A number of regulatory genes involved in mesoderm and/or muscle formation (e.g., Brachyury (Bra), even-skipped (eve), Mox, myosin II heavy chain (mhc)) have been identified chiefly from chordates and the ecdysozoans Drosophila and Caenorhabditis elegans, but data for non-model protostomes, especially those belonging to the ecdysozoan sister clade, Lophotrochozoa (e.g., flatworms, annelids, mollusks), are only beginning to emerge. The mesoderm gives rise to several key morphological features of bilaterian animals including endoskeletal elements and the musculature. 7 Hejnol & Martindale, 2008a Martín-Durán & Romero, 2011 Pocock et al., 2004 Sebé-Pedrós & Ruiz-Trillo, 2017). Since mesodermal expression of Bra appears to be absent in the acoel Convolutriloba longifissura, expression of Bra in the mesoderm may have evolved in the lineage leading to the nephrozoans, with a possible loss of function in various spiralians and nematodes, whereby Bra is absent from the genome of Caenorhabditis elegans altogether ( Fig. 7 Andrikou et al., 2019 Green & Akam, 2014 Kusch & Reuter, 1999 Martín-Durán et al., 2012Nishino et al., 2001 Perry et al., 2015 Peter & Davidson, 2011 Peterson et al., 1999 Satou & Imai, 2015 Takada et al., 2002 Terazawa & Satoh, 1997 Vellutini et al., 2017). Mesodermal expression of Bra has been reported in most nephrozoan taxa (protostomes and deuterostomes), except for a few spiralians, e.g., ectoprocts, phoronids, and chaetognaths, where Bra expression was not detected in the mesoderm ( Fig. This indicates that Bra appears to have a conserved role in blastopore and digestive tract formation amongst bilaterian animals (Fig. Comparison with published data from other groups of animals shows that this high degree of evolutionary conservation often dates back beyond the last common ancestor of Panarthropoda. Our data show that many of the Fox genes likely retained their function during panarthropod evolution highlighting their importance in development. We also provide the first comprehensive analysis of Fox gene expression in an onychophoran species. In this study, we provide the first comprehensive gene expression analysis of arthropod Fox genes including representative species of all main groups of arthropods, Pancrustacea, Myriapoda and Chelicerata. For other arthropods, only selected Fox genes have been investigated. Most of our knowledge about the function of Fox genes comes from vertebrate research, and for arthropods the only comprehensive gene expression analysis is that of the fly Drosophila melanogaster. They represent key-components of multiple gene regulatory networks (GRNs) that are essential for embryonic development. Fox genes represent an evolutionary old class of transcription factor encoding genes that evolved in the last common ancestor of fungi and animals.
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