Cell lineage and gene expression in the development of polychaetes

The developmental strategies of embryos within the various taxa of polychaetes are designed to set up the fate of the cell lines. Some of these traits of pattern formation are considered to be ancestral, but we also find a number of derived developmental characteristics, some of which might be useful indicators for phylogenctic relationships. A combination of ooplasmic segregation and anisotropic cleavage rapidly establishes the fate of several larval cell lines in the polychaete embryo. The setting-up of the primary trochoblasts basically concerns the same cell line (la(2)-ld(2)) in polychaetes and even in molluscs. Such mechanisms may thus be regarded as ancestral. The determination of the mesoderm precursor occurs very late in both equally cleaving annelids and mollusks, indicating that an equal cleavage pattern may be regarded as an ancestral trait. Since both disproportionate cytoplasmic distribution (either by spindle shift or polar lobe formation) and cell cycle asynchronies appear to speed up the development of the mesoderm-forming cell line, these strategies represent derived traits. An analysis of these derived traits of early development is given and is discussed in the light of the phylogenetic relationships among the polychactes. These data are extended by an analysis of some of the postlarval structures in polychaetes and the molecular developmental circuitry involved.