Broad distribution and conserved roles of PKSs in animals. (A) Distribution of the pks genes found by the BLAST searches in the schematic phylogenetic tree of animal kingdom. Red font shows the presence of type I pks gene(s) in the species. Except for fly, frog and mammal, most intensively studied models, pks genes could be overlooked due to incomplete genome information. (B) Whole-mount in situ hybridization of H. pulcherrimus with probes for hppks-1 (Upper Panel) and hppks-2 (Lower Panel). hppks-1 was first detected at the mesenchyme blastula stage in the precursors of the secondary mesenchyme cells (SMCs) at the vegetal pole, and the expression persisted until the prism stage, in the SMCs and then in the ectoderm. The expression was no longer observed in pluteus larvae. hppks-2 expression initiates in PMC precursors at the blastula stage and disappear by late gastrula just after spicule formation starting (mid-gastrulation). (C) Representative results of the MO knockdown experiments in H. pulcherrimus. Images were taken at two stages (24 h and 48 h). Arrows indicate pigment cells. HpPKS-2 first Met MO-injected or its control MO-injected embryos were also observed by a dark-field microscope for visualizing the spicules. Each MO was injected at a concentration of 200 μM. ‘CMO’: Control MO, Scale bars: 50 μm.