Embryonic plasticity through evolution and development in the acoel Hofstenia miamia

Abstract

The single-celled zygote ultimately develops into an adult animal with many diverse terminal cell types and tissues. This totipotency is lost during subsequent cleavages as specification begins in the embryo. In Chapter 1, I reviewed and synthesized literature on the divergent modes of early cleavage across animals. Through this study, it was revealed that many phyla, including Xenacoelomorpha, are understudied in regard to early embryonic plasticity. To investigate the potentials of embryonic cells in xenacoelomorphs, I studied the acoel worm Hofstenia miamia, which enables investigations of embryos in the lab. Though H. miamia adults are known for their tremendous regenerative capacity, no prior study has examined the embryo’s capacity to compensate for missing cells. In Chapter 2, I conducted an exhaustive examination of the post-zygotic totipotency and embryonic plasticity of the early embryo, from the 2-cell stage to the 8-cell stage. I isolated blastomeres at the 2- and 4-cell stage and found that both 2-cell blastomeres and the 4-cell stage macromeres were competent to develop into complete adult worms, exhibiting post-zygotic totipotency. Examination of the progeny of the micromeres and macromeres at the 4- and 8-cell stage showed that micromeres are specified upon birth and cannot develop cell types beyond what they endogenously produce in the wild type embryo, whereas isolated macromeres can develop cell types beyond their endogenous fates. A dissociation and reconstitution assay found that all blastomeres at the 8-cell stage can expand their developmental potential and contribute to exogenous cell types in response to the perturbation, despite earlier specification. The H. miamia embryo exhibits extraordinary plasticity despite having an invariant cleavage program with early, fate-specifying cleavages. In Chapter 3, I attempted to uncover differentially expressed transcripts between the micromeres and macromeres of the early-stage H. miamia embryos. I assembled a new version of the H. miamia transcriptome, enriched for embryonic stages, adding an additional 6,839 transcripts to the previous transcriptome assembly. After remapping newly and previously collected data to the updated transcriptome, I was able to identify cell type marker genes that were expressed earlier during embryogenesis. Altogether, my thesis research provides a summation of previous experimental embryology experiments across animals, a characterization of the plasticity in the early embryo of the acoel H. miamia and identified putative mRNA maternal determinants in the early embryo.