Temporally and spatially dynamic germ cell niches in Botryllus schlosseri revealed by expression of a TGF-beta family ligand and vasa

Abstract

BackgroundGerm cells are specified during early development and are responsible for generating gametes in the adult. After germ cells are specified, they typically migrate to a particular niche in the organism where they reside for the remainder of its lifetime. For some model organisms, the specification and migration of germ cells have been extensively studied, but how these events occur in animals that reproduce both sexually and asexually is not well understood.ResultsWe have identified a novel TGF-β family member in Botryllus schlosseri, tgfβ-f, and found that it is expressed by follicle cell progenitors and the differentiated follicle and support cells surrounding the maturing gametes. Using the expression of tgfβ-f and the germ cell marker vasa, we have found that nearly all germ cells in Botryllus are associated with tgfβ-f-expressing follicle progenitors in clusters consisting solely of those two cell types. These clusters were mostly small, consisting of ten or fewer cells, and generally contained between a 2:1 and 1:1 ratio of follicle progenitors to germ cells. Clusters of germ and follicle progenitor cells were primarily localized to niches in the primary and secondary buds, but could also be found in other locations including the vasculature. We analyzed the location of germ cell clusters throughout the asexual life cycle of Botryllus and found that at the stage when germ cells are first detected in the secondary bud niche, a dramatic change in the size and location of germ/follicle cell clusters also occurred.ConclusionsOur findings suggest that germ/follicle cell clusters have predictable migratory patterns during the weekly asexual developmental cycle in Botryllus. An increased number of small clusters and the presence of clusters in the vasculature coinciding with the appearance of clusters in the secondary bud suggest that fragmentation of clusters and the migration of smaller clusters through the vasculature may be an important aspect of Botryllus reproductive biology, ensuring the transmission of the germline to subsequent asexual generations.Electronic supplementary materialThe online version of this article (doi:10.1186/s13227-016-0047-5) contains supplementary material, which is available to authorized users.

In vivo manipulation of the extracellular matrix induces vascular regression in a basal chordate

“…However, the fact that the level of pY397 FAK is correlated with the concentration of BAPN and the level of pY397 FAK does not change when regression is induced by the downstream inhibition of Raf is strong evidence that apoptosis is due to physical changes in the ECM detected by integrins. In addition, studies in Botryllus using small-molecule inhibitors for both Wnt and transforming growth factor β signaling produced phenotypes in the developing zooids but did not induce vascular regression, indicating that we are inhibiting canonical integrin signaling pathways in the present studies ( Di Maio et al , 2015 ; Langenbacher and De Tomaso, 2016 ).…”

ABC-transporter activity and autocrine eicosanoid-signaling are required for germ cell migration a basal chordate

“…In Botryllus, migration of germ cells to new, germline niches occurs during a defined 48h period 28 . It is common for a migrating cell to require more than one signal to induce this type of directed migration and motility.…”

Gonad development and hermaphroditism in the ascidian Botryllus schlosseri

“…Expression of genes required for germ cell specification (vasa, piwi, and nanos), can be detected by PCR and in situ hybridization in primitive germ cells that are present in both juveniles and adults (Brown et al, 2009;Sunanaga et al, 2010;Kawamura and Sunanaga, 2011;Rinkevich et al, 2013;Rosner et al, 2013;Langenbacher et al, 2015). Germ-line progenitors also circulate between primary and secondary buds, even in juvenile colonies (Langenbacher and De Tomaso, 2016), indicating the ubiquitous and continuous presence of germ-line precursors.…”

“…Langenbacher and De Tomaso () identified a novel homolog of the TGF‐β family member, named tgf‐ β‐ f , that is expressed by follicle cell progenitors, differentiated follicle cells, and support cells enclosing the maturing gametes. Almost all vasa‐positive germ cells in B. schlosseri juveniles associate with tgf‐ β‐ f ‐positive follicle progenitors, together forming clusters consisting of as many as 10 cells with a 1:1 or 2:1 ratio of follicle progenitors‐to‐germ cells.…”