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Phytochromes and Phylogeny in Parasitic Orobanchaceae

Studies of phytochrome evolution in parasitic plants offer the opportunity to address the question of how photoreceptor sequences evolve when development and light cues are to some extent unlinked. The plant family Orobanchaceae provides a useful system because it comprises a single autotrophic lineage that is sister to the remaining family, which in turn comprises sister pairs of lineages that either are photosynthetic root parasites (hemiparasites) or nonphotosynthetic root parasites (holoparasites). apoprotein Both holoparasites and hemiparasites may have prolonged underground stages of skotomorphogenetic development during which they rely completely on the host plant. This developmental pathway contrasts markedly with development in the light-seeking seedlings of autotrophic plants. Upon emergence, holoparasites produce only a nonphotosynthetic inflorescence, remaining host-dependent, while hemiparasites develop a photosynthetic shoot system before flowering. These developmental differences between autotrophs and parasites suggest that the functions of photoreceptors differ among autotrophs, hemiparasites, and holoparasites. Changes in photoreceptor function may be required to maintain the differences in developmental pathways that characterize the three lifestyles. In some cases, origin of a novel function may be important. In others, aspects of photoreceptor function may become superfluous during the transition from autotrophic to parasitic habit. Functional changes may occur through gene loss, through altered expression patterns, by sequence-specific changes in coding regions of the genes, or through a combination of these factors. In order to provide a phylogenetic context for the study of phytochrome evolution in Orobanchaceae, we are using sequences from phytochromes and other loci to infer a phylogenetic hypothesis for the family. This is a collaborative effort with Andi Wolfe's lab, who are gathering plastid and ITS data. This tree will provide novel insight into the phylogenetic history of the family, will reveal closely related pairs of hemi- and holoparasitic lineages, useful for the comparison of phytochrome expression patterns and physiological differences, and will serve as the framework for characterizing patterns of selective constraints among autotrophs, hemi-, and holoparasites.
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