The origin of the germline remains one of the oldest and most perplexing questions in evolutionary as well as developmental biology. Why, if the egg and sperm are not needed until adulthood, are their precursors determined in early embryogenesis? Primordial germ cells (PGCs) arise before gastrulation in most animals, and must undertake an epic migration through the embryo in order to populate the ovary or testes, where they differentiate into male or female gametes. Given the opportunity to impart half of the genome to the next generation, selection for successful development and migration of PGCs must be extraordinary. However, little is known about the mechanisms of their navigation in mammalian embryos. Furthermore, the fate of PGCs that fail to reach the gonad is largely unknown, although human germ cell tumors in extragonadal locations suggest the capacity for continued PGC survival beyond embryonic development. Identifying the mechanisms by which PGCs migrate as well as regulate their proliferation and survival will elucidate self-renewal and trafficking pathways common to other stem cells. We also aim to understand how early developmental events shepherd the genome to the next generation and affect evolutionary selection during gametogenesis.

Major goals: (1) identify the molecular mechanisms that regulate the migration, self-renewal, and survival of mouse PGCs; (2) determine the cell fate potential of committed PGCs.

A forward genetic approach to PGC development

We are leveraging recent advances in mouse genetics to study the machinery of self-renewal, migration, and regulation of PGCs in vivo. In order to discover new genes involved in PGC development, we have screened randomly mutagenized mouse embryos and isolated eight recessive mutants with PGC defects. We are using meiotic recombination mapping together with high throughput sequencing techniques to identify the mutations. Study of each mutant phenotype is a gateway to understanding the function of an individual gene as well as entire pathways. We use imaging, cell sorting, embryological and in vitro approaches to ask questions such as the nature of the allele, the mechanism of defect, and whether it acts autonomously in PGCs. By examining mutant phenotypes in adults, we are learning how embryonic PGC defects might affect fertility, and whether genetic programs are conserved between PGC development and gametogenesis.

Creating fate mapping tools

We are also developing strains of transgenic mice with reporter and recombinase expression uniquely specific to migratory PGCs. By introducing permanent genetic marks in PGCs as well as their cellular progeny, such transgenics will create visual fate maps. We will utilize these tools to address fundamental questions of germ cell lineage commitment and the consequences of PGC mismigration in wild-type backgrounds as well as in the mutants that we generated in our screen.