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Tuesday, 8 September 2015

Do cheaters prosper?

Attempting to selfishly gain an immediate advantage in a situation where others are co-operating is called social cheating. Many people are likely to bitterly recall an experience of this, queue jumping is a classic example, and a wide variety of other organisms undergo the same injustices. Cheaters in theory should have an evolutionary edge, but social co-operation remains at the base of almost all populations. This is a mystery that scientists have been intrigued by for years, as there is very little we really understand about these behaviours and how they co-exist.

Social systems can be ‘modelled’ in much simpler organisms than us; the social amoeba for example. Dictyostelium discoideum (Dicty), are generally alone throughout their lives, but for one 10 hour period. This time is where they become social in order to release spores that will grow into new amoebae. To do this they form ‘fruiting bodies’, where some Dicty give up their lives and harshly but more importantly; their bodies. These form a stalk, the top of which the spores can be released from. However some Dicty cheat - there are amoebae that climb straight to the top to release their spores and contribute less to the stalk. By doing this they release more spores than other, co-operating amoebae and so gain an evolutionary edge, passing on more of their genes. But this advantage cannot be significant, as otherwise they would overrun the co-operators and drive them to extinction.

Photograph of stalked slime mould fruiting bodies, by Lairich Rig (CC BY-SA 2.0)
The Dicty amoebae begin as single celled organisms, before congregating as a multicellular ‘slug’, which then gives rise to fruiting bodies with spores atop long stalks. In the species shown here, each of the sporangia was 2-3mm tall. Image credit: © Lairich Rig, via geograph (CC BY-SA 2.0)

A long term hypothesis has stood that cheaters, whilst gaining an advantage in one aspect of their lives, pay for it by becoming worse in a different contributing factor to their evolution. However, in the case of the Dicty amoeba, research has shown that cheater cells perform equally to their co-operating counterparts in terms of growth and development[1]. This deepens the mystery of a lack of domination from the cheating population. The first Dicty cheating strain to be studied couldn’t form fruiting bodies by itself, so this seemed to answer the question of how co-operating amoebae populations are maintained. Since then though another set of cheaters have been found which can form the stalks for spore release. It would appear these evolved freeloaders could thrive as a population, but in nature they don’t. Perhaps stalks are not be made to the same standard and so none of the amoebae prosper as much. All of them would suffer through a decrease of spore release.

So maybe cheating changes with frequency, and follows one of the rules of other social behaviors; “the success of one individual's strategy depends on how many others are also employing it”[2]. Plus, despite the cheaters not contributing to the co-operation system in place, the co-operators leave them be. The relationship between the amoebae has been likened to “trench warfare”[2], where one population cannot gain a considerable enough advantage over the other to drive them out of the environment. The researchers do not understand the specific mechanisms within this battle; how one suppresses the other to a stalemate, and why.

Bacterial models could hold the answers. Some scientists have shown that bacteria have the potential to inhibit cheaters that are gaining too much of an advantage[3]. In at least one bacterium, ‘police’ have evolved to prevent cheaters becoming a significant force of nature - and others may do the same. The bacteria studied, Myxococcus xanthus (Myxo), has a very similar life cycle to the Dicty amoeba. It too lives a relatively solitary life, but spreads via spores and socially co-operates with other Myxo to form fruiting bodies. The same opportunities are then available and acted upon by cheating populations of the bacteria; a lack of contribution to the stalk and an exaggerated contribution to the spores.

The study involved introducing ‘cheaters’ to different co-operating populations of the bacteria to see how they adapted to deal with them. Instead of sacrificing themselves, some of the bacteria acted to police the cheaters, improving their own evolutionary chances of success. Although this is an individually selfish behaviour, it did improve the spore production of the other co-operating cells. These discoveries led to further questions, as the mechanisms of suppression were unknown. Another unforeseen development was the evolution of cheaters from strains made exclusively of co-operators. When tested with the old cheating and co-operating populations combined, these new cheaters were able to take advantage of the co-operators and the less evolved cheaters. But the researchers were unsure how this came about and why, as still there was no domination by a single population.

Another example of the suppression of cheating has been shown in a biofilm: a complex bacterial community (no, not a nature documentary). These are highly resistant to any type of chemical and grow as a thin ‘film’, hence their name. Pneumonia, meningitis and blood poisoning are all caused by bacteria growing as a biofilm. Research has found that biofilms oscillate in their growth, shrinking and swelling in response to the suppression of cells with the potential to cheat[4]. A biofilm has inner and outer cells, which serve different functions. The inner cells are responsible for antibiotic resistance, preventing the entire film being killed. The outer cells protect the interior and are closest to the nutrients essential for growth. These outer cells are the ones that have the potential to cheat, as they could ‘eat’ all the food outside and starve the inner cells. But they can’t, because of metabolic codependence. This is the name of the relationship between the cell layers that maintains balance and sustains both. The outer cells protect the inside as it is their greatest ally in survival against antibiotics, but the inside has its own control - it releases substances essential for the outer layer’s growth, called metabolites. So if the outside begins to consume too much its growth is stumped by the inside, creating the fluctuating patterns the scientists saw and a natural control against cheating.

Coloured contour lines of oscillating biofilm growth
A biofilm’s growth will change in accordance with an environment and its nutrient availability. It is controlled by the inner cells of the film, regulating the growth of the different layers shown in various colours. The multiple layers of the same colour are examples of a shrinkage or expansion of one type of cell.
Image credit: Suel lab, UC San Diego

Through this suppression, in a way the co-operating populations are working with cheaters, creating another level of co-operation outside of their own. But these interactions are highly complex; evolving and differentiating with every new situation researchers introduce. This is why there are so many open questions about the behaviour - it is very difficult to recreate the diverse ecosystems of nature.

A good example of the disparity between laboratory and natural environments and the effects it has on results can be seen in two studies done on yeast cells[5,6]. The strain of yeast used was picked for it’s reliance on small sugars to grow. The researchers added sucrose to a test tube of the yeast cells and they broke it down by secreting enzymes to make even smaller, consumable sugars. This food source was then available to all surrounding yeast cells for consumption. Freeloaders don’t produce the enzymes, conserving energy and then gaining more by helping themselves to the products of co-operation amongst the other cells. In 2009 the first study indicated that only about 1 in 7 of the yeast cells contributed to the enzyme production[5]. However this was a very simple model of the yeast and so in an attempt to bring it closer to a natural level of complexity a bacterial competitor was added. In this second study, co-operation tripled in the yeast populations to compete for their resources[6]. The research team concluded that cheaters were limited by competition as they couldn’t spread as much. But the study additionally highlighted the importance of improving accurate mimicking of wild ecosystems; we cannot fully understand a social behaviour if it is not being completely exhibited when we study it.

This article was written by Joshua Fleming, a biological sciences student from the University of Leicester conducting a summer internship as a science writer at TWDK.

References
why don't all references have links?

[1] LA Santorelli, A Kuspa, G Shaulsky, DC Queller, and JE Strassmann. 2013. 'A New Social Gene In Dictyostelium Discoideum, chtB'. BMC Evolutionary Biology 13 (4). doi:10.1186/1471-2148-13-4.
[2] 'Cheating — And Getting Away With It'. Washington University in St. Louis Newsroom Jan 9, 2013.
[3] Manhes, P., and G. J. Velicer. 2011. 'Experimental Evolution Of Selfish Policing In Social Bacteria'. Proceedings Of The National Academy Of Sciences 108 (20): 8357-8362. doi:10.1073/pnas.1014695108.
[4] Liu, Jintao, Arthur Prindle, Jacqueline Humphries, Marçal Gabalda-Sagarra, Munehiro Asally, Dong-yeon D. Lee, San Ly, Jordi Garcia-Ojalvo, and Gürol M. Süel. 2015. 'Metabolic Co-Dependence Gives Rise To Collective Oscillations Within Biofilms'. Nature. doi:10.1038/nature14660.
[5] Gore, Jeff, Hyun Youk, and Alexander van Oudenaarden. 2009. 'Snowdrift Game Dynamics And Facultative Cheating In Yeast'. Nature 459 (7244): 253-256. doi:10.1038/nature07921.
[6] Celiker, Hasan, and Jeff Gore. 2012. 'Competition Between Species Can Stabilize Public-Goods Cooperation Within A Species'. Mol Syst Biol 8. doi:10.1038/msb.2012.54.