I have worked on germ cell development since my Ph.D. when I used classical Drosophila genetics to explore the genetic requirements of germ cells during development. Using clonal analysis, I showed that primordial germ cells engage in cell-cell competition prior to gametogenesis, revealing a level of natural selection that operates not only pre-zygotically, but in the very precursors of gametes themselves. This means that allele frequencies can potentially be changed from one generation to the next, not only by natural selection operating on sexually mature adult individuals, but also on the cells responsible for producing the gametes that will ultimately give rise to those individuals. Because of the critical role of germ cells not only in development but also in evolution, I then decided to work on germ cell development in a comparative context.