Localization of Vasa mRNA during early cleavage of the snail Ilyanassa

Abstract

Members of the Vasa family of helicases are specifically localized to germ line lineages in embryos of many animal groups and, in some cases, have been shown to be required for germ line formation. Despite considerable attention to the embryology of gastropod molluscs, the germ line has not been identified in the early cleavage stages of these embryos. We have cloned a Vasa ortholog in the snail Ilyanassa and examined the distribution of IoVasa mRNA during early cleavage. Initially, the transcript is present in all cells and non-specifically localized to centrosomes in a subset of cells. The IoVasa mRNA becomes progressively more enriched in the dorsal quadrant of the embryo, and then becomes restricted to particular cells in the 4d lineage. At the 64-cell stage, IoVasa mRNA is detected in 4dL11, 4dL12, 4dR11, and 4dR12. Following another round of division in the 4d lineage, the mRNA is restricted to two cells: 4dL121 and 4dR121. By the 108-cell stage, IoVasa mRNA is no longer detectable. Because the germ line is thought to arise from the 4d lineage in spiralians, these data are consistent with the hypothesis that the Ilyanassa germ line is marked by inheritance of IoVasa and derived from the cells 4dL121 and 4dR121. Alternatively, IoVasa may be required in somatic lineages where it is expressed, and the germ line may be specified later in development.

Characterization of the piRNA pathway during development of the sea anemone Nematostella vectensis

“…These genes are also expressed in broad regions at early stages of development in multiple species across animal phylogeny including flatworms, mollusks, annelids and arthropods and involvement of these factors in embryogenesis has been proposed. 38,[43][44][45][46][47][48][49][50] The highly similar expression domains of Nematostella piwi and vasa homologs (Fig. 3) are in line with their clustering in the nCounter analysis (Fig.…”

Spiralian model systems

“…Elegant work by Nagy (2002, see also Kingsley et al, 2007) showed that specific mRNAs are localized to particular cells during early cleavage in the snail Ilyanassa obsoleta, and subsequently that some of these mRNAs actually play a role in specifying the fates of the various micromeres (see Swartz et al, 2008, Rabinowitz, et al, 2008Rabinowitz and Lambert, 2010;Chan and Lambert, 2011). These mRNAs become shuttled between the cytoplasm, centrosomes and the cell cortex to ultimately become differentially localized to specific daughter cells during cleavage.…”

A conserved germline multipotency program

“…For example, in the lophotrochozoans Ilyanassa (marine snail) and P. dumerilii (polychaete annelid), and in echinoderms, such as the sea urchin, the germline is segregated after embryogenesis completes. In these animals, multipotent progenitor cells are established during embryogenesis that give rise to both germ and somatic cells in the developing juvenile (Findley et al, 2003;Juliano et al, 2010;Rabinowitz et al, 2008;Rebscher et al, 2007;Swartz et al, 2008;Voronina et al, 2008) (Fig. 1B).…”