Phylogeny of Orobanchaceae Sensu lato Inferred from Phytochromes and Other Data: Implications for the Evolution of Parasitism

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0215780Mathews and Wolfe By definition, plants are autotrophic organisms, capable of producing their own food via photosynthesis. Plants that become parasites suppress or lose their photosynthetic apparatus and come to rely on host species for carbon, nitrogen, and water. With a single exception, parasitism is unique to flowering plants, where parasitic species are found in at least eleven lineages. The plant family Orobanchaceae is a lineage of root parasites that includes the completely parasitic (holoparasitic) broomrapes and the partially parasitic (hemiparasitic) figworts. Within Orobanchaceae, a single autotrophic lineage apparently gave rise to the hemiparasites, and holoparasitic species have originated from different hemiparasitic lineages. Thus, a series of transitions from autotrophism to hemiparasitism and from hemi- to holoparasitism can be investigated to learn about the steps underlying this switch in lifestyle. Preliminary evidence suggests that development of the photosynthetic machinery in these parasites may be suppressed by upstream genetic regulators of photosynthesis. This would allow photosynthetic capacity to be suppressed but not lost. To test this model, an improved hypothesis of phylogenetic relationships within Orobanchaceae is required. DNA sequence data from nuclear and chloroplast genes sampled from the majority of genera in the family will be collected and analyzed to resolve their phylogenetic history. The focus of the nuclear DNA sequencing will be on phytochrome genes, which encode photoreceptors involved in regulating photosynthetic gene expression and development . Thus, data collected to address phylogenetic questions can also be used to test the role of altered phytochrome photoreceptor function in the development of parasites. Specifically, phytochrome gene copy number, molecular evolution, and expression in hemi- and holoparasites and in autotrophic relatives will be assessed to determine whether phytochrome function is altered. The capacity to exist without all or part of the photosynthetic machinery has given rise to parasitic weeds that significantly impact agriculture and forestry throughout the world, but the steps whereby plants become parasites remain unknown. Thus, mechanisms for their control remain underdeveloped. A widely accepted model suggests that gene loss leads to holoparasitism. But this does not fully explain the retention of intact photosynthetic genes by some holoparasites, nor does it explain why most parasitic flowering plants retain photosynthetic capacity. A model involving suppression but not loss of photosynthesis may better explain the data. These studies based on sequences from phytochrome genes, supplemented with sequences from nuclear ribosomal and plastid loci, will improve our understanding of phylogeny in Orobanchaceae and will provide a sound phylogenetic framework for testing models for the evolution of parasitism in plants.