Species Specific Nodal Ligands
Nodal is a widely distributed cytokine. The presence of Nodal is not limited to vertebrates, it is also known to be conserved in other chordates, deuterostomes (cephalochordates, tunicates and echinoderms) and protostomes such as snails, but interestingly neither the nematode C. elegans (another protosome) nor the fruit fly Drosophila (an arthropode) have a copy of nodal. Although mouse and human only have one nodal gene, the zebra fish contain three nodal paralogs: squint, cyclops and southpaw, and the frog five (xnr1,2,3,5 and 6). Even though the zebra fish Nodal homologs are very similar, they have specialized to perform different roles; for instance, Squint and Cyclops are important for mesoendoderm formation, whereas the Southpaw has a major role in asymmetric heart morphogenesis and visceral left-right asymmetry. Another example of protein speciation is the case of the frog where Xnr1 and Xnr2 regulate movements in gastrulation in contrast to Xnr5 and Xnr6 that are involved in mesoderm induction. In mouse, Nodal has been implicated in left-right asymmetry, neural pattering and mesoderm induction (see nodal signaling).
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