We study Botryllodies sp. as a model animal to identify and functionally characterize the molecular pathways behind whole-body regeneration.
B. leachii provides a fantastic opportunity to study the regeneration phenomenon in a chordate system. Botrylloid colonies consist of adult squirts (zooids) sharing a common vascular system embedded in a gelatinous matrix called a tunic. These versatile animals can reproduce either sexually or asexually. Zooids are hermaphrodites that release sperm into the water, which can then enter the incurrent siphons of nearby zooids to fertilize the internal egg. After fertilization, the embryo develops inside a brood pouch within the zooid until it reaches a tadpole stage. All zooids are in unison with embryo development and release tadpole larva at similar times. These larvae swim to find suitable substrates to attach to and then undergo metamorphosis into an adult sea squirt. Once an adult forms, they reproduce asexually to establish a clonal colony. Asexual reproduction is by budding that occurs on a weekly basis. The adult zooid forms a primary bud which itself forms a second bud before completing morphogenesis into an adult this is followed by absorption of the original bud. This cycle of budding and absorption allows the colony to expand rapidly and once large enough the sexual cycle begins. The adults of a colony are organised into rows and connected to each other through a blood vessel network. The blood vessels terminate in termini ampullae that is separate from the adult and it is minute fragments of this vasculature containing only a few hundred blood cells are capable of regenerating a whole functional adult organism within 8-12 days. Only the loss of all the zooids results in regeneration response but very little is known about what triggers regeneration, rather than healing or degeneration in the absence of a colony adult.
Check out some short movies of this novel animal model:
(Left) Blood flowing through a colony fragment, two days after removal of all adults.
(Right) 10 days following induction of regeneration: filter-feeding adult.