ESTABLISHMENT OF THE DORSOVENTRAL AXIS IN NEMERTEAN EMBRYOS - EVOLUTIONARY CONSIDERATIONS OF SPIRALIAN DEVELOPMENT

The Nemertea represent one of a number of invertebrate phyla that disp lay a highly conserved pattern of cell division known as spiral cleava ge. The fates of the early blastomeres are known for representatives o f some spiralian phyla (i.e., molluscs and annelids) and in these spec ies there appears to be a high degree of conservation in the ultimate fates of particular embryonic cells. The first two cleavage planes bea r an invariant relationship to the symmetry properties of the future l arval and adult body plan. To investigate whether these properties of spiralian embryogenesis are shared (conserved) amongst members of othe r spiralian phyla, individual blastomeres in two- and four-cell embryo s of the nemertean, Nemertopsis bivittata, were microinjected with bio tinylated dextran lineage tracers. N. bivittata is a direct-developing hoplonemertean that forms a nonfeeding larva. When individual blastom eres are injected at the two-cell stage, two sets of complementary lab eling patterns (a total of four different patterns) were observed in t he ectoderm of the larvae. When cells were injected at the four-cell s tage, four different patterns were observed that represented subsets o f the four patterns observed in the previous experiment. Unlike the ca se in the annelids and molluscs, in which the first cleavage plane bea rs a strict 45 degrees angular relationship to the future dorsoventral axis, the first cleavage plane in N. bivittata can bear one of two di fferent relationships relative to the larval/adult dorsoventral axis. In half the cases examined, the first cleavage plane corresponded roug hly to the plane of bilateral symmetry, and in the rest, it lay along a frontal plane. A similar result was observed for the embryos of the indirect-developing heteronemertean, Cerebratulus lacteus. These resul ts indicate that the fates of the four cell quadrants in nemerteans ar e not directly homologous to those in other spiralians, such as the an nelids and molluscs. For instance, no single cell quadrant appears to contribute a greater share to the formation of ectoderm, as is the cas e in the formation of the post-trochal region by the D-cell quadrant i n annelids and molluscs. Rather, two adjacent cell quadrants contribut e nearly equally to the formation of dorsal or ventral ectoderm in the larvae. Possible explanations for the determination of dorsoventralit y in nemerteans, as well as implications of these findings regarding t he evolution of spiralian development, are discussed. (C) 1994 Wiley-L iss, Inc.