Identifying the germline in an equally cleaving mollusc: Vasa and Nanos expression during embryonic and larval development of the vetigastropod Haliotis asinina

Abstract

Members of the Vasa and Nanos gene families are important for the specification and development of the germline in diverse animals. Here, we determine spatial and temporal expression of Vasa and Nanos to investigate germline development in the vetigastropod Haliotis asinina. This is the first time these genes have been examined in an equally cleaving lophotrochozoan species. We find that HasVasa and HasNanos have largely overlapping, but not identical, expression patterns during embryonic and larval development, with both being maternally expressed and localized to the micromere cell lineages during cleavage. As embryonic development continues, HasVasa and HasNanos become progressively more enriched in the dorsal quadrant of the embryo. By the trochophore stage, both HasVasa and HasNanos are expressed in the putative mesodermal bands of the larva. This differs from the unequally cleaving gastropod Illyanasa obsoleta, in which IoVasa and IoNanos expression is detectable only in the early embryo and not during gastrulation and larval development. Our results suggest that the H. asinina germline arises from the 4d cell lineage and that primordial germ cells (PGCs) are not specified exclusively by maternally inherited determinants (preformation). As such, we infer that inductive signals (epigenesis) play an important role in specifying PGCs in H. asinina. We hypothesize that HasVasa is expressed in a population of undifferentiated multipotent cells, from which the PGCs are segregated later during 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.…”

A conserved germline multipotency program

“…Both vasa (mRNA) and nanos (mRNA and protein) are uniformly distributed during early cleavage in this snail and then become selectively expressed in the 4d lineage (Rabinowitz et al, 2008;Swartz et al, 2008). In a second sea snail, Haliotis asinina, both vasa and nanos mRNA are detected in the mesodermal region of the larva, thus supporting the conclusion that these genes are expressed in multipotent cells of snails (Kranz et al, 2010). Furthermore, nanos is required to maintain the fate of the 4d lineage in Ilyanassa; loss of nanos protein by morpholino injection into the embryo results in a loss of all 4d-derived structures (Rabinowitz et al, 2008).…”

VASA expression suggests shared germ line dynamics in bivalve molluscs

“…However, evidence from another mollusc, Sphaerium striatinum (bivalvia, subclass Heterodonta), suggests that these cells retain their uncommitted morphology until adult gonad formation (Woods 1931). vasa mRNA is expressed in the 4d lineage (Swartz et al 2008; Kranz et al 2010), supporting the conclusion that these genes are expressed in multipotent cells of these molluscs (Juliano et al 2010). …”

“…More recently, new studies contributed to the discussion on the topic, bringing out an interesting comparison between gastropods, which tend to retain epigenesis, and bivalves, which show preformation as derived character (Obata and Komaru 2012). In parallel with this, genes and molecules involved in germ line specification—in particular vasa homologs — have been identified in several species at transcript (Fabioux et al 2004; Swartz et al 2008; Kranz et al 2010) or protein level (Milani et al 2011). …”