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GROUP 7 PRESENTATION

• Bermudez, Victor Eduardo; Universidad de los Andes, Mathematics Department, Colombia
• Lima, Renato Augusto Ferreira de; University of São Paulo, Ecology Department, Brazil
• Moreno Gamez, Stefany; University of Groningen, Program in Evolutionary Biology, Netherlands
• Porta, Estanislao; Rosario National University, Program in Physics, Argentina
• Sousa, Alfredo Leonardo Porfirio; University of São Paulo, Zoology Department, Brazil
• Souza, Lucas Santana; Federal University of Bahia, Institute of Biology, Brazil

• Diogo Melo
• Renato Coutinho

Behavior can be thought of as the range of actions (or more abstractly states) an organism might exhibit. Upon coming across such definition, one might be prompted to think about animals and their frequently complex sequences of actions, or perhaps even dwell on the often unpredictable outcomes of an active human brain. However, in order to better understand the evolutionary forces that shape the decision-making processes, it is productive to choose a simpler model organism.

The soil dwelling bacterium Bacillus subtilis may very well fit this role. This species derives its nourishment from decaying organic matter, which means it must deal with an irregular nutrient supply: a bout of abundance may follow the arrival of an animal corpse or dead leaves, but the growing bacterial colony will eventually deplete the newfound resources. These bacteria have the ability to form spores, which are metabolically inactive but extremely resistant cells. Just as a time capsule, these spores can avoid present harsh conditions, such as nutrient unavailability, dryness or extreme heat, and reveal their content in a brighter, more hopeful future.

The unavoidable tradeoff to sporulating is the loss of the capacity to incorporate any available organic matter until germination. Therefore, bacterium that sporulate too soon may be outcompeted by strains that remain more active. Accordingly, the decision to become a time traveling spore is critically regulated by the nutrient availability perceived by these bacteria.

• How does the expected frequency of nutrient arrival affect this system?

• How does the unpredictability of nutrient arrival affect this system?

The metabolic cascade that leads to sporulation was long thought to be triggered by environmental factors only. Yet it has been shown that these bacteria are able to sense their populational density and incorporate this information into their decision making process. In addition, there seems to be polymorphism in the peptides and proteins responsible for this density sensing, which results in inefficient density estimation between strains.

• Does the incorporation of information about density to the decision-making process represent an evolutionary advantage?

• Is said polymorphism somehow promoted?

When faced with starvation, individuals in a colony of Bacillus subtilis might sporulate, but they might also resort to a last ditch effort before committing to the sporulation pathway. The individuals that first sense the scarcity of nutrients may delay sporulation by secreting antibiotics that kill the oblivious individuals that haven’t yet perceived the lack of nutrients. The broken cells of the killed individuals release their contents back into the environment, feeding the cannibalistic sporulation avoiders. This behavior is unlike that of many other antibiotic producing bacteria in that given the colony structure, the killing targets closely related cells. This can be understood as the emergence of some form of altruism, not unlike that of the apoptotic cells in multicellular organisms.

What conditions favor the emergence of the cannibalistic-altruistic behavior?

The model may be developed in order to address some of the questions present along the text, or even other questions derived from this complex system.

J. E. González-Pastor, E. C. Hobbs and R. Losick. Cannibalism by Sporulating Bacteria. Science, 25: 301 pp. 510-513, 2003. Available at http://www.sciencemag.org/content/301/5632/510.

A. D. Grossmann and R. Losick. Extracellular control of spore formation in Bacillus subtilis. Proc. Natl. Acad. Sci. USA, 85: 4369-4373, 1988. Available at http://www.pnas.org/content/85/12/4369.abstract.

S. J. Foster and K. Johnstone. Pulling the trigger: the mechanism of bacterial spore germination. Molecular Microbiology, 4: 137-141, 1990. Available at http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2958.1990.tb02023.x/abstract.

J. Errington. Regulation of Endospore Formation in Bacillus Subtiles. Nature Reviews Microbiology, 1: 117-126, 2003. Available at http://www.nature.com/nrmicro/journal/v1/n2/full/nrmicro750.html.

P. Stefanic and I. Mandic-Mulec. Social Interactions and Distribution of Bacillus subtilis Pherotypes at Microscale. J. Bacteriol., 191: 1756-1764, 2009. Available at http://jb.asm.org/content/191/6/1756.long.

P. Schaeffer, J. Millet, and J.-P. Aubert. Catabolic Repression of Bacterial Sporulation. Proc. Natl. Acad. Sci. USA, 54: 704-711, 1965. Available at http://www.pnas.org/content/54/3/704.citation.