Cell commitment prevents larval regeneration in the crinoid Antedon mediterranea

Regeneration is a widespread phenomenon along the phylogenetic tree, however few animal groups display remarkable regenerative abilities as Echinoderms do. Indeed regeneration is consistently present in all the extant classes, both in adults and larval stages. While adult regeneration has been deeply studied, comparatively less studies focused on larval regeneration. Nonetheless the latter is well documented for all echinoderm classes, except for Crinoids. Hence, the aim of this work was to assess if the larval (doliolaria) and post-metamorphic stages of the crinoid Antedon mediterranea are able to regenerate, as the adult form perfectly does at the level of almost any tissue/organ. In normal conditions, free-swimming A. mediterranea doliolaria settle and metamorphose in a temporary sessile and stalked stage (pentacrinoid) which is provided with an apical calix; the latter will eventually develop the ten arms (advanced pentacrinoid) and then detach originating the free-swimming adult individual. Adult specimens of A. mediterranea were collected at Le Grazie (Ligurian Sea, Italy). After hatching, doliolaria larvae were transversally bisected and the obtained anterior and posterior halves were monitored for 2-3 weeks. For each fragment type we defined different post-amputation “developmental” stages which were characterized by light (confocal) and electron microscopy (SEM, TEM). Results showed that none of the surviving halves was able to completely regenerate. Rather, after a wound-healing phase each half continued its “developmental program” so that the obtained post-metamorphic stage lacked structures deriving from the missing half: anterior fragments originated a stalk without a calix whereas the posterior halves produced a calix without a stalk. Noteworthy, this inability to regenerate is progressively rescued upon the transition from pentacrinoid to free-swimming juvenile, as advanced pentacrinoids (close to the stalk detachment event) show albeit minimal regenerative ability while freshly “detached” juveniles are perfectly able to regenerate their tissues/structures (even more than adults). Overall, these data suggest that: doliolaria cells are strictly committed to their original fate; cellular plasticity/dedifferentiation is temporary blocked and/or “stem cells” are missing or in a “stand-by” state. To unswer this question the presence and expression of stemness markers (e.g. c-myc, pou, piwi) is currently under investigation. Considering the basal phylogenetic position of Crinoidea these results are particularly significant to better understand the evolutionary trajectories which led to gain or loss of (larval) regenerative abilities among echinoderms and metazoans.