Vasa and the germ line lineage in a colonial urochordate

Abstract

Germ cell sequestering in Animalia is enlightened by either, launching true germ line along epigenetic or preformistic modes of development, or by somatic embryogenesis, where no true germ line is set aside. The research on germ line-somatic tissue segregation is of special relevancy to colonial organisms like botryllid ascidians that reconstruct, on a weekly basis, completely new sets of male and female gonads in newly formed somatic tissues. By sequencing and evaluating expression patterns of BS-Vasa, the Botryllus schlosseri orthologue of Vasa, in sexually mature and asexual colonies during blastogenesis, we have demonstrated that the BS-Vasa mRNA and protein are not expressed exclusively in germ cell lineages, but appeared in cells repeatedly emerging de novo in the colony, independently of its sexual state. In addition, we recorded an immediate Vasa response to cellular stress (UV irradiation) indicating additional functions to its germ line assignments. To confirm germ lineage exclusivity, we examined the expression of three more stem cell markers (BS-Pl10, Bl-piwi and Oct4). Vasa co-expression with Pl10 and Oct4 was detected in germ line derivatives and with Bl-piwi in somatic tissues. Presumptive primordial germ cells (PGC-like cells), that are Vasa(+)/Pl10(+)/Oct4(+) and 6-12 microm in diameter, were first detected in wrapped-tail embryos, in oozooids, in sexual/asexual colonies, within a newly identified PGC niche termed as 'budlet niche', and in circulating blood borne cells, indicating epigenetic embryogenesis. Alternatively, BS-Vasa co-expression with piwi orthologue, an omnipresent bona fide stemness flag, in non germ line cell populations, may indicate germ cell neogenesis (somatic embryogenesis) in B. schlosseri. Both alternatives are not necessarily mutually exclusive.

Germline cell formation and gonad regeneration in solitary and colonial ascidians

“…In the colonial ascidian, B. schlosseri vasa mRNA is distributed in posterior blastomeres during early embryogenesis, reminiscent of results in Ciona transcript, but in the larvae mRNA and protein are found exclusively in the trunk region (Brown et al, 2009;Rosner et al, 2009). In Ciona, the CiVH signal appears in the trunk of tailbud embryo, while it becomes faint or remarkably reduced in the late tadpole larvae (Takamura et al, 2002;Shirae-Kurabayashi et al, 2006).…”

“…The colony reaches sexual maturity only after several successions of blastogenic generations (Mukai and Watanabe, 1976;Brown et al, 2009;Rosner et al, 2009). Mature colonies usually produce male organs at first and thereafter form both male and female organs.…”

Molecular and Sub-Cellular Gametogenic Machinery of Stem and Germline Cells Across Metazoa

“…In many of the invertebrates, such as cnidarians, acoels, planarians, annelids and colonial urochordates, expression of Piwi and Vasa are not restricted to the germline but are expressed in multipotent stem cells (Alié et al, 2011). Piwi is considered to be an omnipresent stemness flag for self-renewal and maintenance of germ line and stem cells in diverse multicellular organisms (Rosner et al, 2009). Several studies indicate a functional relationship between Vasa and both the small interfering RNA and micro-RNA processing pathways (Gustafson & Wessel, 2010).…”

“…Examples of such pluripotent gametogenic stem cells include sponge archaeocytes, cnidarian interstitial cells, planarian neoblasts, ascidian hemoblasts and stem cells of colonial rhizocephalans (reviews: Isaeva et al, 2008Isaeva et al, , 2009Rinkevich et al, 2009;Sköld et al, 2009;Srouji & Extavour, 2011). In addition to the germline segregation by preformation, or mosaic developmental mode and epigenesis, or regulative mode Extavour & Akam, 2003;Gustafson & Wessel, 2010), somatic embryogenesis was also recognized (Buss, 1987;Blackstone & Jasker, 2003;Gustafson & Wessel, 2010;Rinckevich et al, 2009;Rosner et al, 2009). Earlier, somatic embryogenesis as natural cloning in animals was termed blastogenesis (Berrill, 1961;Ivanova-Kazas, 1996).…”

“…In the life cycle of colonial animals, one generation of oozooid (an individual that has developed from an egg) alternates with numerous generations of blastozooids, with alternating morphogenetic processes: embryogenesis and blastogenesis (Ivanova-Kazas, 1996). Many animals, including placozoans, sponges, cnidarians, platyhelminths, nemerteans, entoproctans, ectoproctans, annelids, hemichordates and urochordates are capable of somatic embryogenesis (Buss 1987;Blackstone & Jasker, 2003;Gustafson & Wessel, 2010;Rosner et al, 2009). Among arthropods, many parasitic rhizocephalan crustaceans (Rhizocephala: Cirripedia: Crustacea) have asexual reproduction, somatic embryogenesis by budding without separation of blastozooids resulting in the emergence of colonial organization (Høeg & Lützen, 1993, 1995Høeg et al, 2005).…”