Discuss the advantages and disadvantages of using microbial systems to investigate the evolution of cooperative behaviour.
“Cooperative behaviour can be defined as a behaviour that benefits another individual and has been selected due to its beneficial effects on the recipient” (Hatchwell, 2018). Cooperation is demonstrated widely in nature, from multicellular organisms to microbial communities (Wingreen and Levin, 2006). Among bacterial clone mates, cooperative behaviour is common and often follows the form of the excretion of “Public Goods”, where non-contributing free riders will have an evolutionary advantage (figure 1). However, cooperation has been a puzzling evolutionary mechanism since Darwin (Czárán and Hoekstra, 2009; Celiker & Gore, 2012). Cooperation is at risk of being exploited by free riders, who do not contribute, but reap the benefits of cooperation, driving cooperating phenotypes to extinction. This concept is known as the tragedy of the commons (Czárán and Hoekstra, 2009; Celiker & Gore, 2012). Examples of cooperation between species include mutualism, whereas altruism is recognized as a cooperative behaviour, known to be costly to the actor at the benefit to the recipient (West, Griffin & Gardner, 2006).
However, cooperation has been a puzzling evolutionary mechanism since Darwin. Cooperation is at risk of being exploited by free riders, who do not contribute, but reap the benefits of cooperation, driving cooperating phenotypes to extinction. This concept is known as the tragedy of the commons.
Cooperation has largely been studied on animals, however the field of socio-microbiology has started to study cooperative behaviours in microorganisms (Czárán and Hoekstra, 2009). The discovery of coordinated multicellular behaviours such as biofilm development has developed the concept that bacteria organize into well-organized communities, a concept that was once restricted to multicellular organisms (Li & Tian, 2012). This review highlights the advantages and disadvantages of using microbial systems into the study of the evolution of cooperative behaviour.
Figure 1: Cooperative investment into Public Goods where the benefits are shared and the costs are borne individually. However, this causes a social dilemma, where there is a temptation to defect, with non-cooperators do better than co-operators (Allen, 2011)
Diversity of microbial systems
The largest biomass on the planet comprises of bacteria and archaea, emphasising microbes wealth of molecular and genomic data (Lyon, 2007). Microbes provide a diverse range of examples to investigate the evolution of cooperative behaviour (Celiker & Gore, 2012). The range of different phenotypes offered by microbe makes it useful for testing evolutionary theory experimentally (Celiker & Gore, 2012). The most well studied example of intraspecies cooperation in microorganisms is quorum sensing (QS), a simple chemical communication system between bacteria (Wingreen & Levin, 2006). QS regulates gene expression in response to changing cell population density using autoinducers (Miller & Bassler, 2001). The concentration of autoinducers increases as a function of cell density determining gene expression levels (Miller & Brassler, 2001; WIngreen & Levin, 2006). QS reduces unnecessary costs when the population density is too spread out, limiting the production of public goods when the quorum threshold is not reached (Ozkaya et al., 2017). There are numerous examples of QS in nature such as in biofilm development (Solano, Echeverz & Lasa, 2014).
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Alternatively, altruism is another example of microbial cooperation as shown in the slime mould Dictyostelium discodium (Wingreen & Levin, 2006). The model amoeba D. discodium, under high nutritional abundance remains single celled (Wingreen & Levin, 2006). However, under limited nutrients D.discodium forms aggregates that pass through multicellular stages of slug and fruiting bodies (Wingreen & Levin, 2006). D. discodium, altruistic behaviour can provide evolutionary benefits if the cooperation is towards individuals that share genes of common interest (Wingreen & Levin, 2006).
QS, biofilm development, and cell transitioning in amoeba Dictyostelium discodium showcase the rich repertoire of cooperative behaviour in microbial systems, where exploitation and cheating are abundant (Damore & Gore, 2012; Czárán & Hoekstra, 2009). Moreover, providing a unique opportunity for scientists interested in the study of the evolution of cooperation and the insight into why cheating mutants don’t spread.
Microbe’s suitability for experimental studies of evolution.
Microorganisms have been evolving and mutating on earth for billions of, yet only recently with the development of socio-biology have microbes been used to study evolution in real time (Santiago & Lenski, 2003; Greenberg, 2011).
Experimentally, microbes offer many practical benefits from rapid reproduction, allowing experiments to run for many generations as well as their ability to be stored in suspended animation and later revived (Santiago & Lenski, 2003). Thus, allowing for evolutionary comparisons from ancestral and evolved types (Santiago & Lenski, 2003). Secondly, microbes short generation times make it relatively easy to select for certain behaviours and experimental repeats can be considerably increased due to microbe’s ability to reproduce asexually and occupy large populations in small spaces (Santiago & Lenski, 2003), compared to animal and plants.
Additionally, microbial genomes are small and therefore easy to genetically manipulate, making microorganisms suitable for experimental studies of evolution in action, for example studying antibiotic resistance in bacteria (Santiago & Lenski, 2003).
Microbes remain good model organisms to study evolution in action, due to fast propagation, genetic manipulation and asexual reproduction (Santiago & Lenski, 2003). The importance of microbes to humans in terms of pathogens and within ecosystems make it even more crucial to understand the mechanisms of microbial cooperation and evolution (Santiago & Lenski, 2003).
Limitations of using microbial systems in a laboratory setting and the implications for research.
Scientist’s ability to study cooperative behaviour of microbes in the laboratory often provides limited insight into the behaviour of microbe’s multicellular communities. This is because microbes are cultured under optimal conditions, which differ to the natural conditions the behaviour evolved in (Palkova, 2004). Therefore, laboratory microorganisms can differ considerably from microbes in natural ecosystems (Palkova, 2004).
Palkova’s study (2004) has shown the difference in cooperative behaviour in laboratory and natural conditions in Saccharomyces cerevisiae. The laboratory strains of S.cerevisiae form smooth colonies with no structured pattern, whereas wild colonies form fluffy structured colonies. The wild colonies cells are joined by an extracellular matrix that provide benefits for the colony, including structural support and nutrient flow. In laboratory environments it is difficult to generate experimental conditions that replicate those of the natural environment. Furthermore, creating biofilms in the lab that are replicates of the true cooperative behaviour in nature, is difficult to achieve. Although the number of studies into microbial cooperation has increased, microbes studied under laboratory conditions don’t always provide a genuine insight into cooperative behaviour.
Potential health benefits of studying into microbial cooperation.
Antibiotic resistance is growing globally and is affecting our ability to treat simple bacterial infections (WHO, 2014). A recent study has shown that competition and cooperation between bacterial communities shapes antibiotic resistance (Brown, 2018). The study analysed mutualistic and competitive interactions between antibiotic resistance and sensitive bacteria strains (Brown, 2018). The mutualistic relationship showed sensitive species treated with antibiotics suppressed the resistant species due to their dependency on one another (Brown, 2018).
These findings hope to use bacteria’s cooperative interactions to improve antibiotics effectiveness in the prevention and treatment of a range of infections (Brown, 2018).
Abundance of microbial cooperation in natural ecosystems.
Cooperation remains a common occurrence in the evolution of cooperative phenotypes in single microbial species (Foster & Bell, 2012). However, the evolution of cooperation is questionable in natural microbial species and rather competition has been shown to dominate (Foster & Bell, 2012). Foster & Bell’s study (2012) studied the positive interactions among bacterial strains from a collective aquatic environment. The findings concluded pair wise combinations of microbial species largely resulted in harmful interactions and no net positive effects were found. It must be noted that these conflicting findings do not exclude the possibility of altruistic or mutualistic cooperative behaviours arising in microbial populations. As discussed, there are many empirical studies demonstrating the diverse cooperative behaviours within microbial communities. These findings show that culturable microbial species largely do not cooperate and are therefore not as useful, compared to the use of animals in studying the evolution of cooperative behaviour.
From their fast reproduction, simple genetics, species diversity, to their short generation times, microbes provide an excellent model for scientists testing evolutionary theory experimentally in the lab (Damore & Gore, 2013). However, the abundance of microbial cooperation in natural microbial ecosystems has been shown within Foster & Bell’s study (2012) to be lower than the array of cooperative behaviour and diversity suggested under Celiker & Gore’s study (2012). Additionally, the difficulties in replicating microbe’s natural environments in the lab can comprise the representation of cooperative behaviour (Palkova, 2004). Although, overall most scientists have now recognize that bacteria communicate and interact like multicellular organisms, and are worth studying alongside other taxa to establish a broad insight into the evolution of cooperative behaviour (Li and Tian, 2012). Future projections look to use our knowledge of microbial cooperation in biotechnological applications, where the efficiency and productivity of many applications are reliant on cooperative interactions and the potential key to solving antibiotic resistance (Cavaliere et al., 2017; Oliveria, Niehus & Foster, 2014). The use of microbes in experimental approaches to study cooperative behaviour has increased in the past decade showcasing microbes laboratory benefits (Celiker & Gore, 2013). Microbes look to be the way forward into greater understanding of the evolution of cooperative behaviour.
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