Abstract submitted to EuroEvoDevo 2012

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Germ cell development in non-spiralian lophotrochozoans: insights from a bryozoan and a brachiopod

Bryozoa and Brachiopoda are two spiralian taxa that, unlike other spiralians, undergo non-spiral cleavage and have unique non-trochophore larvae. Previous morphological studies determined that no distinctive germline is formed during embryonic development and that germ cells first appear relatively late in larval life or after metamorphosis. These observations suggest that the specification of primordial germ cells occurs by late inductive signaling (epigenesis) rather than inheritance of maternal determinants (preformation). The molecular mechanisms involved in germ cell formation in bryozoans and brachiopods are currently unknown. We have therefore used RNASeq data to identify and then clone the conserved germline-specific genes vasa, nanos, and piwi from the bryozoan Membranipora membranacea and the brachiopod Terebratalia transversa. In situ hybridization shows that Mm-nanos transcripts are not detected in the blastomeres during early cleavage, but are localized posteriorly in the internal sac region of the late-gastrula stage and early cyphonautes larvae of M. membranacea. In addition, Mm-piwi2 mRNA is present in the cytoplasm of M. membranacea blastomeres and is broadly expressed in the larval tissues, except for the corona and apical organ. Our preliminary results suggest that the signaling related to the differentiation of bryozoan germ cells may be established earlier in ontogeny than previously thought, possibly during gastrulation. A thorough analysis of the expression patterns will provide clues for understanding the regulatory mechanisms of pluripotent and germ cell development in bryozoans and brachiopods and offer further insights about the developmental diversity of spiralians.