Mollusc shellomes: past, present and future.
In molluscs, the shell fabrication requires a large array of secreted macromolecules including proteins and polysaccharides. Some of them are occluded in the shell during mineralization process and constitute the shell repertoire. The protein moieties, also called shell proteomes or, more simply, 'shellomes', are nowadays analyzed via high-throughput approaches. These latter, applied so far on about thirty genera, have evidenced the huge diversity of shellomes from model to model. They also pinpoint the recurrent presence of functional domains of diverse natures. Shell proteins are not only involved in guiding the mineral deposition, but also in enzymatic and immunity-related functions, in signaling or in coping with many extracellular molecules such as saccharides. Many shell proteins exhibit low complexity domains, the function of which remains unclear. Shellomes appear as self-organizing systems that must be approached from the point of view of complex systems biology: at supramolecular level, they generate emergent properties, i.e., microstructures that cannot be simply explained by the sum of their parts. A conceptual scheme is developed here that reconciles the plasticity of the shellome, its evolvability and the constrained frame of microstructures. Other perspectives arising from the study of shellomes are briefly discussed, including the macroevolution of shell repertoires, their maturation and their transformation through time.