Regeneration ability in four stolidobranch ascidians: Ecological and evolutionary implications

Owing to their impressive regeneration abilities and close phylogenetic relationship to vertebrates, ascidians (phylum: Chordata) are a key model system for regenerative and developmental studies. Nevertheless, ascidian regeneration research has been limited, to date, to only a few model species. In the present work, we investigated the ability of four solitary ascidians of the order Stolidobranchia, Polycarpa mytiligera, Herdmania momus, Microcosmus exasperatus, and Styela plicata, to regenerate their neural complex and siphons. Four treatments were used: a) amputation of the oral siphon, b) amputation of the atrial siphon, c) amputation of both siphons and the neural complex located between the siphons, and d) a control treatment with no amputations. All four species initiated regeneration of the amputated organs, with P. mytiligera demonstrating the most powerful regenerative ability, as it was the only species to survive all amputation procedures and have its neural complex fully regenerated. P. mytiligera's phylogenetic proximity to colonial species suggests that its remarkable regeneration abilities have been maintained as an ancestral feature, and that this species may constitute a unique phylogenetic transition between coloniality and a solitary life style. P. mytiligera's distinctive regeneration traits may have also provided this species with an ecological advantage, as evidenced by its abundance in the Red Sea. Our findings highlight P. mytiligera's potential to serve as a most valid model organism for research into the evolution of regeneration within chordates and into regenerative biology in general.