Dictyostelium Discoideum (DD) are a fascinating single-cellular organism. When nutrients are plentiful, the DD cells act as autonomous individuals foraging their local vicinity. At the onset of starvation, a few (<0.1%) cells begin communicating with others by emitting a spike in the chemoattractant protein cyclic-AMP. Nearby cells sense the chemical gradient and respond by moving toward it and emitting a cyclic-AMP spike of their own. Cyclic-AMP activity increases over time, and eventually a spiral wave emerges, attracting hundreds of thousands of cells to an aggregation center. How DD cells go from autonomous individuals to a collective entity remains an open question for more than 60 years--a question whose answer would shed light on the emergence of multi-cellular life. Recently, trans-scale imaging has allowed the ability to sense the cyclic-AMP activity at both cell and colony levels. Using both the images as well as toy simulation models, this research aims to clarify whether the activity at the colony level is in fact initiated by a few cells, which may be deemed "leader" or "pacemaker" cells. In this talk, I will demonstrate the use of information-theoretic techniques to classify leaders and followers based on trajectory data, as well as to infer the domain of interaction of leader cells. We validate the techniques on toy models where leaders and followers are known, and then try to answer the question in real data--do leader cells drive collective behavior in DD colonies?

Chemotaxis is the process where cells move in response to external chemical gradients. It has mainly been viewed as a foraging and defense mechanism, enabling bacteria to move towards nutrients or away from toxins. We recently found that the Gram-positive bacterium Bacillus subtilis performs chemotaxis towards DNA. While DNA can serve as a nutrient for B. subtilis, our results suggest that the response is not to DNA itself but rather to the information encoded within the DNA. In particular, we found that B. subtilis prefers DNA from more closely related species. These results suggest that B. subtilis seeks out specific DNA sequences that are more abundant in its own and related chromosomes. In this talk, I will discuss the mechanism of DNA sensing and chemotaxis in B. subtilis. I will conclude by discussing the physiological significance of DNA chemotaxis with regards to natural competence and kin identification.