Angiosperms, or flowering plants, form the largest group of plants, with more than 350,000 extant species. They exhibit extensive diversity in shape, size, color, structure and organization of their reproductive organs contained within the flowers. With regard to the sexual nature of the flowers, males, females and hermaphrodites occur in nature, with hermaphroditism being the norm. About 6% of flowering plants are dioecious and 5% monoecious, supporting the widely accepted view that bisexual flowers are the ancestral condition. Unisexuality evolves from hermaphroditism by the process of random mutations affecting the female organ (carpel) or male organ (stamen) abortion. The ABCE model of floral development provides a basic underlying developmental framework of individual floral whorls across species. However, it stops short of universally explaining the occurrence of selective organ abortion, as in the case of unisexual flowers. Abortion of either one of the sexual organs is the first step towards evolving a sexually dimorphic species, mostly by a loss-of-function mutation, rarely by a gain-of-function mutation, occurring in any of a number of genes and regulatory elements involved. Ontogenic similarities between the lateral organ leaf and the flower has also led to research demonstrating increasing roles played by the plant hormone auxin in the initiation and patterning of these organs. Reproductive organs are structurally complex and critical to survival, and have been under intense research for the last three decades. However, the genetic elements and interactions between that sculpt the organs are relatively poorly understood, and neither of these models sufficiently explain the occurrence of different sex types in plants. Stable dioecy results when the two functional genes affecting carpel and stamen development are linked in close proximity and their recessive alleles are linked in repulsion phase. The relatively low frequency of female sterile mutants in nature is indicative of the evolutionary constraints on the female organs by dint of their role in bearing the ovules and providing nutrition and protection to the next generation. It is also caused by the lesser probability of female mutations being fixed in a population as the sedentary recipient nature of carpels would drive the population to extinction. The model plant species Arabidopsis also reflects this dearth of female sterile mutants in laboratory studies. Being a hermaphrodite species with perfect flowers, and with a fully sequenced genome, Arabidopsis is ideally suited to study floral organ development.Carica papaya variety AU9 is an improved dioecious variety with male and female sex types controlled by a pair of nascent sex chromosomes, and with a sequenced genome. It makes for an ideal system to study the underlying genetic basis for floral sex organ development. Arabidopsis and papaya are both in the order Brassicales, with papaya having 2 fewer whole genome duplications than Arabidopsis. To explore and identify genomic regions and geneAdvisors/Committee Members: Ming, Ray R (advisor), Ming, Ray R (Committee Chair), Zielinski, Raymond E (committee member), Tranel, Patrick J (committee member), Clough, Steven J (committee member).