A novel role for sox7 in Xenopus early primordial germ cell development: mining the PGC transcriptome

Xenopus primordial germ cells (PGCs) are determined by the presence of maternally derived germ plasm. Germ plasm components both protect PGCs from somatic differentiation and begin a unique gene expression program. Segregation of the germline from the endodermal lineage occurs during gastrulation, and PGCs subsequently initiate zygotic transcription. However, the gene-network(s) that operate to both preserve and promote germline differentiation are poorly understood. Here, we utilized RNA-sequencing analysis to comprehensively interrogate PGC and neighboring endoderm cell mRNAs after lineage segregation. We identified 1,865 transcripts enriched in PGCs compared to endoderm cells. We next compared the PGC-enriched transcripts to previously identified maternal, vegetally-enriched transcripts, and found that >50% of maternal transcripts were enriched in PGCs, including sox7. PGC-directed sox7 knockdown and over-expression studies revealed an early requirement for sox7 in germ plasm localization, zygotic transcription, and PGC number. We identified oct60 as the most highly expressed and enriched OCT3/4 homologue in PGCs. We compared the Xenopus PGC transcriptome with human PGC transcripts and showed that 80% of genes are conserved, underscoring the usefulness of Xenopus for human based studies.