CHEMICAL ECOLOGY can be defined as the chemical exploration of natural molecules (natural products) that influence behavior within or between species, genera, phylla or even Kingdoms

For example, sex, trail, alarm or aggregation pheromones can drive behavioral responses between members of the same species, in turn delivering a valuable survival advantage. On occasion the same chemical can exert contradictory responses across different species. A case study, in 1996 I was part of a team that identified the alarm pheromone of the Australian meat ant, Iridomyrmex purpureus, and demonstrated that this chemical was also a selective attractant for the spider Habronestes bradleyi, a specialist predator of meat ants. In this instance, the alarm response elicited a defensive posture that protected the nest, while the attractant response killed a handful of individual ants – on balance the pheromone enhanced nest survival. Chemical ecology plays out within and between many living organisms, including microbes, plants, insects and animals. Chemical ecology research undertaken by my group seeks to explore and understand the ecological role of natural products, to gain knowledge, to develop protocols and tools, to enhance our efforts in microbial, pharmaceutical and agrochemical biodiscovery.  Our chemical ecology research can be considered against several sub-themes (see below).

 
Microbial. We are keen to explore microbial chemical ecology as it plays out between microbes, as well as between microbes and plants, microbes and animals, and microbes and insects.

Microbes vs Microbes: In a trial study, we determined (for the first time) that ~10% of fungal cultivations treated with very low concentrations (0.6 ng/mL) of the Gram –ve cell wall constituent lipopolysaccharride (LPS) responded by altering the transcriptional status of biosynthetic gene clusters (BGCs) – either enhancing or accelerating the production of selected natural products, or activating the production of entirely new natural products, including new antibacterials. Similarly, treatment with the mycobacterial cell wall constituent mycolic acid stimulated production of new anti-mycobacterial natural products. In a broader study, detailed investigations into the co-cultivation of bacteria + bacteria, bacteria + fungi, and fungi + fungi revealed great promise. We documented co-cultivation events where a chemical cue produced by microbe A activated the transcription of an otherwise silent defensive natural product(s) in microbe B. On occasion this chemical dialogue continued, with the activated defensive chemical(s) from microbe B inducing a counter defensive response by microbe A. Our efforts to detect, identify and operationalize the routine use of microbial chemical cues that activate microbial silent biosynthetic gene clusters (BGCs), has the potential to inform innovative new strategies for microbial, pharmaceutical and agrochemical biodiscovery.


Microbes vs Plants: In collaborative study we demonstrated (for the first time) that bacteria isolated from the rhizosphere of plants are stimulated by the presence of a plant pathogen (Phytophthora) to produce a chemical cue that induces a biochemical change in pathogen cells, leading to the release of a chemical that stimulates the plant innate immune system – thereby fighting off infection. Building on this discovery, we have successfully synthesised the bacterial chemical cue, confirmed its chemical stability in soil, and demonstrated that pre-treatment of either soil or seedlings defends against infection. We are currently exploring the scope of this discovery against an array of plant pathogens, prior to patent protection.

Microbes vs Animals: In a proof-of-concept study we recovered bacterial and fungal isolates from sheep feces and pastures heavily infected with the livestock parasite Haemonchus contortus. We went on to demonstrate (for the first time) that selected cultivations respond to chemical cues from H. contortus, activating the transcription of silent BGCs to produce new anthelmintics. This discovery represents a potential paradigm shift in anthelmintic discovery that we hope to exploit, to enhance our efforts in agrochemical biodiscovery.

Microbes vs Insects: In a proof-of-concept study we isolated a fungal strain from a specimen of mud dauber wasp. Chemical analysis of this strain across a panel of cultivation conditions (the Matrix) identified conditions that produced water-soluble metabolite identified as a beetle contact sex pheromone (CA). Remarkably, CA had only previously been isolated in trace quantities from beetles. Our discovery that CA was fungal in origin suggested an unprecedented “pheromonal” relationship between beetles and beetle-associated fungi. As bark beetles are a major cause of damage to plantation timbers, we are exploring the possibility that the sexual cycle of bark beetles can be disrupted in trees inoculated with our wasp-associated  fungal strain. If we can successfully use a fungal strain to protect trees from bark beetle infestation this would be of enormous value to both native forests and plantations under beetle assault. We hope to exploit the chemical ecology between microbes and insects, to enhance our efforts in agrochemical biodiscovery.

Venom. While better known for bioactive peptides, we are exploring the natural product (small molecule) chemical ecology of cone snail, spider, centipede and scorpion venoms. To date we have isolated and synthesized a selection of bioactive natural products, and are evaluating their biological properties. Building on an interest in microbial chemical ecology (see above), we have isolated multiple taxonomically distinct bacterial strains from different species of cone snail. Surprisingly, extracts prepared from all isolates exhibit identical secondary metabolite profiles – characterized by a suite of known antifungal polyketide macrolactams.  Consistent with this level of antifungal protection, we failed to recover fungal isolates from any cone snail samples. The discovery of a highly conserved chemical ecology between cone snails and associated bacteria suggests a level of co-evolution. We are currently exploring this at a genomic level.

Cane Toad. Since their 1935 introduction to northern Queensland as a failed biological control for cane beetles, the South American cane toad Rhinella marina (formerly Bufo marinus) has invaded west into the Northern Territory and across to Western Australia, and south into northern New South Wales. The ecological impact of the cane toad invasion has been profound, leading to many deaths among native predator species (e.g. lizards, snakes, crocodiles and mammals), as well as domestic pets. This assault on Australian animals and ecosystems is of great concern to the Australian public, none more so than indigenous peoples who are confronted with the poisoning of animal species and the despoiling of areas of cultural significance. Despite public and government concern, the cane toad invasion of Australia continues unabated. Disappointingly, historic cane toad control in Australia (and elsewhere) has largely been limited to the hand and trap collection of accessible adult toads, and the occasional use of barriers to impede and redirect toad movement. For a nation as vast as Australia we need more.

Our research focuses on understanding the chemical ecology of the cane toad, to identify weaknesses that might be exploited as a means of control. In this regard we have been very successful, with three very promising lines of enquiry.

Tadpole Trapping: After identifying the pheromone cane toad tadpoles use to hunt down an eat cane toad eggs, we used this knowledge to develop an tadpole attractant bait that when used in combination with a funnel trap can selectively capture and remove tadpoles from managed water bodies (e.g. dams, lakes, creeks etc…). This technology has been patented and licensed, and is currently under commercial development.

Toad Detox: Following a detailed analysis of cane toad toxin we determined (for the first time) that during a predatory attack pro-toxins stored within the parotoid gland, bufotoxins, are co-secreted with an enzyme, bufotoxin hydrolase (BtH), and are rapidly converted to toxic bufagenins. Using transcriptomic and proteomic analyses we identified and sequenced BtH, and went on to isolate and characterise the kinetics and substrate specificity of the BtH hydrolysis of bufagenins. In exploring this phenomena further we isolated bacteria from within the parotoid gland and demonstrated that they were capable of selectively biotransforming bufagenins, but not bufotoxins. Knowledge of the bufotoxin, bufagenin, BtH cycle is pivotal to a new line of research initiated with CSIRO, where we propose using CRISR-Cas9 technology to selectively knock out the BtH gene, generating a less toxic toad (i.e. where bufotoxins cannot be converted to bufagenins following a predatory attack). Gene drive technology will be evaluated as a possible means to introduce this detoxified toad phenotype into the natural population, thereby reducing pressure on native predator species.

Toad Bacteria: As noted above we have determined that selected bacteria isolated from the cane toad parotoid gland are very effective at degrading bufagenins, but are incapable of degrading bufotoxins. We are exploring means to discover/develop sub-strains of these bacteria that are capable of degrading bufotoxins. Toads inoculated with these sub-strains would potentially have their pro-toxin reserves degraded, and would be rendered less (non) toxic.

Cane Toad Challenge (CTC). The CTC is a community engagement and citizen science program aimed at raising our profile, and attracting corporate and public sponsorship/donations/grants, to support our ongoing cane toad control research. The CTC also serves as a mechanism for informing, inspiring and partnering with, and supplying tadpole attractant baits and advice direct to the public. This initiative is gaining remarkable traction, and is poised to deliver unprecedented access to cane toad control solutions, direct to the public, community organizations, industry and government agencies.

For further information on cane toads consider reading the following; 

 
An ugly menace
(UQ Research Impact)  
 
Myth-busting cane toads
(Issues, 2010, p. 38-41)
 
Chemists vs cane toads
(Engineering World, 2009, p. 22-25)

Myth busting - cane toads in Australia
(Chemistry in Australia, 2009, p. 3-6)

Use of chemical ecology for control of the cane toad?(
Chemical Signals in Vertebrates 11, J.L. Hurst, et al., Editors. 2008, Springer: U.S.A. p. 409-417) 
 
Cane toad chemical ecology: Getting to know your enemy
(Science of Cane Toad Invasion and Control in Proceedings of the Invasive Animals CRC/CSIRO/Qld NRM&W Cane Toad Workshop. 2006. Rydges Resort, Brisbane: Invasive Animals Cooperative Research Centre)