© 1975 by The Society for Integrative and Comparative Biology
Transcripts of Reiterated DNA Sequences in the Determination of Blastomeres and Early Differentiation in Echinoid Larvae
Departments of Microbiology and Zoology and the Friday Harbor Laboratories, University of Washington Seattle, Washington 98195
RNA transcribed from repetitive DNA sequences during the early cleavages and RNA present in unfertilized eggs of the sand dollar Dendraster excentricus have been studied by means of molecular hybridization. Egg RNA, capable of competing in the hybridization reactions, is localized on particles in the cytoplasm. RNA populations synthesized during the first four cleavages are found to be very similar to egg RNA, are nuclear in origin, heterogeneous in size, and probably contain histone mRNA. Egg-type RNAs are present, and are synthesized in embryos throughout early development; the relative amounts of these RNAs increase to the late blastula stage and then decrease to a low level at the prism stage.
Blastomeres were separated at the 16-cell stage to yield pure preparations of micromeres and a meso-plus macromere fraction. Competition-hybridization experiments showed that egg-type RNA populations present in embryos at the fourth cleavage were distributed almost equally in the isolated micromere and meso-plus macromere fractions. However, experiments with H3-RNA extracted from micromeres, obtained after pulse-labeling the 16-cell stage embryos, showed qualitatively new transcriptions from repeated DNA sequences. Such new transcripts could not be detected in H3-RNA isolated from the meso-plus macromere fraction. These observations are discussed in relation to current information on the synthesis of RNA and its involvement in early echinoderm development.