact with substances promotes the contamination of nestmates, even in the absence of feeding, serving as a model for further studies on the contamination of workers with the active ingredients of insecticides.