EVOLUTION OF PATTERNING GENES IN LAND PLANTS John L. Bowman1,2, Sandra K. Floyd1, Keiko Sakakibara1, John P. Alvarez1, Chris Zalewski2, Karen Yip2, Pia Sappl1, Dyani Lewis1, Lukas Brand1, Eduardo FloresSandoval1 1 School of Biological Sciences, Monash University, Clayton Campus, Melbourne VIC 3800, Australia; 2Department of Plant Biology, University of California Davis, Davis, CA, 95616, USA As more plant genome sequences become available, researchers are increasingly using comparative genomics to address some of the major questions in plant biology. Such questions include the evolution of photosynthesis and multicellularity, and the developmental genetic changes responsible for changes in body plan and the origin of important plant innovations such as roots, leaves, and vascular tissue. We have focused on tracing the evolutionary history of genes involved in pattern formation. In particular, we have investigated the history of genes involved in establishing leaf polarity in Arabidopsis. We find that some genes are evolutionarily recent, evolving coincident with the evolution of leaves in seed plants. However, other genes predate the evolution of leaves, implying a co-option from a more ancestral role. Expression and functional studies in early diverging lineages of land plants provide clues as to ancestral functions, and thus to the evolution of land plant morphology. For example, Class III HD-Zip genes act to promote meristem development, adaxial leaf development and vascular development in Arabidopsis. Since the latter two tissues do not exist in early diverging land plant lineages, apical growth and meristem development may represent an ancestral function. Expression analyses in moss and liverworts support this hypothesis, but also suggest a more direct role in response to light, an environmental parameter that is key in molding the plastic development of plants, and establishing polarity in liverworts in particular. Thus, ancestral roles of patterning genes may lie in interactions with environmental parameters critical in the transition from an aquatic algal ancestor to a land plant.