Volvox photoreceptors. Photograph of multicellualr Volvox and domain composition of its photoreceptors according to the Pfam database. Two different cell types, i.e., large dark green reproductive cells and small pale biflagellate somatic cells are located below and at the surface, respectively. Photoreceptors are categorized in three groups, i.e., somatic-, reproductive- and non-cell-specific photoreceptors, on the basis of their cell-type specific transcript levels (Additional file 2: Figure S2 and Additional file 4: Figure S3). Proteins are drawn approximately to scale. Domain abbreviations are photly. (photolyase domain), FAD (flavin adenine dinucleotide binding domain), LOV (Light-oxygen-voltage), Ser/ThrK (serine/threonine kinase), RBS (retinal binding site), RHO (rhodopsin), HisK (histidine kinase), RR (response regulator), Cycl (adenylate/guanylate cyclase domain), RCC1 (regulator of chromosome condensation).