p266

動物の陸上への進出と腸内細菌---見解修正

「動物の陸上進出も(セルロースを消化利用するための)腸内微生物との共生がカギであったに違いない」と書いたとき，動物にみられるセルラーゼは微生物起源であろうという誤った見解にもとづていた．話はそれほど簡単ではない．

Bilateriaの共通祖先が endo-β-1,4-glucanase(とglucosidase)を持っていたことには疑問の余地がない:

N. Lo, H. Watanabe, and M. Sugimura, “Evidence for the presence of a cellulase gene in the last common ancestor of bilaterian animals,” 

 Proc. Roy. Sc.  270 , S69 (2003)

Positional identity in the introns of cellulase genes from a termite, a sea squirt and an abalone provided compelling evidence that a similar gene was present in the last common ancestor of protostomes and deuterostomes. At least one family of of endogenous cellulases may be more widespread in animals than previously thought. 

Hirofumi Watanabe, Gaku Tokuda, Cellulolytic Systems in Insects, 

 Ann Rev Entomology 55 609 (2009)

が最近の関連した総説であり，そこには上記のセルラーゼが水平移動によってもたらされたものでないことが記されている．

しかし，では，自前のセルラーゼだけで充分であったか．現生のシロアリで鞭毛虫などを共生させてない種類は自前のセルラーゼで生きていると見られていたが，抗生物質で腸内細菌を駆除するとセルラーゼ活性が大きく減る:

Tokuda et al., “Hidden cellulases in termites: revision of an old hypothesis,” 

 Biol. Lett.  3 , 336 (2007). 

75% of all termite species do not harbor the cellulolytic flagellates; the endogenous cellulase secreted from the midgut tissue has been considered a sole source of

cellulases in these termites Antibiotic treatment administered to Nasutitermes takasagoensis significantly reduced cellulase activity in the hindgut, suggesting that these cellulases were produced by symbiotic bacteria.

腸内細菌のメタゲノミクスについては

Warnecke et al., “Metagenomic and functional analysis of hindgut microbiota of a wood-feeding higher termite,” 

 Nature  450 , 560 (2007)

Metagenomic analysis of the bacterial community resident in the hindgut paunch of a wood-feeding ‘higher’ Nasutitermes species (which do not contain cellulose-fermenting protozoa) to show the presence of a large, diverse set of bacterial genes for cellulose and xylan hydrolysis.

腸内細菌の窒素源としての役割も重要であったかもしれない:

Hongoh et al., “Genome of an Endosymbiont Coupling N 2 Fixation to Cellulolysis Within Protist Cells in Termite Gut,” 

 Science  322 , 1108 (2008).

The complete genome sequence of the uncultured Bacteroidales endosymbiont of the cellulolytic protist Pseudotrichonympha grassii , which accounts for 70% of the bacterial cells in the gut of the termite Coptotermes formosanus .

∗ Functional annotation of the chromosome (1,114,206 base pairs) unveiled its ability to fix dinitrogen and recycle putative host nitrogen wastes for biosynthesis of diverse amino acids and cofactors, and import glucose and xylose as energy and carbon sources.

∗ Nitrogen fixation and cellulolysis are coupled within the protist’s cells.

Chaffron et al., “Termites in the woodwork,” Genome Biol.  8 , 229 is a review: Microbes found in the termites’ hindguts possess just the right tools.

木質部を利用するにはリグニンの消化が必要であるが，それが可能なのは限られた菌類のみである:

Only a small group of fungi—termed white rot fungi—are able to degrade lignin, and as a consequence, these fungi play an important role in the global carbon cycle. [Birren et al., “Genomics of the fungal kingdom: Insights into eukaryotic biology,” Genome Res.  15 , 1620 (2005)]

そこで当然，木質部を利用する昆虫は特殊な菌類との共生が必須である:

Geib et al., “Lignin degradation in wood-feeding insects,” 

 PNAS  105 , 12932 (2008).

∗ Lignin degradation by two insect species, the Asian longhorned beetle ( Anoplophora glabripennis ) and the Pacific dampwood termite ( Zootermopsis angusticollis ).

∗ In A. glabripennis identified a single species of fungus in the Fusarium solani/Nectria haematococca species complex. This is a soft-rot fungus that may be contributing to wood degradation.

ついでながら，パンダゲノム解析の論文

Li et al., The sequence and de novo assembly of the giant panda genome, 

 N 463 311 (2010). 

によるとパンダのタケ，ササへの適応は腸内細菌の特殊化によるところが大きいようである．