Patterning a spiralian embryo: A segregated RNA for a Tis11 ortholog is required in the 3a and 3b cells of the Ilyanassa embryo

Spiralian embryogenesis is found in a number of animal phyla, but the molecular mechanisms that pattern these embryos remain poorly understood. A hallmark of spiralian development is the production of tiers of cells, called quartets, that share distinct developmental potentials. Many RNAs have been discovered that are segregated into particular quartets, raising the possibility that such RNAs could be involved in establishing quartet-specific developmental potentials. In the spiralian embryo of the mollusc Ilyanassa, the IoTis11 RNA is segregated into the second and third quartets, then decays in nearly all lineages except for the ventral–anterior cells of the third quartet, 3a and 3b. Previously published fate-mapping studies, extended here, show that 3a and 3b make bilaterally symmetrical contributions to the esophagus, head ectoderm, and larval musculature. Deletion of either 3a or 3b has only mild effects on development, but ablating both cells impairs development of the esophagus and several other organs. Knockdown of IoTis11 with a translation-blocking morpholino oligonucleotide causes a very similar set of phenotypes as ablation of 3a and 3b, showing that translation of this transcript is required for normal development of 3a and 3b. These results show that a segregated RNA is necessary for the cells that inherit it in a spiralian embryo. Given that RNAs are asymmetrically segregated in nearly all the early cleavages in this embryo, these results suggest that the embryo is extensively patterned by segregated factors. Our experiments also uncovered two previously unappreciated non-autonomous events during Ilyanassa development. First, we found that the embryo can regulate to develop normal esophagus after deletion of either 3a or 3b. Second, we found that the 3a or 3b lineages are required for normal development of the digestive glands, which arise from the fourth order macromeres.

Research highlights
► The IoTis11 RNA is segregated to the second and third quartets of micromeres, but degrades everywhere except the ventral third quartet cells 3a and 3b.
► Fate mapping studies show that these cells generate head ectoderm, mesoderm and esophagus.
► Ablation of either 3a or 3b alone has only mild effects on development, but ablation of both results in more severe defects, notably the loss of normal esophagus and digestive glands.
► Knockdown of IoTis11 translation with a morpholino oligonucleotide results in loss of esophagus and digestive glands.
► These results suggest that differences between micromere quartets are influenced by segregated factors.