Pyrrolizidine alkaloids (PAs) are secondary metabolites found sporadically in many species of flowering plants. They are synthesized as a part of plant chemical defense against herbivores. The enzyme Homospermidine synthase (HSS) catalyzes the first step of PA biosynthesis. HSS is known to have originated from a gene duplication event from the ubiquitous eukaryotic enzyme deoxyhypusine synthase (DHS) involved in the activation of translation initiation factor. In our study we are focusing on the Convolvulaceae (Morning glory) family, wherein PAs occur only in a few isolated species. The recruitment of HSS to PA biosynthesis in these species has shown to be an efficient model to study the molecular evolution and functional divergence of enzymes after gene duplication. Our recent studies indicate a single duplication event in a common ancestor has led to the HSS genes in PA producing species as well as putative HSS copies in PA-free species. Hence, leading to interesting questions like, did the common ancestor of all PA-producing plants possess the pathway, followed by many independent losses? Or did the PA-biosynthesis evolve repeatedly in different lineages? We are using molecular phylogenetic analyses combined with biochemical characterization to study the factors that influenced the evolution of HSS. On the broader level we are trying to understand the role of gene duplication on the origin of new function and metabolism.