Dynamic patterning of maternal mRNAs in the Early C. elegans embryo

Abstract

Asymmetric segregation of maternally-encoded proteins is essential to cell fate determination during early cell divisions of the Caenorhabditis elegans (C. elegans) embryo, but little is known about the patterning of maternal transcripts inside somatic lineages. In the first Chapter of this thesis, by detecting individual mRNA molecules in situ, we measured the densities of the two maternal mRNAs pie-1 and nos-2 in non-germline cells. We find that nos-2 mRNA degrades at a constant rate in all somatic lineages, starting approximately 1 cell-cycle after each lineage separated from the germline, consistent with a model in which the germline protects maternal mRNAs from degradation. In contrast, the degradation of pie-1 mRNAs in one somatic lineage, AB, takes place at a rate slower than that of the other lineages, leading to an accumulation of that transcript. We further show that the 3' untranslated (UTR) region of the pie-1 transcript at least partly encodes the AB-specific degradation delay. Our results indicate that embryos actively control maternal mRNA distributions in somatic lineages via regulated degradation, providing another potential mechanism for lineage specification. The evolutionary fate of an allele ordinarily depends on its contribution to host fitness. Occasionally, however, genetic elements arise that are able to gain a transmission advantage while simultaneously imposing a fitness cost on their hosts. Seidel et al. previously discovered one such element in C. elegans that gains a transmission advantage through a combination of paternal-effect killing and zygotic self-rescue. In the second Chapter of this thesis we demonstrate that this element is composed of a sperm-delivered toxin, peel-1, and an embryo-expressed antidote, zeel-1. peel-1 and zeel-1 are located adjacent to one another in the genome and co-occur in an insertion/ deletion polymorphism. peel-1 encodes a novel four-pass transmembrane protein that is expressed in sperm and delivered to the embryo via specialized, sperm-specific vesicles. In the absence of zeel-1, sperm-delivered PEEL-1 causes lethal defects in muscle and epidermal tissue at the two-fold stage of embryogenesis. zeel-1 is expressed transiently in the embryo and encodes a novel six-pass transmembrane domain fused to a domain with sequence similarity to zyg-11, a substrate-recognition subunit of an E3 ubiquitin ligase. zeel-1 appears to have arisen recently, during an expansion of the zyg-11 family, and the transmembrane domain of zeel-1 is required and partially sufficient for antidote activity. Although PEEL-1 and ZEEL-1 normally function in embryos, these proteins can act at other stages as well. When expressed ectopically in adults, PEEL-1 kills a variety of cell types, and ectopic expression of ZEEL-1 rescues these effects. Our results demonstrate that the tight physical linkage between two novel transmembrane proteins has facilitated their co-evolution into an element capable of promoting its own transmission to the detriment of the rest of the genome. The Apical Epidermal Ridge (AER) in vertebrates is essential to the outgrowth of a growing limb bud. Induction and maintenance of the AER reply heavily on the coordination and signaling between two surrounding cell types: ectodermal and mesenchymal cells. In morphogenesis during embryonic development, a process called the epithelial-mesenchymal transition (EMT) occurs to transform epithelial cells into mesenchymal cells for increased cell mobility and decreased cell adhesion. To check whether the AER, which originated from the ectodermal layer, undergoes EMT for enhanced cell motility and invasiveness at an early stage of the limb outgrowth, we examined expression of biomarkers of the epithelial and mesenchymal cell types in the AER of a mouse forelimb at embryonic day 10.5 in Chapter three of this thesis. We also customized correlation-based image registration algorithm to perform image stitching for more direct visualization of a big field of tissue sample. We found that the AER surprisingly expresses both the epithelial marker and the mesenchymal marker, unlike a normal non-transitioning epithelial cell or a cell undergoing EMT. Our finding serves as a basis for potential future cell isolation experiments to further look into cell type switching of the AER and its interaction with the surrounding ectodermal and mesenchymal cells.