Ascidian metamorphosis

Solitary ascidians have a long legacy as model organisms for investigating developmental mechanisms because of the simplicity of their embryogenesis and their basal phylogenetic position within the chordates (Berrill, 1929; Cameron et al., 2000; Conklin, 1905; Jeffery and Swalla, 1997; Kowalevsky, 1866; Satoh, 1994). The chordate traits that unite ascidians with the cephalochordates and vertebrates include the larval notochord and dorsal nerve cord (Fig. 1A), as well as the adult pharyngeal gill slits and endostyle (Fig. 1B) (Jeffery and Swalla, 1997). Recent investigations of ascidian development have focused almost exclusively on embryogenesis (Di Gregorio et al., 2001; Hotta et al., 2000; Jeffery and Swalla, 1997; Nishida and Sawada, 2001; Satoh, 1994). However, the majority of ascidian development, including the differentiation of the adult endoderm, body wall musculature, heart, blood cells and nervous system takes place during metamorphosis (Cloney, 1978; Hinman et al., 2000, Jeffery and Swalla, 1997; Satoh, 1994). The metamorphosis of solitary ascidian tadpole larvae into sessile filter-feeding juveniles can be divided into two phases: a rapid settlement reaction followed by a prolonged period of juvenile differentiation. The initial settlement phase of ascidian metamorphosis has been thoroughly characterized by Cloney (Cloney, 1978; Cloney, 1982). After a rapid embryogenesis lasting 8-48 hours, solitary ascidian larvae hatch and swim within the plankton for a variable period of a few hours to five days. Solitary ascidian larvae have a well-developed tail including the notochord, dorsal neural tube as well as a larval CNS including an otolith and ocellus (Jeffery and Swalla, 1997) (Fig. 1A). However, they possess only rudimentary adult structures, including a single-layered endothelial rudiment of the gut and a pocket of trunk mesenchyme that will form the adult mesoderm (Hirano and Nishida, 1997) (Fig. 1B). Initially, hatched larvae are unable to respond to either natural or artificial settlement cues. Within a discrete, speciesspecific number of hours after hatching, larvae become competent to respond to settlement cues and commence a series of rapid morphological changes (Degnan et al., 1997). Within the first 20 minutes of settlement, the papillae facilitate initial attachment, the tail is rapidly resorbed and the trunk 4739 Development 129, 4739-4751 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV5984