The first goal of this study is to identify sex and stage-specific novel and known genes involved in gonad differentiation and development in sea lamprey. The second goal is to identify the expression of germline-specific genes in a sex-specific manner and to determine the function of these genes in sea lamprey gonadogenesis. Finally, the third goal is to identify the early evolutionary history of hypothalamic-pituitary-gonadal axis genes and analyze tissue and stage-specific expression of these genes.