Early germ line development in the hemipteran insect Oncopeltus fasciatus

Abstract

The cell lineage that forms eggs and sperm is referred to as the germ line, and the integrity of this cell lineage is critical for the successful reproduction and the survival of a species. The earliest progenitors of the germ line are known as primordial germ cells (PGCs), and PGCs form during embryonic development. In most animals, germ cells are formed in a location that is distant from the location where gonads (testes or ovaries) will develop, and therefore germ cells must migrate, often long distances, during embryonic development. I investigate the mechanisms for two major processes in early germ line development: specification and migration. To study these processes, I look to insects, the most diverse group of animals, where we can study mechanisms that govern germ line development in a comparative context and generate evolutionary hypotheses about how these mechanisms evolve. While most mechanistic studies of developmental processes have been in holometabolous insects (insects that undergo metamorphosis), hemimetabolous insects have many features that are more representative of an ancestral form of insect development. Here, I study the hemimetabolous insect, the large milkweed bug Oncopeltus fasciatus (Hemiptera), which is in a group sister to the monophyletic Holometabola. It is therefore in an ideal phylogenetic position to make comparisons to mechanisms that have been well-studied in holometabolous insects such as the fruit fly Drosophila melanogaster (Diptera). In Chapter 1, I review evidence in the literature showing the roles of the Bone Morphogenetic Protein (BMP) pathway in reproductive system development and function across animals. I propose that there is an ancient association with BMP signaling in the development of both the germ line and supporting somatic gonad tissue that may have arisen with the segregation of the germ line early in metazoan evolution. In Chapter 2, I investigate whether the BMP pathway has a role in PGC specification in O. fasciatus where the signaling factors specifying PGCs are currently unknown. I provide evidence that O. fasciatus PGCs are competent to receive BMP signal, but future work is needed to assess whether this pathway is required for PGC formation in this insect. In Chapter 3, I characterize the dynamics of PGC migration in O. fasciatus and use a candidate gene approach to find genes involved in regulating PGC migration. I provide evidence that at least one member of the JAK/STAT signaling pathway is involved in PGC migration in O. fasciatus. To my knowledge, this is the first functional genetic study into the mechanism of PGC migration in a hemimetabolous insect.