The formation of left-right (L-R) asymmetry is a crucial event in embryonic development and this body axis is especially important for proper function and placement of the visceral organs.

The main pathway in L-R asymmetry is the Nodal signalling pathway. The main ligand Nodal, its inhibitors and other regulatory elements, together restrict the expression of Nodal to the left side of the lateral plate mesoderm (LPM), a structure important for the future development of the visceral organs.

The pathway is highly evolutionarily conserved. However, there are some variations in the number of genes involved in the pathway when looking at different species. For example, the mouse has homologues of the main inhibitor of Nodal called Lefty, where as the chicken only has one Lefty homologue. Nodal co-receptors of the EGF-CFC family are necessary for proper binding of Nodal to its target cell surface receptors. The mouse has two EGF-CFC genes whereas the chicken only has one.

It was expected that marsupials would resemble eutherian mammals such as humans and mice in their mechanisms of L-R determination. This study investigates how the establishment of the L-R body axis and asymmetrical gene expression in the LPM takes place in the tammar and whether the later developmental pattern more closely resembles the chicken or the mouse. The early development of the tammar morphologically resembles that of the chick somewhat, as it develops from a planar disc.

We found that, as in the chicken, the tammar has only one homologue of LEFTY and EGF-CFC. NODAL, LEFTY, PITX2, CERBERUS1, FOXH1, SNAIL1 and other genes associated with the Nodal signalling pathway are expressed between day 10 and 17 of the developing tammar. Thus while many of the gene pathways are conserved with those of other mammals, there appears to be some evolutionary plasticity in the way that marsupial L-R symmetry is established.