ORTHOSCOPE Analysis Reveals the Presence of the Cellulose Synthase Gene in All Tunicate Genomes but Not in Other Animal Genomes

Abstract

Tunicates or urochordates-comprising ascidians, larvaceans, and salps-are the only metazoans that can synthesize cellulose, a biological function usually associated with bacteria and plants but not animals. Tunicate cellulose or tunicine is a major component of the outer acellular coverage (tunic) of the entire body of these organisms. Previous studies have suggested that the prokaryotic cellulose synthase gene (CesA) was horizontally transferred into the genome of a tunicate ancestor. However, no convenient tools have been devised to determine whether only tunicates harbor CesA. ORTHOSCOPE is a recently developed tool used to identify orthologous genes and to examine the phylogenic relationship of molecules within major metazoan taxa. The present analysis with this tool revealed the presence of CesA orthologs in all sequenced tunicate genomes but an absence in other metazoan genomes. This supports an evolutionary origin of animal cellulose and provides insights into the evolution of this animal taxon.

Keywords: CesA genes; ORTHOSCOPE; horizontal gene transfer; tunicates.

Figure 1

The composition of tunicate CesA, shown by a schematic of the domains. Tunicate CesA is a seven transmembrane (TM) domain protein, and a fusion protein of CesA and GH6 domains that are separated at the end of TM7. Red indicates sequences used for CesA domain identification and blue indicates sequences used for GH6 domain identification. Molecular signatures of CesA and GH6 domains follow Nakashima et al. [10].

Figure 2

Molecular phylogenic tree of the CesA domain obtained using ORTHOSCOPE analysis (884 nucleotide sites). The CesA domains of four CesA genes from three tunicates (indicated with arrows) were used as query sequences. The Streptomyces sequence (CAB44539) was selected for tree rooting. Because all the non-tunicate metazoans examined did not contain CesA orthologs, the tree includes those of Streptomyces and Arabidopsis in addition to tunicates. The numbers beside the nodes indicate bootstrap probabilities (>50%). To count orthologs, identical sequences derived from ORTHOSCOPE were replaced with query sequences. Sequences ending with “-gene” are raw data, whereas those ending with “-domain” are partial gene sequences. Color coding for taxonomic groupings is used in the other figures. Underlined sequences were manually added to the sequence alignment.

Figure 3

Molecular phylogenic tree of the GH6 domain obtained using ORTHOSCOPE analysis (438 nucleotide sites). The GH6 domains of four CesA genes of three tunicates (indicated with arrows) were used as query sequences. The Streptomyces gene (CAB65568) was selected for tree rooting. Because all the non-tunicate metazoans examined do not contain GH6 orthologs, the tree includes only tunicates. It should be noted that tunicate genomes contain not only CesA genes, including the GH6 domain, but also another tunicate-specific GH6 gene.