Multicellular organisms consist of populations of cells that are tightly coordinated by genetic circuits coupled to intercellular signals, physical and mechanical interactions between cells, and the environment in which they grow and reproduce. Unicellular organisms such as bacteria, fungi and algae also grow in populations, colonies, or biofilms that are coordinated by chemical signalling, genetic regulation, and physics (see for example the volvox ‘embryo’). They differentiate into distinct ‘cell types’ arranged in spatial patterns, develop morphological structures, and follow a form of reproductive cycle. However these unicellular aggregates are usually considered communities rather than organisms and certainly do not display the level of diversity and complexity seen in true multicellular systems.
The evolutionary origins of multicellular organisms are likely in these kinds of community associations between cells. Unicellular ancestors may share functional genes found in multicellular relatives but do not form organisms. These genes are involved in critical processes of intercellular coordination. Stuart Newman [REF] proposed a set of such processes, or modules, for multicellular development – Dynamical Patterning Modules. These include cell-cell adhesion, intercellular signalling, cell asymmetry, cell growth and death. While the specific evolved molecular mechanisms that implement these DPMs in extant species may be conserved, this perspective suggests that they are not essential. Rather it is the processes that they generate and their interactions that give rise to multicellular complexity as an emergent phenomenon.
Microbial cell communities exhibit all of the processes of DPMs in some form, suggesting that they have the potential to become multicellular. We use bacteria and other simple unicellular organisms to characterise and engineer mechanisms of intercellular coordination with two aims. Firstly by rewiring unicellular systems we can examine more clearly the principles of how cells interact to coordinate the population as a whole. Secondly, using synthetic biology we can connect cellular processes (signalling, adhesion, gene regulation) in novel ways to create new population structures.
An interesting question is whether using these techniques we can generate emergent multicellularity de novo and, more philosophically, would this constitute a new organism?