A Colony of Clones
The Star Ascidian, Botryllus schlosseri, forms thin, gelatinous sheets that encrust rocks, algae, and boat hulls in shallow marine waters. What looks like a single, colorful organism is a colony of hundreds of genetically identical individuals called zooids. Each zooid is a complete animal, typically 2-4 mm in size, with its own heart, digestive system, and gills. These zooids are arranged in star-shaped clusters of 5 to 20 individuals around a shared exit siphon. The entire colony is embedded in a common matrix called a tunic, which is made of a cellulose-like substance.
All the zooids in a colony are interconnected by a shared network of blood vessels. This common circulatory system allows for the distribution of nutrients and stem cells throughout the colony. The colony itself reproduces asexually through a synchronized, weekly cycle. Older, filtering adult zooids are systematically broken down and resorbed through apoptosis, a form of programmed cell death, while a new generation of buds matures to take their place. This constant regeneration, driven by a population of ever-present stem cells, effectively makes the colony immortal, though individual zooids are transient.
To Fuse or to Fight
When two separate Botryllus schlosseri colonies grow into contact, a dramatic identity check occurs. The terminal tips of their blood vessels, called ampullae, touch and initiate a natural transplantation process. The outcome, fusion or rejection, is dictated by a single, highly polymorphic region of the genome known as the fusion/histocompatibility (Fu/HC) locus. In some populations, this locus can have hundreds of different versions, or alleles.
If the two colonies share at least one allele at this locus, they are recognized as kin. Their blood vessels merge, creating a single, larger chimeric organism that shares blood, nutrients, and stem cells. This fusion can be beneficial, as larger colonies may be more resilient and reach sexual maturity faster. However, a sinister competition can follow. Stem cells from one colony can migrate to the other and take over its reproductive organs, effectively turning its former rival into an incubator for its own offspring.
If the colonies share no alleles at the Fu/HC locus, they reject each other in an inflammatory battle. Specialized immune cells called morula cells migrate to the point of contact and release cytotoxic compounds. This creates a necrotic barrier, a scar of dead tissue that permanently separates the two warring colonies. This complex self/non-self recognition system is an example of convergent evolution with the vertebrate immune system, making Botryllus schlosseri an important model organism for immunology and stem cell research.