Developmental mechanisms of evolution: loss of reproductive structures in birds

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The genetic mechanisms that regulate development of the genitourinary system (reproductive and urologic organs) are among the least understood in developmental biology. Paradoxically, birth defects of the external genitalia are among the most frequently occurring malformations in humans, ranking second only to heart defects. Some of the most common genital malformations in humans -- such as an open urethral tube and a persistant cloaca (a single outlet for urinary and digestive tracts) -- resemble the natural variation seen across vertebrates. Thus, identification of the mechanisms that direct genitourinary development in non-mammalian vertebrates will have implications for our understanding of abnormal genitourinary development in humans. Moreover, comparative developmental studies will identify the genetic mechanisms that drive evolutionary change. This project investigates the developmental basis of external genital evolution in birds, a group that exhibits a broad spectrum of anatomical patterns that range from an almost mammal-like phallus to complete absence of external genitalia. The approach will combine molecular, cellular, and experimental embryological methods to determine the extent to which the genetic circuits that regulate genital development are conserved between birds and mammals. It is expected that the genetic program that directs genital development in mammals also operates in the most primitive birds, which would point to a surprisingly deep origin of a character that had been thought to be exclusive to mammals. In addition, it is predicted that in a specific group of birds (known as the galliformes), failure to form external genitalia results from disruption of an ancient cell survival mechanism, and excessive cell death causes a developmental arrest. Comparative studies using non-traditional model systems, such as birds and reptiles, are needed to develop new models for disease, and this project will contribute this infrastructure to the research community. Identification of new avian (bird) embryo models is especially important given the prohibitive cost of rodent models for many researchers at small colleges and in countries with limited resources. The impacts of the proposed activities extend beyond scientific discovery. By identifying actual mechanisms of evolutionary change, this research will contribute to society's understanding of how evolution works, which will improve our national literacy in evolution. The project dovetails with educational outreach activities involving students from elementary school to undergraduates, and contains specific action items to increase participation of women and other groups underrepresented in science.