Date Published: January 8, 2010
Publisher: Public Library of Science
Author(s): Nemuri Todaka, Tetsushi Inoue, Kanako Saita, Moriya Ohkuma, Christine A. Nalepa, Michael Lenz, Toshiaki Kudo, Shigeharu Moriya, I. King Jordan. http://doi.org/10.1371/journal.pone.0008636
Abstract: The relationship between xylophagous termites and the protists resident in their hindguts is a textbook example of symbiosis. The essential steps of lignocellulose degradation handled by these protists allow the host termites to thrive on a wood diet. There has never been a comprehensive analysis of lignocellulose degradation by protists, however, as it has proven difficult to establish these symbionts in pure culture. The trends in lignocellulose degradation during the evolution of the host lineage are also largely unknown. To clarify these points without any cultivation technique, we performed meta-expressed sequence tag (EST) analysis of cDNA libraries originating from symbiotic protistan communities in four termite species and a wood-feeding cockroach. Our results reveal the establishment of a degradation system with multiple enzymes at the ancestral stage of termite-protistan symbiosis, especially GHF5 and 7. According to our phylogenetic analyses, the enzymes comprising the protistan lignocellulose degradation system are coded not only by genes innate to the protists, but also genes acquired by the protists via lateral transfer from bacteria. This gives us a fresh perspective from which to understand the evolutionary dynamics of symbiosis.
Partial Text: Cellulosic biomass is now regarded as a very exciting candidate source for bio-fuel. Currently, the use of ethanol as fuel incurs a cost to the food supply, as ethanol production requires both starch and sucrose. Woody biomass can be used as an alternative, but doing so entails the daunting challenge of saccharifying cellulose with the enzymes from the biomass. The most crucial step is the treatment of lignin, a component that resists enzymatic degradation and prevents enzymes from accessing cellulose. The symbiotic relationship with the protistan community within the termite gut seems to endow termites with the ability to degrade cellulose from complex natural ligno-cellulose that is composed of lignin, hemi-cellulose and cellulose –. The termite system may thus provide useful clues for the establishment of an artificial process for saccharifying woody biomass.
We investigated the trends in lignocellulose degradation during the evolution of the host lineage using four representative termite species from four families of wood-feeding termites (Reticulitermes speratus (Rhinotermitidae), Hodotermopsis sjostedti (Termopsidae), Neotermes koshunensis (Kalotermitidae), and Mastotermes darwiniensis (Mastotermitidae)) and Cryptocercus punctulatus, a wood-feeding cockroach that is a member of a genus thought to be in a sister clade to the termite lineage .