The evolution of larval body plans

Most marine invertebrates have a complex life cycle divided into a pelagic, planktotrophic larval stage and a benthic adult stage. The larval forms of these indirect developing animals are often so radically different from adults that they are said to possess their own body plan. How can a single genome orchestrate the sequential development of two body plans within the same life cycle? Are larval and adult body plans built upon a shared molecular architecture? What are the molecular mechanisms that underlie morphological differences between larvae and adults?

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In order to explore these issues, I study the development of the hemichordate enteropneust (or  acorn worm) Schizocardium californicum. Acorn worms are infaunal deposit feeders specialized for  life in the mud. As an adult, S. californicum possesses the typical tripartite body plan of enteropneust hemichordates, with an anterior proboscis (used for digging and transporting food  to the mouth), a collar (which encloses the buccal cavity)  and a very long trunk that encloses the gut.

late_tornaria_2In contrast, the tornaria larva of S. californicum looks nothing like a worm. Tornaria larvae are specialized for a pelagic lifestyle. They are small (1-2mm), transparent, balloon-shaped organisms that swim and feed using cilia. These larvae spend weeks to months in the plankton before metamorphosing rapidly into a miniature version of the adult.

The developmental programs that build  these two radically different bodies are encoded in a single genome. My goal is to identify hidden molecular similarities that underlie the development of these two body plans, as well as the genetic and developmental basis of their differences.