The Onset of Whole-Body Regeneration in Botryllus schlosseri: Morphological and Molecular Characterization

Speed vs completeness: a comparative study of solitary and colonial tunicate embryogenesis

Solitary ascidians, such as Ciona robusta, have been used for over a century as model systems for embryological studies. These species are oviparous, producing many relatively small and transparent eggs, which are released and fertilized outside the parent body. Embryos develop rapidly in a stereotyped manner and reach the larva stage in less than 1 day (at 20°C). The larvae then settle and metamorphose into sessile juveniles in approximately 2 days. On the other hand, colonial ascidians are ovoviviparous, with heavily yolked eggs that develop inside the parent body. In the colonial Botryllus schlosseri, embryos are connected to the parental body via a cup-like placenta and develop into larvae within a week (at 20°C). These larvae, which possess both typical larval organs and prospective juvenile organs, are released into seawater, where they settle very rapidly, sometimes after only 15 minutes of free swimming. Then, they metamorphose into juvenile oozooids. The ability to study embryo development in colonial ascidians within the parent body is limited. To address this, we developed a method for in vitro culturing B. schlosseri embryos outside the parental body and combined it with time-lapse and confocal microscopy to describe the embryonic developmental stages. Moreover, we used histological analysis based on serial sections to investigate late-stage development, when embryo opacity made other techniques ineffective. We identified 19 stages of development, from the fertilized egg to the swimming larva, and described the stage of organ appearance and differentiation. Comparing the embryonic development timeline of B. schlosseri with that of C. robusta, we found heterochrony in development, particularly in the timing of organ appearance and growth rate. We hypothesize that this difference in maturation timing between solitary and colonial ascidians reflects a shift in the regulation of key developmental pathways that contributed to ascidian diversification. This heterochronic evolution likely facilitated a significant (approximately four-fold) shortening of the metamorphosis time in B. schlosseri by allowing embryos to remain in a safe ovoviviparous environment five times longer than those in C. robusta before hatching.

Deep quantitative proteomics of North American Pacific coast star tunicate (Botryllus schlosseri)

Botryllus schlosseri, is a model marine invertebrate for studying immunity, regeneration, and stress-induced evolution. Conditions for validating its predicted proteome were optimized using nanoElute® 2 deep-coverage LCMS, revealing up to 4930 protein groups and 20,984 unique peptides per sample. Spectral libraries were generated and filtered to remove interferences, low-quality transitions, and only retain proteins with >3 unique peptides. The resulting DIA assay library enabled label-free quantitation of 3426 protein groups represented by 22,593 unique peptides. Quantitative comparisons of single systems from a laboratory-raised with two field-collected populations revealed (1) a more unique proteome in the laboratory-raised population, and (2) proteins with high/low individual variabilities in each population. DNA repair/replication, ion transport, and intracellular signaling processes were distinct in laboratory-cultured colonies. Spliceosome and Wnt signaling proteins were the least variable (highly functionally constrained) in all populations. In conclusion, we present the first colonial tunicate's deep quantitative proteome analysis, identifying functional protein clusters associated with laboratory conditions, different habitats, and strong versus relaxed abundance constraints. These results empower research on B. schlosseri with proteomics resources and enable quantitative molecular phenotyping of changes associated with transfer from in situ to ex situ and from in vivo to in vitro culture conditions.