The goal of the studies described in this thesis was to investigate the mechanism of embryonic head development in Drosophila melanogaster. The work presented here can be divided into two sections, each focused on a different level of the genetic hierarchy that establishes the head. In the experiments in the first part, I investigated the role of the cephalic gap gene orthodenticle in specifying head segments. In particular, I tested the hypothesis that orthodenticle, cooperates with other cephalic gap genes to specify head segment identities in a combinatorial fashion. My results contradict this model and suggest instead that orthodenticle specifies intrasegmental cell fates by regulating segment polarity gene expression. The second part of this thesis examines the interactions among genes of the segment polarity class, which act downstream of the cephalic gap genes. Interactions among these genes specify pattern within the trunk segments, but little is known about their role in head development. My results indicate that regulatory interactions among the segment polarity genes wingless, engrailed, hedgehog, and patched are significantly different in the head than in the trunk. This work has revealed novel roles for these genes in embryonic development. I also found that the regulatory pathways in which the segment polarity genes participate, unlike in the trunk, differ among head segments. This complexity may underlie the high degree of structural diversity characteristic of the embryonic head. Homologues of both orthodenticle and the segment polarity genes are expressed in the developing mammalian forebrain. Therefore, it will be important to determine whether the regulatory relationships described in this thesis are applicable to higher organisms
