T. Gregor, K. Fujimoto, N. Masaki and S. Sawai, Science 328, 1021-1025 (2010).
Here is a very nice feature on John Bonner, Princeton emeritus professor and Dictyostelium hero. With a standing activity of over 70 years of research, he is probably the longest living research contributor to the fascinating world of the social amoebae.
He discovered in 1947 that Dictyostelium cells are attracted by and chemotax towards a chemical called cyclic adenosine monophosphate or cAMP. And it is only now, more than 60 years later that we can actually visualize and measure the concentration of this chemical in living Dictyostelium cells using an optical technology called FRET.
If you want to read more on him please go here.