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).
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.
(Animation by Ethan Levesque.)
- Mathews, S. 2005. Phytochrome evolution in green and nongreen plants. Journal of Heredity. 96(3): 1-8. [pdf]