Chapter 5. Patterning the spiralian embryo: insights from Ilyanassa

Abstract

The spiralian developmental program is a highly conserved mode of early development that is characterized by regularities in cleavage pattern, fate map, and larval morphology. It is found in a number of animal phyla, and was likely present in the last common ancestor of the large superphylum Lophotrochozoa. Despite this key position for understanding the evolution of development in animals, and the intrinsic advantages for using spiralian embryos to study embryonic development and asymmetric cell division, very little is known about the molecular mechanisms of spiralian embryogenesis. The snail Ilyanassa has typical spiralian development, as well as a number of practical and experimental advantages that have made it a useful model for understanding spiralian embryogenesis and basic processes in metazoan development. Here, I describe the key embryological experiments that inform our understanding of spiralian development in Ilyanassa, and I review recent progress in understanding the molecular basis of patterning this embryo.