Systematics Section / ASPT
Morawetz, Jeffery J. , Wolfe, Andrea D. .
The evolution of rbcS in Orobanchaceae: a preliminary investigation.
The parasitic habit in plants ranges from species which are still photosynthetic, obtaining only mineral nutrients and water from their hosts (hemiparasites), to those which have fully lost photosynthetic ability, relying completely on their hosts for all nutrition (holoparasites). The evolution of holoparasitism places the genes involved in photosynthesis under relaxed functional constraint, allowing them to evolve rapidly. The Rubisco holoenzyme consists of eight small subunit (SSU) proteins, encoded by the nuclear gene family rbcS, and eight large subunit (LSU) proteins, encoded by the plastid gene rbcL. Formation of rbcL pseudogenes has occurred in some holoparasitic genera in Orobanchaceae, but several holoparasitic genera also retain an apparently functional copy of this gene. While the evolution of rbcL has been well studied in parasitic plants, no data exist on the evolution of rbcS. In those holoparasitic taxa with rbcL pseudogenes, rbcS may be free to evolve from functional constraints as well. Alternatively, the enzyme may be degrading independently within the chloroplast while the nuclear encoded SSU retains an intact open reading frame (ORF). In those holoparasites with intact rbcL ORFs, there may be compensatory changes in rbcS at sites of nonsynonymous substitution to preserve the tertiary structure of the Rubisco polypeptide to retain its functionality. We examined rbcS in 10 taxa spanning the range of nutritional dependencies within Orobanchaceae from non-parasites through holoparasites, including one member of Scrophulariaceae for comparison. rbcS appears to exist as three copies (800 bp, 650 bp, 250 bp) in many hemiparasitic taxa (including Sopubia and Cycnium), but as a single copy in the holoparasitic Hyobanche (1200 bp), Harveya (250 bp) and the hemiparasitic Buchnera (800 bp).
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1 - Ohio State University, Department of Evolution, Ecology and Organismal Biology, 300 Aronoff Laboratory, 318 W. 12th Ave., Columbus, Ohio, 43210-1293, USA
Presentation Type: Oral Paper:Papers for Sections
Location: Stevens 2/Hilton
Date: Tuesday, July 10th, 2007
Time: 11:00 AM