The development and maintenance of the sexually dimorphic phenotype in vertebrates is regulated by the signaling interactions of steroid hormones like androgens, estrogens, and progestins. While considerable advances have elucidated the molecular mechanisms responsible for formation of the male and female phenotype in mammalian models, there still remain many outstanding questions regarding this regulation in different vertebrates. This dissertation explores facets of sexually dimorphic development in an ancient vertebrate model system by focusing on copulatory organ morphogenesis, as well as the evolution of the steroid receptors.

The earliest known vertebrate copulatory organs are claspers, paired penis-like structures that are associated with evolution of internal fertilization and viviparity in Devonian placoderms. Today, only male chondrichthyans possess claspers, and these intromittent organs represent an outstanding model of sexually dimorphic development. Using the model organism Leucoraja erinacea (little skate), we show that Sonic hedgehog (Shh) signaling is necessary for male clasper development and is sufficient to induce clasper cartilages in females. We next demonstrate that androgen receptor (AR) controls the male-specific pattern of Shh in the claspers, potentially via direct regulation of Hand2, a gene directly upstream of Shh. This work demonstrates a deep conservation of a gene regulatory network, and suggests a potentially novel mechanism of gene regulation by AR.

We next examine the evolution of the steroid receptor family by focusing on estrogen receptors (ERs). Most vertebrates possess two ERs (genes Esr1, Esr2) that are resultant of a gene duplication that occurred during evolution of this lineage, though the timing of this event is unclear. Additionally, ER signaling is indispensable for sexual development in mammals, though its functional role in other clades is not well understood. To address these questions, we clone novel ER genes within skate, and demonstrate by robust phylogenetic reconstruction that an ER duplication event likely occurred at the gnathostome divergence. Analysis of Esr1 and Esr2 transcripts during skate development reveal highly overlapping expression domains, especially when compared to zebrafish and mouse. This study reconciles the evolution of ERs within vertebrates, and suggests that divergence of these steroid receptors by cis-regulatory elements occurred after the chondrichthyan split.