Cane toad toxins: mystery revealed

Abstract

Cane toads (Bufo marinus) were introduced to Australia as a biocontrol agent for the beetle pests of sugarcane. An ill-advised attempt at biocontrol, the cane toad experiment proved remarkably unsuccessful. Not only did cane toads fail to control the beetles, they became an invasive poisonous pest and threat to many native predator species and ecosystems across northern Australia. All subsequent attempts to control the cane toad invasion have proved unsuccessful. In an attempt to address this challenge we recently set out to investigate cane toad chemical ecology – arguing that better knowledge of cane toad toxins and pheromones would reveal weaknesses that might be exploited for control.In preliminary investigations into cane toad toxins Capon group reported that the chemical composition of cane toad toxin secretions was more complex than previously reported and that bacteria isolated from the cane toad parotoid gland were capable of in vitro biotransformation of cane toad toxins. In our current investigations we have (i) characterized an array of bufadienolides and arginyl amides, and developed analytical methods for quantification in cane toad toxins (ii) Analysis of cane toad toxins determined the variation of chemical diversity based on various handling methods (iii) cane toad toxins are hydrolysed by BtH during secretion from conjugates (bufotoxins) to non-conjugates (bufagenins), (iv) there are two types of microbes present in parotoid gland, a former that can degrade bufagenins and a latter that can biotransform bufagenins to other analogues (vii) cane toad eggs contains bufagenins and bufolipins (conjugates with fatty acids) and (viii) evaluation of pharmacological toxicity of cane toad toxins (an SAR study) using in vitro cytotoxicity against mammalian cells and bacteria and activation of mNOS pathway by arginyl amides.Cane toad toxin as always been identified as a single entity and the mystery (such as protoxin to toxin conversion of cane toad toxins, role of BtH and relationship between biotransformation and degradation bacteria) has been elaborated in the current thesis. Knowledge of the full story behind cane toad toxins, inclusive of bufagenins and bufotoxins, and the role of biotransforming enzymes and bacteria, is the first step in “knowing the enemy”. We are hopeful that this knowledge will inform future control solutions.