Postembryonic epigenesis of Vasa‐positive germ cells from aggregated hemoblasts in the colonial ascidian, Botryllus primigenus

Abstract

We investigated whether Vasa was a germline-specific marker in the colonial ascidian Botryllus primigenus, and whether it was inducible epigenetically in the adult life span. We cloned a Botryllus Vasa homologue (BpVas). The deduced open reading frame encoded 687 amino acid residues. It was expressed specifically by germline cells such as the loose cell mass, oogonia and juvenile oocytes in the ovary, and the primordial testis (compact cell mass), spermatogonia and juvenile spermatocytes in the testis. The loose cell mass, the most primitive germline cells, showed an ultrastructure of undifferentiated cells known as hemoblasts. The hemoblasts did not contain electron-dense materials or a mitochondrial assembly in the cytoplasm. These organelles appeared later in the oogonia and oocytes. When the loose cell mass and developing germ cells were eliminated by extirpating all zooids and buds from the colonies, BpVas transcripts disappeared completely from the vascularized colonies. After 14 days, when the colonies regenerated by vascular budding, BpVas-positive cells reappeared in some cases, and in 30 day colonies, BpVas-positive germ cells were observed in all the regenerated colonies. These results show that in B. primigenus, germ cells are inducible de novo from the Vasa-negative cells even at postembryonic stages.

Body Muscle-Cell Differentiation from Coelomic Stem Cells in Colonial Tunicates

“…It is referred to as a hemoblast and is characterized by the presence of a large nucleus with a prominent nucleolus and narrow cytoplasm filled with polysomes (Wright, 1981). A variety of tissues and organs such as the gonad (Mukai and Watanabe, 1976;Sunanaga et al, 2006Sunanaga et al, , 2007, the pericardium (Nunzi et al, 1979), the pyloric gland (Kawamura and Nakauchi, 1986), and bud vesicle (Oka and Watanabe, 1957;Freeman, 1964;Sabbadin et al, 1975) differentiate from undifferentiated coelomic cells.…”

Cell proliferation dynamics of somatic and germline tissues during zooidal life span in the colonial tunicateBotryllus primigenus

“…During asexual reproduction, coelomic undifferentiated cells, the hemoblasts, differentiate into various tissues such as muscle cells, pyloric gland, germ cells, and multipotent epithelium (Mukai and Watanabe, 1976;Nunzi et al, 1979;Kawamura and Nakauchi, 1986a;Sunanaga et al, 2006Sunanaga et al, , 2007Sugino et al, 2007). In Symplegma reptans, hemoblasts cluster around the siphon anlagen of de- Outer and inner pharyngeal epithelia and inner longitudinal vessels were stained.…”

“…A strong signal was also observable in loose and compact cell masses in the gonads. Both the bud primordium and germline progenitor cells are known to have an undifferentiated configuration (Manni et al, 1994;Sunanaga et al, 2006). It is, therefore, an interesting question as to whether BpMyc is related to cell proliferation or to the undifferentiated state of cells.…”

“…The fixed specimens were rinsed in PBST (PBS containing 0.1% Tween 20), and were dehydrated in graded series of methanol and stored in 75% methanol at Ϫ20°C. In situ hybridization was performed according to Sunanaga et al (2006). In brief, a digoxigenin-labeled antisense RNA probe was used for hybridization.…”

Germline cell formation and gonad regeneration in solitary and colonial ascidians

“…Some investigators have observed that they derive from the germinal epithelium in Styela plicata (Tucker, 1942), Styela clava (Ermak, 1976), Ciona savignyi (Sugino et al, 1987(Sugino et al, , 1990, and C. intestinalis (Okada and Yamamoto, 1993). Other investigators have suggested that the primary follicle cells come from free hemoblasts of the hemocoel (Peres, 1954;De Vincentiis, 1962;Mancuso, 1965;Newberry, 1968;Mukai and Watanabe, 1976;Manni et al, 1993Manni et al, , 1995Sunanaga et al, 2006). In B. primigenus, the primary follicle cells appear to have the same origin as the PGCs, because they express a PGC-specific molecular marker at the earliest stage of differentiation (Sunanaga et al, 2006).…”