Left-right asymmetric expression of BbPtx, a Ptx-related gene, in a lancelet species and the developmental left-sidedness in deuterostomes

The long-standing question of how asymmetric development or asymmetric body structures in lancelets (amphioxus) are phylogenetically related to the bo dy plan of other animals is still untouched. Three anterior structures, the preoral pit, club-shaped gland and mouth, are remarkable asymmetric featur es in developing lancelets that all open on the left side of the body. A Pt x-related gene, BbPtx is the first identified transcription factor gene wit h an asymmetrical expression pattern in lancelets similar to that in verteb rates, and thus it may provide a clue for the above question. Expression of the BbPtx gene is first detected at the dorsal margin of the blastopore in early mid-gastrulae and then becomes restricted to the left anterodorsal w all of the primitive gut and to the developing left somitocoelomic system. Expression continues on the left side in the developing preoral pit, club-s haped gland and mouth as well as in the mesoderm at the caudal end. Unlike D-Ptx1 in Drosophilla, BbPtx is not coexpressed with a fork head gene in la ncelets; instead the two genes are expressed in a complementary fashion on the left side of the embryo. The expression pattern of BbPtx is not compati ble with the calcichordate hypothesis of Jefferies, in which the proposed a ncestor of chordates rotated its tail 90 degrees counterclockwise in relati on to the head/trunk. The expression of both BbPtx and vertebrate Pitx2 in tissues derived from the coelom implies that the left-right asymmetric deve lopment has a common origin between cephalochordates and vertebrates. Consi dering the development of the coelom in deuterostomes, however, left-right asymmetric development involving Pitx2-related genes is rather likely to be a primitive character shared among deuterostomes.