# Mass Spectrometry: A Rosetta Stone to Learn How Fungi Interact and Talk
## 2 Fungi and Their Specialized Metabolites
### Alkaloids
#### 2.2.2 Ecological Role of Specialized Metabolites  
Deterrents and Self-Protective Compounds  
Fungi have evolved a variety of defense strategies in order to protect themselves against adverse abiotic (e.g., UV radiation) and biotic environmental conditions (e.g., feeding animals, infections by bacteria, other fungi and viruses) [7; 13]. For instance, melanin, which is an indolic polymer, and other pigments, which are typically found in spores and hyphae, protect fungi against UV damage and antioxidant chemicals [13; 57]. Fungi have developed compounds with bitter or pungent properties (e.g., some indole alkaloids) that can be also in some occasions linked to toxicity [6; 40], and can deter potential predators and pathogens, or overtake the defenses of potential preys and host organisms. Indeed, it has been shown that fungivorous animals can learn to avoid fungi with pungent properties, such as the case of the opossum _Didelphis virginiana_, who learned to avoid fruiting bodies containing the toxin muscimol [58; 59; 60]. Furthermore, fungal volatile compounds can act as repellents, e.g., 1-cent-3-ol is a deterrent of the banana slug _Ariolionax columbianus_ from consuming the mushroom _Clitopilus prunulus_[61]. Many fungal metabolites act as toxins, therefore they are known as mycotoxins, and further classify as aflat, ochratoxins, zearalenone, trichothecenes, cytochalasins and fumonisins [40]. Although their ecological role in nature has been rarely tested, it is reasonable to think that they evolved as a mean to protect fungi against potential predators and overcome the defenses of potential preys.  
Chemical Communication in Ecological Associations, and Mutualistic and Symbiotic Interactions  
Fungi evolved diverse associations with fungal and non-fungal organisms. Fungi communicate and interact either via VOCs or secreted substances [14; 15]. Examples of metabolic exchange include cell surface recognition, quorum-sensing, biofilm formation and secretion of antibiotics, among others [62; 63; 64]. Each of these types of interaction plays a vital part in the fungi metabolic exchange and provides the basis for their survival. VOCs also play a major role in intraspecific interactions as they trigger certain behaviors such as attraction between gametes for mating in sexual reproduction, sporulation and induction of morphological changes. For instance, in ascomycetes and basidiomycetes, there are metabolites that have a function of mating pheromones, which can allow the discrimination among fungi genotypes [65; 66]. Some fungi attract animals to facilitate the dispersion of their spores; this attraction is usually achieved through individual VOCs such as dimethyl sulfide that is releasedby, e.g., truffles (i.e., _Tuber_ spp.) to attract mammals and insects [67; 68], or mimicking complete blends of VOCs such as the parasitic _Puccinia monoica_, that mimics the scent of co-occurring plants [69]. Both VOCs and non-VOCs also participate in the communication within strong mutualistic associations such as plants and mycorrhizal fungi. Metabolites of mycorrhizal fungi are able to trigger plant defense responses [40; 70]. Fungi also seem to have a predominant role in the ecological interactions of lichens, symbiotic associations of fungi and algae organisms, as most of the lichen specialized metabolites are from fungal origin. These lichen metabolites (e.g., atranorin, usnic acid and vulpinic acid) may act as allelopathic compounds, repellents of potential herbivores and protective against radiation, among others [71; 72].