XB-ART-51574Zoological Lett January 1, 2015; 1 33.
On the origin of vertebrate somites.
INTRODUCTION: Somites, blocks of mesoderm tissue located on either side of the neural tube in the developing vertebrate embryo, are derived from mesenchymal cells in the presomitic mesoderm (PSM) and are a defining characteristic of vertebrates. In vertebrates, the somite segmental boundary is determined by Notch signalling and the antagonistic relationship of the downstream targets of Notch, Lfng, and Delta1 in the anterior PSM. The presence of somites in the basal chordate amphioxus (Branchiostoma floridae) indicates that the last common ancestor of chordates also had somites. However, it remains unclear how the genetic mechanisms underlying somitogenesis in vertebrates evolved from those in ancestral chordates. RESULTS: We demonstrate that during the gastrula stages of amphioxus embryos, BfFringe expression in the endoderm of the archenteron is detected ventrally to the ventral limit of BfDelta expression in the presumptive rostral somites along the dorsal/ventral (D/V) body axis. Suppression of Notch signalling by DAPT (a γ-secretase inhibitor that indirectly inhibits Notch) treatment from the late blastula stage reduced late gastrula stage expression of BfFringe in the endodermal archenteron and somite markers BfDelta and BfHairy-b in the mesodermal archenteron. Later in development, somites in the DAPT-treated embryo did not separate completely from the dorsal roof of the archenteron. In addition, clear segmental boundaries between somites were not detected in DAPT-treated amphioxus embryos at the larva stage. Similarly, in vertebrates, DAPT treatment from the late blastula stage in Xenopus (Xenopus laevis) embryos resulted in disruption of somite XlDelta-2 expression at the late gastrula stage. At the tail bud stage, the segmental expression of XlMyoD in myotomes was diminished. CONCLUSIONS: We propose that Notch signalling and the Fringe/Delta cassette for dorso-ventral boundary formation in the archenteron that separates somites from the gut in an amphioxus-like ancestral chordate were co-opted for anteroposterior segmental boundary formation in the vertebrate anterior PSM during evolution.
PubMed ID: 26613046
PMC ID: PMC4660845
Article link: Zoological Lett
Genes referenced: dlc dll1 gsc lfng lims1 myod1 notch1 tbx1 tbxt
Article Images: [+] show captions
|Fig. 1 Development of amphioxus rostral somites. a At the early neurula stage, the dorsal roof of the anterior archenteron begins to expand dorsolateral to the ectoderm. b At the mid-neurula stage, the rostral somites swell and form a U-shape that remains part of the dorsal roof of archenteron. c At the late neurula stage, the somites pinch off from the archenteron roof. nc notochord|
|Fig. 2 Notch signalling controls the pinching off process of the rostral somites. a–b BfFringe expressed in the anterior endoderm (ventral part of the archenteron) and BfDelta was expressed in the presumptive rostral somites at the late gastrula stage. Blastopore views with the dorsal side up. The white dotted circle indicates the archenteron. s somite, ar archenteron. Scale bar, 50 μm. c–f Effect of 100 μM DAPT treatment on BfFringe (n = 7, 100 %) or BfDelta (n = 8, 100 %). Anterior views with the dorsal side up. g In the DMSO-treated control larval embryo, the segmental boundary between the somite and the dorsal gut roof was clear (n = 1). h In larval embryos treated with DAPT, the boundary was unclear and ectopic expression of Muscle-actin was observed (n = 1). Transverse sections with the dorsal side up. White arrowheads indicate somite and gut fusion locations. The white dotted circle indicates the border between the gut and somites|
|Fig. 3 DAPT treatment affects somitogenesis in amphioxus embryos. Effect of treatment with 100 μM DAPT at the late blastula stage on mesodermal gene expression (a, b Delta; c, d, Hairy-b; e, f, Gsc; g, h, Brachyury; n = 10 each, 100 %) at the gastrula stage. Dorsal views with the anterior side up. s, somite. Scale bar, 50 μm (a)|
|Fig. 4 Effect of DAPT treatment on BfMuscle-actin expression. a, c, e DMSO-treated control embryo (n = 5, 100 %). Anterior to the left. Dorsal view. nt, notochord; s, somite; m, myotome. CellMask (Red) labelled the plasma membrane. BfMuscle-actin (Green) was stained by fluorescence in situ hybridization. The white dotted circle indicates a myotome. The white line indicates the segmental border between myotomes. b, d, f DAPTtreated embryo (n = 5, 100 %). Anterior to the left. Dorsal view. The white arrowhead indicates a possible segmental border between myotomes. Scale bars, 20 μm|
|Fig. 5 Filament formation of actin is suppressed by DAPT treatment. a DMSO-treated control embryos display filamentous actin in the rostral somites (n = 12, 100 %). b F-actin staining in the somites. Treatment with 100 μM DAPT from the late gastrula stage onwards (n = 10, 100 %). The images were taken using an LSM 710 confocal microscope (Zeiss). c Dorsal view of DMSO-treated control embryos. d Dorsal view of DAPT-treated embryos. s somite, nt notochord, n neural tube, g, gut. Nuclei were labelled in blue and the plasma membrane was labelled in red|
|Fig. 6 Inhibition of Notch signalling results in loss of segment formation in Xenopus embryos. a–f DAPT treatment did not affect the expression pattern of Gsc (a n = 42, 93 %; b n = 38, 87 %) and Brachyury (c n = 20, 95 %; d n = 14, 94 %), whereas it disrupted the paired-stripe expression pattern of Delta-2 (e n = 33, 100 %; f n = 47, 51 %). a–b Anterior views with dorsal side up. c–f Dorsal views with the anterior side up. g–h DAPT treatment resulted in loss of segmental expression of Mrf1 in amphioxus (g n = 12, 100 %; h n = 11, 100 %. Dorsal views with anterior to the left), MyoD in Xenopus (i n = 23, 100 %; j n = 15, 93 %), and Tbx1 (k n = 23, 100 %; l n = 13, 54 %). i, j The lower left panel displays the magnification of the somite expression of MyoD|
|Fig. 7 A new evolutionary scenario of the chordate somite formation. a In amphioxus, mesoderm/endoderm boundary formation is organized by Fringe/Delta cassette under Notch signalling. b The Notch signalling-dependent mechanism of segmental boundary formation in (a) was co-opted to the future segments in the anterior PSM of amniotes embryos. The mesoderm is presented in green. The endoderm is in light blue. The dotted line indicates a segmental border|