What sponges can tell us about the evolution of developmental processes

Sponges are one of the simplest, and probably the oldest (earliest branching) multicellular lineage of extant animals. Although their embryonic development has been intensively studied in the late 19th and early 20th century, they have been mostly neglected by modern developmental biology. Recent interest in the evolution of development, aided by advances in sequencing technology, has brought the sponges back into the spotlight. It is known that the developmental toolkit of sponges includes signalling pathways, transcription factors and cell adhesion molecules that are employed during development ofmorecomplex animals (i.e. bilaterians). We are now beginning to understand how these conserved regulatory genes are used during the development of sponges. Methodological resources are now being developed for model species representing all major sponge lineages, potentially allowing us to gain insight into the evolutionary origin of animal developmental mechanisms.