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In this study, we aimed to identify molecular mechanisms involved in the specification of the 4d (mesentoblast) lineage in  Platynereis dumerilii . We employ RT-PCR and  in situ  hybridization against the  Platynereis dumerilii twist  homolog ( Pdu-twist ) to reveal mesodermal specification within this lineage. We show that  Pdu-twist  mRNA is already maternally distributed. After fertilization, ooplasmatic segregation leads to relocation of  Pdu-twist  transcripts into the somatoblast (2d) lineage and 4d, indicating that the maternal component of  Pdu-twist  might be an important prerequisite for further mesoderm specification but does not represent a defining characteristic of the mesentoblast. However, after the primordial germ cells have separated from the 4d lineage, zygotic transcription of  Pdu-twist  is exclusively observed in the myogenic progenitors, suggesting that mesodermal specification occurs after the 4d stage. Previous studies on spiral cleaving embryos revealed a spatio-temporal correlation between the 4d lineage and the activity of an embryonic organizer that is capable to induce the developmental fates of certain micromeres. This has raised the question if specification of the 4d lineage could be connected to the organizer activity. Therefore, we aimed to reveal the existence of such a proposed conserved organizer in  Platynereis  employing antibody staining against dpERK. In contrast to former observations in other spiralian embryos, activation of MAPK signaling during 2d and 4d formation cannot be detected which questions the existence of a conserved connection between organizer function and specification of the 4d lineage. However, our experiments unveil robust MAPK activation in the prospective nephroblasts as well as in the macromeres and some micromeres at the blastopore in gastrulating embryos. Inhibition of MAPK activation leads to larvae with a shortened body axis, defects in trunk muscle spreading and improper nervous system condensation, indicating a critical function for MAPK signaling for the reorganization of embryonic tissues during the gastrulation process.

Maternal Inheritance of Twist and Analysis of MAPK Activation in Embryos of the Polychaete Annelid Platynereis dumerilii