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Here, we will outline mechanisms of germ cell formation in solitary and colonial
ascidians, as well as the regeneration of germ cells in colonial species. We will also
briefly discuss observations about de novo formation of germ cells during postem-
bryonic development.
8.4.1 Embryonic Germ Line Specification in Solitary
and Colonial Ascidians
In ascidians, germ cells originate from the part of the embryo that contains a specific
cytoplasm, called the postplasm. The postplasm accumulates many maternal RNAs
and proteins (Sardet et al. 2005 ), and contains a structure termed the centrosome
attracting body (CAB), which shows morphological similarity to germinal granules.
These granules are a hallmark of germ cells in a wide range of animal species. In the
solitary ascidian Ciona intestinalis, RNA and protein of the Ciona vasa homolog
Fig. 8.8 Germ cell specification in Ascidians: distribution of vasa-expression during embryogen-
esis, metamorphosis and larval stages to juveniles. Vasa-mRNA is broadly distributed in unfertil-
ized eggs, and accumulates at the posterior cortex of the fertilized one-cell stage embryo. During
the 4 to 32-cell stages, it is incorporated into a pair of blastomeres located at the posterior pole. In
64- and 110-cell stage embryos, Vasa-protein is concentrated in the posterior-most cells. After
gastrulation, four Vasa-protein-positive cells are formed by the late tailbud stage. In the tadpole
larva, Vasa-positive cells in the endodermal strand are located in the posterior half of the tail.
During metamorphosis, eight Vasa-positive cells align in the space between the intestine and stom-
ach. In juvenile zooids, Vasa-positive cells increase in number and form a cluster next to the pyloric
gland, and later give rise to the gonad rudiment
T. Aguero et al.