Centrosome-mediated RNA segregation : a novel patterning mechanism in Ilyanassa

Thesis (Ph. D.)--University of Rochester. Dept. of Biology, 2010.All multicellular organisms require asymmetric cell divisions for normal patterning during embryogenesis. Spiralian embryos are thought to be particularly reliant on autonomous cues for embryonic patterning and thus represent potentially useful models for understanding asymmetric cell division. The series of asymmetric divisions that produce the micromere quartets are particularly important for patterning, because they subdivide the animal-vegetal axis into tiers of cells with different developmental potentials. Recently, a novel segregation mechanism has been found in the embryo of the spiralian mud snail Ilyanassa obsoleta: centrosome-mediated RNA segregation. This mechanism results in each quartet inheriting a unique subset of patterning RNAs and has been proposed to be the molecular event responsible for patterning the quartets. In the experiments reported here, I examine how this mode of asymmetric RNA division is established in the Ilyanassa embryo and the development impact of these segregated RNAs. In Chapter 1 I discuss the pertinent background information for this work and discuss the advantages of using Ilyanassa as a model to study RNA segregation. In Chapter 2 I report the work on a conserved patterning gene, Nanos, which was used to develop the tools necessary to do molecular studies in the snail. The major focus of my thesis is discussed in Chapter 3. I show that one of the segregated RNAs, IoLR5, is required in the lineage which inherits it and I characterize two cis-acting elements in localized RNAs, IoLR5 and IoLR1, that mediate transport to the centrosomes. Importantly, these results demonstrate that micromere quartet identity, a hallmark of the ancient spiralian developmental program, is controlled in part by specific RNA localization motifs. I report my attempts to identify trans-acting proteins required for centrosome-mediated RNA segregation in Chapter 4 and summarize my conclusions in Chapter 5