How sea squirts got their safety sac

Sea squirts — filter-feeders stuck to the rocky ocean floor — evolved their tough tubular ‘tunic’ after obtaining a bacterial gene.

Genes can jump between unrelated organisms, but cannot act on the new host unless they adapt to its unique gene expression systems. A team including researchers from the University of Tsukuba has discovered how cellulose synthase, the gene responsible for growing the protective pouch on sea squirts, was successfully acquired from actinobacteria, a common bacterial group found in soils and oceans.

The group found that AP-2, a protein coding gene, specifically controls the expression of cellulose synthase in the protective outer layer of sea squirts. AP-2 also preferentially recognizes guanine and cytosine (GC) rich DNA. Since the actinobacteria genome is 70 per cent GC, AP-2 could rapidly detect and express the GC-rich cellulose synthase in a beneficial way.

Horizontal gene transfer, such as this, may occasionally succeed due to a natural compatibility between two organisms, bringing evolutionary advantages that would otherwise depend upon rare mutations, the authors say.