![]() ![]() Initially, their origin in the arthropods was attributed to a date before the divergence of termites and cockroaches, approximately 250 MYA. GHF9 has been relatively widely studied in the Metazoa, following the surprising discovery of endogenous GHF9 genes in termites (phylum Arthropoda Watanabe et al. However, the fifth family of metazoan glycosyl hydrolase genes-GHF9 endo-beta-1,4-glucanases-is exceptional because the core gene sequence is both relatively long (over 430 amino acids) and conserved. Even in the case of GHF5 and GHF6 genes, phylogenetic resolution is quite poor, presumably because the genes are short and saturated for substitution ( Lo, Watanabe, and Sugimura 2003 Matthysse et al. Phylogenetic analysis to test for an ancient origin using these genes is compromised by a lack of data. Finally, GHF45 cellulases have been described from a beetle ( Girard and Jouanin 1999) and two mollusks ( Xu, Janson, and Sellos 2001 Harada, Hosoiri, and Kuroda 2004), and a GHF10 cellulase has been isolated from a mollusk ( Wang et al. Again, there is reasonable phylogenetic evidence that the GHF6-like domain was gained by horizontal transfer ( Matthysse et al. The sea squirts Ciona intestinalis and Ciona savignyi have a protein with a putative GHF6-like domain ( Matthysse et al. There is reasonable phylogenetic evidence that both of these genes are derived from bacteria by horizontal gene transfer ( Yan et al. 2003) express GHF5 cellulases ( table 1). 1998) and a phytophagous beetle ( Sugimura et al. Tylenchine plant-parasitic nematodes ( Smant et al. For four (GHF5, GHF6, GHF10, GHF45), very few animal-derived members have been identified. Five of these families have representatives in Metazoa ( table 1). Fourteen families of glycosyl hydrolases (GHF) are able to degrade cellulose (GHF5, 6, 7, 8, 9, 10, 12, 26, 44, 45, 48, 51, 61, and 74 Henrissat 1991 see ). 2004).īefore concluding that genes have been gained by horizontal transfer, it is necessary to rigorously investigate the evidence, preferably using a gene-by-gene approach ( Ochman, Lawrence, and Groisman 2000 Genereux and Logsdon 2003). 1998 Lo, Watanabe, and Sugimura 2003 Matthysse et al. The alternative explanation for the presence of cellulases in these diverse animals is that they are derived from genes in an ancient ancestral eukaryote and have persisted only in some metazoan lineages ( Yan et al. ![]() In consequence, it is commonly believed (e.g., Morris 2003) that most animals are unable to digest cellulose except when assisted by these commensals and that “surprising” exceptions in termites, nematodes, and sea squirts have acquired their cellulolytic endoglucanases by horizontal gene transfer from prokaryotes ( Smant et al. The majority of decomposing degradation is carried out by bacteria, fungi, and protozoa, most famously as commensals in the guts of herbivorous animals. Plants, some bacteria, fungi, protozoa, and sea squirts (ascidians) synthesize cellulose and also need to be able to degrade or modify it during growth and development. Therefore, to understand global carbon cycling the dynamics of cellulose synthesis and degradation must be understood. In contrast, GHF9 subgroup E2 genes are relatively rare in bacteria.Ĭellulase, expressed sequence tag, glycosyl hydrolase, horizontal gene transfer IntroductionĬellulose is the most abundant organic compound on Earth. Several paralogous subfamilies of GHF9 can be identified in plants, and genes from primitive species tend to arise basally to angiosperm representatives. We also found that sequences isolated from the same animal phylum tend to group together, and in some deuterostomes, GHF9 genes are characterized by substitutions in catalytically important sites. As several intron positions are also conserved between four metazoan phyla then, contrary to the still widespread belief that cellulases were horizontally transferred to animals relatively recently, GHF9 genes must derive from an ancient ancestor. We also demonstrate that eukaryotic GHF9 gene families are ancient, forming distinct monophyletic groups in plants and animals. Using information from expressed sequence tags, we show that GHF9 genes (subgroup E2) are widespread in Metazoa because at least 11 classes in five phyla have expressed GHF9 cellulases. \)).While it is widely accepted that most animals (Metazoa) do not have endogenous cellulases, relying instead on intestinal symbionts for cellulose digestion, the glycosyl hydrolase family 9 (GHF9) cellulases found in the genomes of termites, abalone, and sea squirts could be an exception. ![]()
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