Gametogenesis and the maternal control of embryogenesis have been studied extensively in animal systems at the genetic, molecular, and cellular level. These studies have yielded insights into fundamental molecular mechanisms involved in the establishment and interpretation of positional information, cell-cell interactions, signal transduction processes, and the control of cell proliferation. In contrast, very little is known about the genetic and molecular control of these processes in plants. Unlike in animals, where meiotic products differentiate directly into gametes, the haploid spores of plants form multicellular organisms (gametophytes), which in turn produce the gametes. The simple but highly polar and specialized structure of the female gametophyte make it an ideal system to study the role of positional information and signaling in development. This project focuses on an elucidation of key events in the sexual reproduction of plants. Over the last years we have concentrated on the identification and characterisation of mutants and genes that are required for female gametogenesis, and on the maternal control of seed development in the model plant Arabidopsis thaliana. In this project, we use a combination of genetic, molecular, and biochemical approaches, focussing on two related areas: A) To elucidate the genetic and molecular mechanisms that control female gametophyte development, with specific attention to cell specification and fertilization, by1) molecularly characterizing WYRD, a gene affecting cell specification that was cloned through positional methods 2) investigating developmental dynamics of cell specification based on microgenomics, regulatory network analysis, and reverse genetic approaches3) characterizing novel components of the FERONIA signaling pathway, identified using genetic or molecular approachesB) To elucidate the molecular mechanisms of genomic imprinting at the MEA locus and the function of MEA at fertilization, by1) identifying novel factors regulating imprinting at the MEA locus in a targeted genetic screen2) molecularly isolating an activator of MEA that was characterized at the genetic level3) biochemically characterizing the function of MEA at fertilization, which is independent of the other components of the FIS-PRC2 complex These studies will yield insights into fundamental questions that transcend plant biology: How are cell types specified at the molecular level? To what extent is cell-cell signaling involved in cell specification and fertilization? What is the molecular basis of genomic imprinting, which occurs only in mammals and seed plants? While some of the investigated processes are conserved between animals and plants, others are likely to be specific, such that the mechanisms can be compared and contrasted to gain new insights into fundamental developmental concepts.