Effects of GSK3β inhibition in the regeneration of Syllis malaquini (Syllidae, Annelida)

Wnt/β-catenin signaling promotes posterior axial regeneration in non-regenerative tissue of the annelid Capitella teleta

To rescue regeneration, the mechanisms underlying regeneration failure must be identified and overcome. In the annelid Capitella teleta, a transverse cut triggers asymmetric responses across the amputation plane: head fragments regenerate the tail, but tail fragments do not regenerate. We compare regeneration of head fragments (successful regeneration) to that of tail fragments (unsuccessful regeneration) using cell proliferation assays, immunolabeling, and in situ hybridization. Surprisingly, following amputation, a dynamic response of the nervous system occurs in the non-regenerating tail fragments of C. teleta that has not previously been described in annelids. Wnt/β-catenin signaling plays a conserved role in patterning the primary axis of some bilaterians during regeneration, but this role has never been demonstrated in annelids. Wnt/β-catenin pathway components are expressed in the blastema of head fragments but not at the cut site of tail fragments in C. teleta. Experimental activation of Wnt/β-catenin signaling following amputation of tail fragments (24 - 72 hr post amputation) induces expression of stem cell markers, increases cell division at the wound site, and produces differentiated muscle and hindgut. Furthermore, activation of Wnt/β-catenin signaling induces ectopic posterior identity at the amputation site, as it does in other bilaterians. Inhibition of Wnt/β-catenin signaling does not rescue head regeneration. Our results indicate that C. teleta tail fragments have latent regenerative potential that is activated by Wnt/β-catenin signaling. However, the incomplete regenerative response suggests that additional cell signaling pathways are required for this complex process. Comparing tissues with different regenerative abilities elucidates the mechanisms underlying regeneration regulation, thereby enabling the prospect of rescuing or increasing regeneration ability in regeneration-deficient tissues.

Monsters reveal patterns: bifurcated annelids and their implications for the study of development and evolution

During recent decades, the study of anatomical anomalies has been of great relevance for research on development and its evolution. Yet most animal groups have never been studied under this perspective. In annelids, one of the most common and remarkable anomalies is anteroposterior axis bifurcation, that is animals that have two or more heads and/or tails. Bifurcated annelids were first described in the 18th century and have been occasionally reported since then. However, these animals have rarely been considered other than curiosities, one-off anomalies, or monsters, and a condensed but comprehensive analysis of this phenomenon is lacking. Such an analysis of the existing knowledge is necessary for addressing the different patterns of annelid bifurcation, as well as to understand possible developmental mechanisms behind them and their evolution. In this review we summarize reports of annelid bifurcation published during the last 275 years and the wide variety of anatomies they present. Our survey reveals bifurcation as a widespread phenomenon found all over the annelid tree. Moreover, it also shows that bifurcations can be classified into different types according to anatomy (lateral versus dorsoventral) or developmental origin (embryonic versus postembryonic, the latter occurring in relation to regeneration, reproduction, or growth). Regarding embryos, three different types of bifurcation can be found: conjoined twins (in clitellates); Janus embryos (two posterior ends with a single head which shows duplicated structures); and duplicitas cruciata embryos (with anterior and posterior bifurcation with a 90° rotation). In adults, we show that while lateral bifurcation can result in well-integrated phenotypes, dorsoventral bifurcation cannot since it requires the discontinuity of at least some internal organs. The relevance of this distinction is highlighted in the case of the Ribbon Clade, a group of syllid annelids in which some species reproduce by collateral and successive gemmiparity (which involves dorsoventral bifurcation), while others grow by branching laterally. Although most known cases of bifurcation came from accidental findings in the wild or were unintentionally produced, experimental studies resulting in the induction of bifurcation of both embryos and adults are also reviewed. In embryos, these experimental studies show how mechanical or chemical disruption of the zygote can result in bifurcation. In adults, the ventral nervous system and the digestive tract seem to play a role in the induction of bifurcation. Based on the reviewed evidence, we argue that the long-forgotten study of annelid developmental anomalies should be incorporated into the growing field of annelid EvoDevo and examined with modern techniques and perspectives.