Cellular proliferation dynamics during regeneration in Syllis malaquini (Syllidae, Annelida)

BACKGROUND: In syllids (Annelida, Syllidae), the regenerative blastema was subject of many studies in the mid and late XX(th) century. This work on syllid regeneration showed that the blastema is developed by a process of dedifferentiation of cells near the wound, followed by their proliferation and redifferentiation (cells differentiate to the original cell type) or, in some specific cases, transdifferentiation (cells differentiate to a cell type different from the original). Up to date, participation of stem cells or pre-existing proliferative cells in the blastema development has never been observed in syllids. This study provides the first comprehensive description of Syllis malaquini’s regenerative capacity, including data on the cellular proliferation dynamics by using an EdU/BrdU labelling approach, in order to trace proliferative cells (S-phase cells) present before and after operation. RESULTS: Syllis malaquini can restore the anterior and posterior body from different cutting levels under experimental conditions, even from midbody fragments. Our results on cellular proliferation showed that S-phase cells present in the body before bisection do not significantly contribute to blastema development. However, in some specimens cut at the level of the proventricle, cells in S-phase located in the digestive tube before bisection participated in regeneration. Also, our results showed that nucleus shape allows to distinguish different types of blastemal cells as forming specific tissues. Additionally, simultaneous and sequential addition of segments seem to occur in anterior regeneration, while only sequential addition was observed in posterior regeneration. Remarkably, in contrast with previous studies in syllids, sexual reproduction was not induced during anterior regeneration of amputees lacking the proventricle, a foregut organ widely known to be involved in the stolonization control. CONCLUSIONS: Our findings led us to consider that although dedifferentiation and redifferentiation might be more common, proliferative cells present before injury can be involved in regenerative processes in syllids, at least in some cases. Also, we provide data for comparative studies on resegmentation as a process that differs between anterior and posterior regeneration; and on the controversial role of the proventricle in the reproduction of different syllid lineages. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12983-021-00396-y

Comparative transcriptomics in Syllidae (Annelida) indicates that posterior regeneration and regular growth are comparable, while anterior regeneration is a distinct process

Background: Annelids exhibit remarkable postembryonic developmental abilities. Most annelids grow during their whole life by adding segments through the action of a segment addition zone (SAZ) located in front of the pygidium. In addition, they show an outstanding ability to regenerate their bodies. Experimental evidence and field observations show that many annelids are able to regenerate their posterior bodies, while anterior regeneration is often limited or absent. Syllidae, for instance, usually show high abilities of posterior regeneration, although anterior regeneration varies across species. Some syllids are able to partially restore the anterior end, while others regenerate all lost anterior body after bisection. Here, we used comparative transcriptomics to detect changes in the gene expression profiles during anterior regeneration, posterior regeneration and regular growth of two syllid species: Sphaerosyllis hystrix and Syllis gracilis; which exhibit limited and complete anterior regeneration, respectively. Results: We detected a high number of genes with differential expression: 4771 genes in S. hystrix (limited anterior regeneration) and 1997 genes in S. gracilis (complete anterior regeneration). For both species, the comparative transcriptomic analysis showed that gene expression during posterior regeneration and regular growth was very similar, whereas anterior regeneration was characterized by up-regulation of several genes. Among the upregulated genes, we identified putative homologs of regeneration-related genes associated to cellular proliferation, nervous system development, establishment of body axis, and stem-cellness; such as rup and JNK (in S. hystrix); and glutamine synthetase, elav, slit, Hox genes, β-catenin and PL10 (in S. gracilis). Conclusions: Posterior regeneration and regular growth show no significant differences in gene expression in the herein investigated syllids. However, anterior regeneration is associated with a clear change in terms of gene expression in both species. Our comparative transcriptomic analysis was able to detect differential expression of some regeneration-related genes, suggesting that syllids share some features of the regenerative mechanisms already known for other annelids and invertebratesThis research was supported by MINECO/FEDER, UE funds (Grant: CGL2015– 63593-P, “Macroevolutionary transitions in Syllidae” project, PI: MTA). RPR is supported by the program “Contratos predoctorales para Formación de Personal Investigador, FPI-UAM,” Universidad Autónoma de Madrid. GPS is supported by the “Contratos Predoctorales para la Formación de Doctores 2016” program of the MINECO, Spain (code: BES-2016-076419), cofinanced by the European Social Found. We acknowledge support by German Research Foundation and the Open Access Publication Funds of the Georg-AugustUniversität Göttinge

Investigaciones acerca de la innovación en el plan corporal en Syllidae (Annelida): control genético de la reproducción y la evolución del plan corporal ramificado

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Biología. Fecha de Lectura: 08-02-2022Esta Tesis tiene embargado el acceso al texto completo hasta el 08-08-202

Sex-specific gene expression differences in reproducing Syllis prolifera and Nudisyllis pulligera (Annelida, Syllidae)

Syllidae is an annelid family characterized by its complex life cycles involving some of the most outstanding annelid reproductive strategies. Syllid reproductive modes sometimes imply the modification of the posterior body to form independent reproductive units (schizogamy) or the development of swimming adults (epigamy). These modes of sexual reproduction have been studied for more than 150 years, and yet, little is known regarding their molecular background. Notably, while several studies during the last three decades have revealed details about molecular mechanisms involved in the reproduction of some few model annelids, studies focusing on syllids remain limited. Thus, we performed differential gene expression analyses of female, male, and non-reproducing individuals of Syllis prolifera (schizogamic) and Nudisyllis pulligera (epigamic), as representatives of two different reproductive strategies. For that, transcriptomes from specimens of three conditions (non-reproducing, male, female) were de novo assembled and annotated for S. prolifera and N. pulligera. We found rather similar gene expression profiles for female and non-reproducing individuals, while male gene expression is clearly different. Although previous studies have suggested that femininity in syllids might require additional signalling, our analyses support a scenario, where masculinity may also involve several specific genetic processes

Integrative anatomical study of the branched annelid Ramisyllis multicaudata (Annelida, Syllidae)

The sponge-dwelling Syllidae Ramisyllis multicaudata and Syllis ramosa are the only annelid species for which a branched body with one head and multiple posterior ends is known. In these species, the head is located deep within the sponge, and the branches extend through the canal system of their host. The morphology of these creatures has captivated annelid biologists since they were first discovered in the late XIXth century, and their external characteristics have been well documented. However, how their branched bodies fit within their symbiotic host sponges and how branches translate into internal anatomy has not been documented before. These features are crucially relevant for understanding the body of these animals, and therefore, the aim of this study was to investigate these aspects. In order to assess these questions, live observation, as wells as histology, immunohistochemistry, micro-computed tomography, and transmission electron microscopy techniques were used on specimens of R. multicaudata. By using these techniques, we show that the complex body of R. multicaudata specimens extends greatly through the canal system of their host sponges. We demonstrate that iterative external bifurcation of the body is accompanied by the bifurcation of the longitudinal organ systems that are characteristic of annelids. Additionally, we also highlight that the bifurcation process leaves an unmistakable fingerprint in the form of newly-described “muscle bridges.” These structures theoretically allow one to distinguish original and derived branches at each bifurcation. Last, we characterize some of the internal anatomical features of the stolons (reproductive units) of R. multicaudata, particularly their nervous system. Here, we provide the first study of the internal anatomy of a branched annelid. This information is not only crucial to deepen our understanding of these animals and their biology, but it will also be key to inform future studies that try to explain how this morphology evolved