Do NMPs Exist Outside the Vertebrate Clade?

A major unresolved question in the field is whether NMPs are a vertebrate innovation. The vast majority of bilaterians undergo a posterior growth process during embryo- genesis (Martin and Kimelman, 2009). Based on the diversity of extant organisms that undergo posterior growth, this mode of body plan formation is thought to have been present in the so-called urbilaterian (the ancestor to all bilaterians) (Martin and Kimelman, 2009). Those posteriorly growing metazoans that are also segmented exhibit a progressive posterior addition of new segments, similar to vertebrate somite addition. Many aspects of the posterior growth and the segmentation process are conserved between vertebrates and other bilaterians, such as the requirement for canonical Wnt signaling during posterior growth of both vertebrates and ecdysozo- ans (Martin and Kimelman, 2009).

Despite several commonalities between the posterior growth and segmentation process of vertebrates and invertebrates, a posteriorly localized cell type that gives rise to both neural and pre-segmentation mesoderm has yet to be formally identified (see Chapters 3 and 4). Some of the genes and signaling pathways play a conserved role during posterior elongation. In ecdysozoans, canonical Wnt signaling is localized to the segment addition zone, and disruption of the Wnt pathway leads to truncation of the body axis and a loss of posterior segments, similar to the effect of Wnt inhibition in vertebrates. Wnt signaling is also critical for the posterior elongation of the hemichor- date Saccoglossus kowalevskii, and like vertebrates, functions in an autoregulatory loop with the Brachyury transcription factor (Fritzenwanker et al., 2019). Canonical Wnt signaling and Brachyury function in an autoregulatory loop in the sea urchin as well, suggesting that this regulatory relationship was present at the base of all deutero- stomes (Sethi et ah, 2012). Although canonical Wnt signaling is required for posterior elongation and segment addition in ecdysozoans, here it does not function in an autoregulatory loop with Brachyury. Loss of Brachyury function in Tribolium and crickets has no effect on posterior growth and segmentation (Berns et ah, 2008; Shinmyo et ah, 2006). Thus, the role of Wnt signaling during posterior growth is conserved across bilaterians, but Brachyury regulation of Wnt signaling appears to be a deuterostome innovation. Given that the essential role of Brachyury during zebrafish axis extension is to maintain Wnt signaling, and that Brachyury is dispensable during zebrafish axis extension as long as Wnt signaling is present (Martin and Kimelman, 2012), the absence of an important role for Brachyury during ecdysozoan axis extension could be due to another factor maintaining posterior Wnt signaling in this lineage.

The other factor that defines NMPs besides Brachyury expression is the expression of Sox2, which is a member of the SoxB subfamily of Sox transcription factors. Relatively little is known about the role of SOX2 in the context of NMPs, partially due to the early lethality of mouse Sox2 mutants and lack of genetic knockouts in other models (Avilion et al., 2003), although conditional analysis suggests it plays a role in neural induction (Takemoto et al., 2011). Recent work has examined the role of SoxB transcription factors during axial elongation and segment addition in the spider Parasteatoda tepidariorum (Paese et al., 2018). Loss of SoxB function results in truncated embryos, with a loss of segments. Although this is a distinct phenotype from the mouse, it shows that SoxB factors play an important role in axial elongation outside of the vertebrate lineage. Thus, there appears to be important functions of both SoxB transcription factors and canonical Wnt signaling during axial elongation and segmentation in diverse organisms. Whether a germ-layer plastic progenitor homologous to the vertebrate NMPs exists outside of the vertebrate lineage remains to be determined.

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