CONSERVATION OF THE SPIRALIAN DEVELOPMENTAL PROGRAM - CELL LINEAGE OFTHE NEMERTEAN, CEREBRATULUS-LACTEUS

Lineage tracers were injected into individual blastomeres in embryos o f the indirect-developing nemertean Cerebratulus lacteus through the f ormation of the fourth quartet of micromeres. Subsequent development w as followed to the formation of feeding pilidium larvae to establish t heir ultimate fates. Results showed that these blastomeres have unique fates, and their clones give rise to highly predictable regions of th e larval body. As in other spiralians, four discrete cell quadrants ca n be identified. For the most part, their identities are homologous to the typical spiralian A, B, C, and D cell quadrants. In some respects their fates differ from the typical spiralian fate map; however, thes e can be understood in terms of simple modifications of the early clea vage program. Unlike most spiralians, the first quartet micromeres in the eight-celled embryo are larger than the corresponding vegetal macr omeres, and generate most of the larval ectoderm. All four of these mi cromeres contribute to the apical organ and generate four bilaterally situated domains of ectoderm, where the progeny of the la and Id micro meres lie to the left of the median plane while those of Ib and Ic lie to the right. Unlike the progeny of the first quartet, those of the s econd quartet are situated in left (2a), ventral (2b), right (2c), and dorsal (2d) positions. The third quartet micromeres generate clones s ituated in a bilaterally symmetrical fashion similar to those of the f irst quartet. The alternating axial relationships exhibited by success ive micromere quartets are a characteristic of spiralian development. Unlike other spiralian larvae possessing a ciliary band, the pilidium larval ciliary band is formed by all blastomeres of the first and seco nd micromere quartets, as well as 3c and 3d. Ectomesoderm is derived f rom two blastomeres (3a and 3b), which give rise to the extensive arra y of the larval muscle cells. C. lacteus also possesses a true mesento blast (4d) which gives rise to a pair of mesodermal bandlets, and scat tered mesenchymal cells. The dual origin of the mesoderm, as both ecto mesoderm and endomesoderm, appears to be a condition present in all sp iralians. The gut is formed by all the fourth quartet micromeres as we ll as the vegetal macromeres (4A, 4B, 4C, 4D). Despite differences in the determination of axial properties and some modifications in quadra nt fates, nemerteans appear to be constructed on the typical spiralian developmental platform. (C) 1998 Academic Press.