Recent elucidation of the expression and function of developmental genes in different animal groups has made it possible to determine the molecular developmental mechanisms underlying the evolution of various animal body

Recent elucidation of the expression and function of developmental genes in different animal groups has made it possible to determine the molecular developmental mechanisms underlying the evolution of various animal body plans. Among numerous evolutionary developmental processes, we have been focusing on the evolution of chordate body plans (Satoh and Jeffery, 1995; Satoh, 1995; Yasuo et al., 1995). The phylum Chordata comprises the subphylum groups urochordates, cephalochordates and vertebrates (cf. Fig. 8). They share several characteristic features, including a notochord, a dorsal hollow neural tube, pharyngeal gill slits and an endostyle (e.g., Brusca and Brusca, 1990; Nielsen, 1995; Gee, 1996; Hall, 1998). In addition, chordates form a superphyletic group of deuterostomes together with two other non-chordate invertebrate groups, echinoderms and hemichordates (cf. Fig. 8). Recent molecular phylogenic studies support the idea that echinoderms, hemichordates and chordates form a monophyletic group (Wada and Satoh, 1994; Turbeville et al., 1994). Therefore, chordate body plans evolved from the ancestor by developing their characteristic features mentioned above. This suggests that the elucidation of molecular developmental mechanisms underlying the formation of these organs will lead to a better understanding of the evolution of chordate body plans. Several studies have focused on developmental mechanisms underlying the formation of the notochord (Yasuo and Satoh, 1993; Harada et al., 1995; Tagawa et al., 1998) and neural tube (e.g., Wada et al., 1998). For example, Brachyury is a key developmental gene in the formation of the notochord (reviewed by Di Gregorio and Levine, 1998; Satoh et al., 1999). The pharyngeal gill slits and endostyle are organs characteristic of filter feeding, which might has arisen when primitive chordates shifted to internal feeding. Therefore, the pharyngeal gill is also an important structure for understanding the evolution of chordate body plans. In a previous study, we characterized two pharyngeal gill (endodermally derived 2539 Development 126, 2539-2550 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV2400