Identification and Expressional Analysis of Putative PRDI-BF1 and RIZ Homology Domain-Containing Transcription Factors in Mulinia lateralis

SIMPLE SUMMARY: The formation of primordial germ cells (PGCs) is the basis of molluskan reproduction, but limited information is reported on this topic. PRDI-BF1 and RIZ homology domain-containing proteins (PRDMs), especially PRDM1 (also known as BLIMP1) and PRDM14, have been reported to be essential for the formation of PGCs. In this study, we systematically characterized the putative PRDMs in a bivalve mollusk species, Mulinia lateralis, and analyzed their sequence structures, phylogenetic relationships, and expressional profiles. Furthermore, we analyzed the temporal–spatial expression patterns of Ml-prdm1 and Ml-prdm14 RNA in early embryos and larvae. Our study suggests that Ml-prdm1 may function as an important regulator of PGC formation in M. lateralis. ABSTRACT: Mollusca represents one of the ancient bilaterian groups with high morphological diversity, while the formation mechanisms of the precursors of all germ cells, primordial germ cells (PGCs), have not yet been clarified in mollusks. PRDI-BF1 and RIZ homology domain-containing proteins (PRDMs) are a group of transcriptional repressors, and PRDM1 (also known as BLIMP1) and PRDM14 have been reported to be essential for the formation of PGCs. In the present study, we performed a genome-wide retrieval in Mulinia lateralis and identified 11 putative PRDMs, all of which possessed an N-terminal PR domain. Expressional profiles revealed that all these prdm genes showed specifically high expression levels in the given stages, implying that all PRDMs played important roles during early development stages. Specifically, Ml-prdm1 was highly expressed at the gastrula stage, the key period when PGCs arise, and was specifically localized in the cytoplasm of two or three cells of blastula, gastrula, or trochophore larvae, matching the typical characteristics of PGCs. These results suggested that Ml-prdm1-positive cells may be PGCs and that Ml-prdm1 could be a candidate marker for tracing the formation of PGCs in M. lateralis. In addition, the expression profiles of Ml-prdm14 hinted that it may not be associated with PGCs of M. lateralis. The present study provides insights into the evolution of the PRDM family in mollusks and offers a better understanding of the formation of PGCs in mollusks.