Protein-Ligand Binding and Structural Modelling Studies of Pheromone-Binding Protein-like Sol g 2.1 from Solenopsis geminata Fire Ant Venom

Sol g 2 is the major protein in Solenopsis geminata fire ant venom. It shares the highest sequence identity with Sol i 2 (S. invicta) and shares high structural homology with LmaPBP (pheromone-binding protein (PBP) from the cockroach Leucophaea maderae). We examined the specific Sol g 2 protein ligands from fire ant venom. The results revealed that the protein naturally formed complexes with hydrocarbons, including decane, undecane, dodecane, and tridecane, in aqueous venom solutions. Decane showed the highest affinity binding (Kd) with the recombinant Sol g 2.1 protein (rSol g 2.1). Surprisingly, the mixture of alkanes exhibited a higher binding affinity with the rSol g 2.1 protein compared to a single one, which is related to molecular docking simulations, revealing allosteric binding sites in the Sol g 2.1 protein model. In the trail-following bioassay, we observed that a mixture of the protein sol g 2.1 and hydrocarbons elicited S. geminata worker ants to follow trails for a longer time and distance compared to a mixture containing only hydrocarbons. This suggests that Sol g 2.1 protein may delay the evaporation of the hydrocarbons. Interestingly, the piperidine alkaloids extracted have the highest attraction to the ants. Therefore, the mixture of hydrocarbons and piperidines had a synergistic effect on the trail-following of ants when both were added to the protein.

Characterization and Localization of Sol g 2.1 Protein from Solenopsis geminata Fire Ant Venom in the Central Nervous System of Injected Crickets (Acheta domestica)

Solenopsis geminata is recognized for containing the allergenic proteins Sol g 1, 2, 3, and 4 in its venom. Remarkably, Sol g 2.1 exhibits hydrophobic binding and has a high sequence identity (83.05%) with Sol i 2 from S. invicta. Notably, Sol g 2.1 acts as a mediator, causing paralysis in crickets. Given its structural resemblance and biological function, Sol g 2.1 may play a key role in transporting hydrophobic potent compounds, which induce paralysis by releasing the compounds through the insect's nervous system. To investigate this further, we constructed and characterized the recombinant Sol g 2.1 protein (rSol g 2.1), identified with LC-MS/MS. Circular dichroism spectroscopy was performed to reveal the structural features of the rSol g 2.1 protein. Furthermore, after treating crickets with S. geminata venom, immunofluorescence and immunoblotting results revealed that the Sol g 2.1 protein primarily localizes to the neuronal cell membrane of the brain and thoracic ganglia, with distribution areas related to octopaminergic neuron cell patterns. Based on protein-protein interaction predictions, we found that the Sol g 2.1 protein can interact with octopamine receptors (OctRs) in neuronal cell membranes, potentially mediating Sol g 2.1's localization within cricket central nervous systems. Here, we suggest that Sol g 2.1 may enhance paralysis in crickets by acting as carriers of active molecules and releasing them onto target cells through pH gradients. Future research should explore the binding properties of Sol g 2.1 with ligands, considering its potential as a transporter for active molecules targeting pest nervous systems, offering innovative pest control prospects.

Biological Activities and Ecological Significance of Fire Ant Venom Alkaloids

Venoms produced by arthropods act as chemical weapons to paralyze prey or deter competitors. The utilization of venom is an essential feature in the biology and ecology of venomous arthropods. Solenopsis fire ants (Hymenoptera: Formicidae) are medically important venomous ants. They have acquired different patterns of venom use to maximize their competitive advantages rendered by the venom when facing different challenges. The major components of fire ant venom are piperidine alkaloids, which have strong insecticidal and antibiotic activities. The alkaloids protect fire ants from pathogens over the course of their lives and can be used to defend them from predators and competitors. They are also utilized by some of the fire ants' natural enemies, such as phorid flies to locate host ants. Collectively, these ants' diverse alkaloid compositions and functions have ecological significance for their survival, successful invasion, and rapid range expansion. The venom alkaloids with powerful biological activities may have played an important role in shaping the assembly of communities in both native and introduced ranges.

Synthesis and Insecticidal Activity of Fire Ant Venom Alkaloid-Based 2-Methyl-6-alkyl-Δ1,6-piperideines

2,6-dialkylpiperideines found in the venom of Solenopsis (Hymenoptera, Formicidae) fire ants are a range of compounds possessing various biological activities. A series of racemic 2-methyl-6-alkyl-Δ^1,6-piperideines were synthesized for chemical confirmation of the natural products found in fire ant venom, and the evaluation of their biological activity. Synthetic Δ^1,6-piperideines and the natural compounds in the cis-alkaloid fraction of Solenopsis invicta had identical mass spectra and retention times. Their insecticidal activities against the third-instar larvae of cotton bollworm (Helicoverpa armigera) were evaluated by using injection and topical application methods. All three compounds exhibited no lethal effect at concentrations of 0.05–0.4 mol/L by topical treatment, but moderate lethal effect at 0.4 mol/L through injection treatment. Compound 6a showed significantly higher activity than the natural insecticide nicotine. The differences in activity among compounds 6b, 6c and nicotine were not significant. The elongation of the carbon chain at the 6-position of the piperideine ring appears to decrease insecticidal activity.

Novel alkaloids from the fire ant, Solenopsis geminata

South American fire ants, Solenopsis richteri and Solenopsis invicta, were accidently introduced into the southern USA in the 1900s and 1930s, respectively. The rapid spread and high population densities of S. invicta, and its potent sting, resulted in broad economic impacts and a variety of research efforts. In the 1970s, their venom alkaloids were identified as a complex blend of trans-2-methyl-6-alkyl- and alkenyl-piperidines. Solenopsis geminata is a worldwide tramp species but a native of the southern coastal regions of the USA. It was found to only produce cis- and trans-2-methyl-6-undecyl-piperidines. These alkaloids were considered the Solenopsis ancestral alkaloid profile since they were identified from female sexuals (potential queens) of all Solenopsis species in South and North America. The dramatic modification of alkaloids in Solenopsis invicta was attributed to their response to evolutionary pressure and the lack of change in S. geminata alkaloids due to no response to evolutionary pressure. Here we report the unexpected discovery of 6-undecyl-pyridine, 2-methyl-6-undecyl-pyridine and 2-methyl-6-(1)-undecenyl-pyridine as components of S. geminata worker venom, suggesting that S. geminata like its South American relatives have responded to evolutionary pressures. Our results will stimulate future research on S. geminata populations throughout the tropical/subtropical world.

On the Biological Diversity of Ant Alkaloids

Ants have outstanding capacity to mediate inter- and intraspecific interactions by producing structurally diverse metabolites from numerous secretory glands. Since Murray Blum's pioneering studies dating from the 1950s, there has been a growing interest in arthropod toxins as natural products. Over a dozen different alkaloid classes have been reported from approximately 40 ant genera in five subfamilies, with peak diversity within the Myrmicinae tribe Solenopsidini. Most ant alkaloids function as venom, but some derive from other glands with alternative functions. They are used in defense (e.g., alarm, repellants) or offense (e.g., toxins) but also serve as antimicrobials and pheromones. We provide an overview of ant alkaloid diversity and function with an evolutionary perspective. We conclude that more directed integrative research is needed. We suggest that comparative phylogenetics will illuminate compound diversification, while molecular approaches will elucidate genetic origins. Biological context, informed by natural history, remains critical not only for research about focal species, but also to guide applied research.

A trail pheromone mediates the mutualism between ants and aphids

Mutualisms, such as the ones between ants and aphids, evolve and persist when benefits outweigh the costs from the interactions between the partners. We show here that the trail pheromone of the red imported fire ant, Solenopsis invicta, can enhance these benefits by suppressing aphid dispersal and stimulating their reproduction. The ant's mutualistic partner, the cotton aphid Aphis gossypii, was found to readily perceive and respond to two specific trail pheromone components. Two pheromone components, Z,E-α-farnesene and E,E-α-farnesene, both suppressed walking dispersal of apterous aphids, whereas only the major pheromone component, Z,E-α-farnesene, also increased aphid reproduction rate. The ants, as well as the aphids, benefit from this inter-species function of the trail pheromone. For the ants it increases and prolongs the availability of honeydew as a key food source, whereas the aphid colony benefits from faster population growth and continuous ant-provided protection. These findings reveal a hitherto unknown mechanism by which ants and aphids both increase the benefits that they provide to each other, thereby likely enhancing the stability of their mutualistic relationship.

Highlights of animal venom research on the geographical variations of toxin components, toxicities and envenomation therapy

Geographical variation of animal venom is common among venomous animals. This kind of intraspecific variation based on geographical location mainly concerned from envenomation cases and brought new problems in animal venom studies, including venom components regulatory mechanisms, differentiation of venom activities, and clinical treatment methods. At present, food is considered as the most related factor influencing venom development. Related research defined the variational venomous animal species by the comparison of venom components and activities in snakes, jellyfish, scorpions, cone snails, ants, parasitoid wasps, spiders and toads. In snake venom studies, researchers found that antivenom effectiveness was variated to different located venom samples. As described in some snake venom research, developing region-specific antivenom is the development trend. The difficulties of developing region-specific antivenom and theoretical solutions have been discussed. This review summarized biological studies of animal venom geographical variation by species, compared venom components and major biological activities of the vary venom from the same species, and listed the basic methods in comparing venom protein compositions and major toxicity differences to provide a comprehensive reference.

Fire Ant Venom Alkaloids Inhibit Biofilm Formation

Biofilm formation on exposed surfaces is a serious issue for the food industry and medical health facilities. There are many proposed strategies to delay, reduce, or even eliminate biofilm formation on surfaces. The present study focuses on the applicability of fire ant venom alkaloids (aka ‘solenopsins’, from Solenopsis invicta) tested on polystyrene and stainless steel surfaces relative to the adhesion and biofilm-formation by the bacterium Pseudomonas fluorescens. Conditioning with solenopsins demonstrates significant reduction of bacterial adhesion. Inhibition rates were 62.7% on polystyrene and 59.0% on stainless steel surfaces. In addition, solenopsins drastically reduced cell populations already growing on conditioned surfaces. Contrary to assumptions by previous authors, solenopsins tested negative for amphipathic properties, thus understanding the mechanisms behind the observed effects still relies on further investigation.

Queen venom isosolenopsin A delivers rapid incapacitation of fire ant competitors

Fire ant venom contains insecticidal alkaloids named 'solenopsins'. Whilst species-specific differences are reported, little attention has been given to caste-specific venom adaptations. The venom of fire ant queens has remained particularly poorly studied, though studies have shown it to be strikingly similar across different species, in being primarily composed of the alkaloid isosolenopsin A, regardless of the chemical configuration in workers. We predict that this is the evolutionary outcome of stabilising selection, implying that a shared mechanism is responsible for the conserved venom composition among fire ant queens. The present investigation tests whether venom plays a role in nest founding, when queens must succeed in isolation in the field against competitor species. Here, we report that fire ant queen venom and isosolenopsin A are faster to incapacitate alien ants than the venom of fire ant workers. Representative sympatric competitor ant species were selected and exposed on their heads to the venom of workers and queens of the invasive fire ant species Solenopsis invicta and S. geminata. Queen venom was found to incapacitate rival foragers quicker than worker venom. The effects were reproduced using synthetic solenopsins, establishing that solenopsin A analogues are particularly effective contact neurotoxins. Overall, the venom of S. invicta is more lethal than of S. geminata, regardless of the incapacitation speed. We believe these are fundamental aspects of the chemical ecology of the invasive ants which remain overlooked, and emphasise the need for further studies into the venom biology of founding queens.

Pseudacteon Phorid Flies: Host Specificity and Impacts on Solenopsis Fire Ants

Human commerce has resulted in the spread of the imported fire ants, Solenopsis species, worldwide. Six species of parasitic Pseudacteon phorid flies that are highly host specific to the Solenopsis saevissima complex of Solenopsis fire ants have been successfully released in the southern United States. The presence of Pseudacteon phorid flies, in addition to having direct mortality effects on their host ants, modifies foraging behavior and disrupts interspecific competition between host species and other ant species in the community. Fire ant workers have evolved effective methods to cope with parasitism pressure, which may relieve population-level impacts of introduced phorid flies. This review focuses on the mechanisms underlying host location, host preference, and host-size selection of Pseudacteon phorid flies and highlights their direct and indirect effects on fire ant populations. Knowledge gained from parasitoid-ant interactions will enhance use of natural enemies as biological control agents for invasive social insects.

Cuticular Hydrocarbon Profiles Differentiate Tropical Fire Ant Populations (Solenopsis geminata, Hymenoptera: Formicidae)

The cuticular hydrocarbons (CHCs) from hexane rinses of workers from two Florida populations (dark and red forms) of the tropical fire ant, Solenopsis geminata, were separated by silica gel chromatography and identified by GC/MS analysis. Both the dark form and the red form produce similar CHCs with carbon chain lengths ranging from 17 to 35. However, the relative percentages of these CHCs were consistently different between the two color forms. The largest CHC component in the dark form is tricosane, and (Z)-9-tricosene for the red form. There were several significant differences in percent composition. For example, the dark form was characterized by a low tricosene:tricosane ratio (ca. 0.25), whereas this ratio was > 2.5 for the red form. The ratio of tricosene:tricosane can be used as a diagnostic biomarker to delimit the dark and red forms. Cluster analysis showed that the CHCs patterns of dark form colonies are completely separated from the CHC pattern of red form colonies. Differences in social behaviors like nestmate recognition and polygyny between workers from this dark form and the red form await further investigation.