Molecular genetic insights into deuterostome evolution from the direct-developing hemichordateSaccoglossus kowalevskii

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Abstract
Progress in developmental biology, phylogenomics and palaeontology over the past five years are all making major contributions to a long-enduring problem in comparative biology: the early origins of the deuterostome phyla. Recent advances in the developmental biology of hemichordates have given a unique insight into developmental similarities between this phylum and chordates. Transcriptional and signalling gene expression patterns between the two groups during the early development of the anteroposterior and dorsoventral axes reveal close similarities, despite large morphological disparity between the body plans. These genetic networks have been proposed to play conserved roles in patterning centralized nervous systems in metazoans, yet seem to play a conserved role in patterning the diffusely organized basiepithelial nerve net of the hemichordates. Developmental genetic data are providing a unique insight into early deuterostome evolution, revealing a complexity of genetic regulation previously attributed only to vertebrates. While these data allow for key insights into the development of early deuterostomes, their utility for reconstructing ancestral morphologies is less certain, and morphological, palaeontological and molecular datasets should all be considered carefully when speculating about ancestral deuterostome features.