15-P001 The marsupial CDX2 is not expressed during early embryonic development until the late unilaminar blastocyst stage, preceding trophoblast–trophectoderm transition and hypoblast formation

The first lineage allocation during mouse development results in the trophectoderm and inner cell mass at the blastocyst stage. The caudal-related transcription factor, CDX2, is upregulated and required for the repression of inner cell mass genes Oct 4 and Nanog in the trophectoderm to specify the two lineages. Mouse embryos are precocious implanters requiring early establishment of trophectoderm. It is unclear whether this is conserved between mammals, particularly in embryos that implant late during embryonic development, such as marsupial embryos. Using nested PCR, we have cloned a partial CDX2 sequence from the stripe-faced dunnart (Sminthopsis macroura) based on known mammalian CDX2 sequences. Dunnart CDX2 shares 97.8% and 86.7% sequence homology to the gray short-tailed opossum and mouse CDX2 sequence, respectively, suggesting conserved homology among mammalian species. Using gene expression analysis, dunnart CDX2 is expressed in the gut and the ovary, consistent with studies in mice. However, unlike in mice when CDX2 can be detected as early as the 8-cell stage, dunnart CDX2 is not expressed during early embryonic development until the late unilaminar blastocyst stage, Day 5.5 of a 10.7 day gestation period. This difference in expression may be related to later trophectoderm differentiation, hypoblast allocation and implantation in the dunnart. This is the first study of CDX2 in a marsupial species and suggests that CDX2 expression is involved in lineage determination in the dunnart.