Insights into the early evolution of SOX genes from expression analyses in a ctenophore

SOX genes encode transcription factors acting in various developmental processes in bilaterian animals, such as stem cell maintenance and the control of specification and differentiation of cell types in a variety of contexts, notably in the developing nervous system. To gain insights into the early evolution of this important family of developmental regulators, we investigated the expression of one subgroup B, two subgroup E, one subgroup F and two divergent SOX genes in the cydippid larva and in the adult of the ctenophore Pleurobrachia pileus. Transcripts of the two unclassified SOX (PpiSOX2/12) were detected in the female germ line and in various populations of putative somatic stem cells/undifferentiated progenitors. The remaining genes had spatially restricted expression patterns in ciliated epithelial cells, notably within neuro-sensory territories. These data are compatible with an ancient involvement of SOX proteins in controlling aspects of stem cell maintenance, cellular differentiation and specification, notably within neuro-sensory epithelia. In addition, the results highlight the complexity of the ctenophore anatomy and suggest that the SOX played an important role in the elaboration of the unique ctenophore body plan during evolution, through multiple gene co-option.