The long-standing question of how asymmetric development or asymmetric body structures in lancelets

The study of lancelets can provide important clues to understanding the evolution of chordates. A number of genes shared with vertebrates have been studied developmentally in lancelets (Holland et al., 1992, 1995, 1996; Holland and Holland, 1996; Shimeld, 1997, 1999; Terazawa and Satoh, 1997; Glardon et al., 1998; Langeland et al., 1998). In these studies, the expression pattern of the lancelet gene has been found to be similar to that of the vertebrate counterpart, supporting the close affinity of cephalochordates to vertebrates. Phylogenetic systematics using molecular tools also support a view that the cephalochordate is the sister group of vertebrates (Turberville et al., 1994; Wada and Satoh, 1994) and have been cited frequently in the study of gene expressions. In spite of the above studies, it is also true that lancelets retain many anatomical features whose phylogenetic relationships have yet to be elucidated (Gans and Northcutt, 1983; Yasui et al., 1998) and there are some examples of gene expression patterns that are quite different from vertebrates. The study of AmphiEn in a Florida lancelet, Branchiostoma floridae, has demonstrated that metameric somites, which have been considered a shared character between cephalochordates and vertebrates, utilize the same molecular cue to establish the repeated pattern as in Drosophila (Holland et al., 1997). AmphiBMP2/4 and BbBMP2/4 present another example (Panopoulou et al., 1998; I. Semba, H. S., M. U. and K. Y., unpublished data). The vertebrate BMP4 protein plays an important role in establishing the dorsoventral polarity in mesoderm (Suzuki et al., 1994; Schmidt et al., 1995), whereas AmphiBMP2/4 and BbBMP2/4 do not apparently relate to this function. Likewise, molecular systematics can yield different results depending on the molecules to be utilized (Turberville et al., 1994; Nikoh et al., 1997; Naylor and Brown, 1998). Left-right asymmetric development of lancelets is one of the most problematic features in investigating phylogenetic relationships. Three anterior structures, the preoral pit, clubshaped gland and mouth, show remarkable asymmetry in developing lancelets. Although their homology with organs or 187 Development 127, 187-195 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV1447