Botryllus humilis: A promising new ascidian model for aging, stem cell dynamics, and whole-body regeneration

Model organisms are essential in science as they provide systems for studying conserved biological processes. In addition to widely used multicellular and unicellular models, tunicates have emerged as valuable model organisms in various biological fields due to their distinctive characteristics. As chordates with both sexual and asexual reproductive strategies and the capacity for whole-body regeneration (WBR), tunicates provide a unique system for regeneration studies. Although the majority of organisms display varying levels of regenerative ability, only a few can perform WBR; notably, botryllid ascidians (Stolidobranchia) are the sole documented group of chordates with WBR. Tunicates, a subphylum of over 3000 identified marine invertebrate species, are the closest living relatives of vertebrates. Botryllids comprise approximately 160 colonial tunicate species within the genera Botryllus and Botrylloides, less than ten of which have been shown to undergo WBR from vascular tissue. However, it remains unknown whether all botryllids are capable of WBR or not. Botryllus humilis was first observed in New Caledonia and has recently been identified in the Mediterranean Sea. In the present study, the blastogenic cycle and WBR capacity of B. humilis were monitored for the first time following the ablation of all zooids and buds. Two temperature conditions and two staging systems were used to record the blastogenetic cycle. Colonies completed their blastogenic cycle in 7 days at 21 °C and 38 ppt salinity, while this duration shortened to 4 days at 26 °C under the same salinity. Colony age appeared to influence the number of budded zooids during the blastogenic cycle. I also assessed the WBR capacity in B. humilis for the first time by inducing WBR at different stages of the blastogenic cycle. Regeneration was completed within 120-138 h post-surgery, and the first post-regeneration blastogenic cycle was completed within 2 days. This study provides valuable insights into B. humilis as a highly valuable model species for aging, stem cell dynamics, and WBR studies.