Localization of polyadenylated RNAs during teloplasm formation and cleavage in leech embryos

In the embryos of glossiphoniid leeches, as in many annelids, cytoplasmic reorganization prior to first cleavage generates domains of yolk-deficient cytoplasm (called teloplasm) that are sequestered during the first three cell divisions to the D' macromere. Subsequently, the D' macromere generates a set of embryonic stem cells (teloblasts) that are the progenitors of the definitive segmental tissues. The hypothesis that fate-determining substances are localized within the teloplasm and segregated to the D' macromere during cleavage is supported by experiments in which a redistribution of yolk-defcient cytoplasm changes the fate of blastomeres that inherit it (Astrow et al. 1987; Devries 1973; Nelson and Weisblat 1992). As a step toward identifying fate-determining factors in teloplasm, we describe the distribution of polyadenylated RNAs (polyA+ RNA) in the early embryo of the leech, Helobdella triserialis, as inferred from in situ hybridization using tritiated polyuridylic acid (3H-polyU). Our results indicate that polyA+ RNA colocalizes with teloplasm during cytoplasmic rearrangements resulting in teloplasm formation, and that it remains concentrated in the teloplasm during the cell divisions and a second cytoplasmic rearrangement during early embryogenesis. Lesser amounts of polyA+ RNA appear to be localized in cortical cytoplasm at most stages.