Starting with one, then

few cell types, these diversified more and more in the various evolutionary lineages. Cell types that evolved from the same immediate precursor in a given lineage are referred to as sister cell types [7]. If, in two emergent sister cell types, the cellular modules are basically retained (but modified to some extent), structure and function of these cell types initially remain the same but diverge with time. Good examples for such ‘divergence of function’ are the rods and cones of the vertebrate retina (that both retained and modified the ciliary photoreceptor module in different directions [7]). If, instead, cellular modules are lost in one or both sister cell types, structure and function of these cell types become distinct. Illustrating such ‘segregation of function’, the bipolar cells of the Selleckchem PLX4032 vertebrate retina appear to have lost the photoreceptor module that was present in their ancient precursors [7 and 8]. This process is also referred to as ‘division of labour’ [9••]. Notably, divergence and segregation of function can

co-occur in the same diversification event [7]. Can we Olaparib purchase track the evolutionary process that gave rise to today’s diversity of modules and cell types? Given that the assembly of all cellular modules follows information encoded in the genome, comparative genomics has great potential in unravelling the genealogies of these modules. In particular, genome sequencing allows inferring when a given module has come into place in the course of evolution; we can then infer, from the cell type(s) present in these ancestors, what the first function of this module has been and how this relates

to the later functions exerted by the module; furthermore, sequence comparisons reveal whether similar modules in distinct evolutionary lineages are the result of homology or convergence. Our minireview surveys recent comparative Mirabegron genomics studies that address the evolution of key modules of neurons [10, 11, 12• and 13] and muscle cells [14••], such as synapses and acto-myosin filaments, and of cell types of the immune system [15•• and 16••]. These studies track the emergence of the molecular components that constitute cell type specific modules through animal evolution and provide excellent case studies for functional divergence and division of labour. Furthermore, they exemplify a general principle that appears to govern cell type evolution: that, in many cases, novel cell types such as neurons and myocytes evolve by specialized usage of pre-existing modules rather than by the de novo-emergence of new modules. Illustrating this, our Figure 1 maps the gradual emergence of key cellular modules antedating the emergence of neurons and muscle cells on a simplified animal evolutionary tree, as deduced from these studies.