LOCALIZATION OF POLYADENYLATED RNAS DURING TELOPLASM FORMATION AND CLEAVAGE IN LEECH EMBRYOS

In the embryos of glossiphoniid leeches, as in many annelids, cytoplas mic reorganization prior to first cleavage generates domains of yolk-d eficient cytoplasm (called teloplasm) that are sequestered during the first three cell divisions to the D' macromere. Subsequently, the D' m acromere generates a set of embryonic stem cells (teloblasts) that are the progenitors of the definitive segmental tissues. The hypothesis t hat fate-determining substances are localized within the teloplasm and segregated to the D' macromere during cleavage is supported by experi ments in which a redistribution of yolk-deficient cytoplasm changes th e fate of blastomeres that inherit it (Astrow et al. 1987; Devries 197 3; Nelson and Weisblat 1992). As a step toward identifying fate-determ ining factors in teloplasm, we describe the distribution of polyadenyl ated RNAs (polyA+ RNA) in the early embryo of the leech, Helobdella tr iserialis, as inferred from in situ hybridization using tritiated poly uridylic acid (3H-polyU). Our results indicate that polyA+ RNA colocal izes with teloplasm during cytoplasmic rearrangements resulting in tel oplasm formation, and that it remains concentrated in the teloplasm du ring the cell divisions and a second cytoplasmic rearrangement during early embryogenesis. Lesser amounts of polyA+ RNA appear to be localiz ed in cortical cytoplasm at most stages.