Cellulose production and the evolution of the sessile lifestyle in ascidians

Abstract

The outstanding characteristics of the marine invertebrate chordates, ascidians, are a sessile adult stage and cellulose production. These characteristics are not seen in other chordate groups. Molecular studies have suggested that these two characteristics are tightly linked. Ascidians possess the gene encoding cellulose synthase in their genomes. The disruption of the cellulose synthase gene results in abnormal metamorphosis and failure in adhesion, suggesting that cellulose is necessary for starting and continuing sessile life. Ascidian cellulose synthase is suspected to have been transferred to the tunicate ancestor from a bacterial group by horizontal gene transfer. It was suggested that by this transfer, the ancestor of tunicates was given both the gene body and an epidermal enhancer from the bacterium. The simultaneous transfer of these two genetic elements is thought to have facilitated the success of the gene transfer, which enabled the ancestor to utilize cellulose for evolving sessile ascidians.