Expression of muscle‐related genes and two MyoD genes during amphioxus notochord development

Abstract

The notochord is one of the diagnostic features of the phylum Chordata. Despite the similarities in the early morphogenetic patterns of the notochords of various chordates, they are strikingly distinct from one another at the histological level. The amphioxus notochord is one example of an evolutionary novelty because it is made up of muscle cells. Our previous expressed sequence tag analysis, targeting messenger RNAs expressed in the adult amphioxus notochord, demonstrated that many muscle-related genes are expressed there. To characterize amphioxus notochord cells and to gain insights into the myogenic program in the notochord, we determined the spatial and temporal expre-ssion patterns of these muscle-related genes during amphioxus development. We found that BbNA1 (notochord actin), Amphi-Trop I (troponin I), Amphi-TPmyosin (tropo-myosin), Amphi-MHC2 (myosin heavy chain), Amphi-nMRLC (notochord-specific myosin regulatory light chain), Amphi-nTitin/MLCK (notochord-specific titin/myosin light chain kinase), Amphi-MLP/CRP3 (muscle LIM protein), and Amphi-nCalponin (notochord-specific calponin) are expres-sed with characteristic patterns in notochord cells, including the central cells, dorsally located cells, and ventrally located cells, suggesting that each notochord cell has a unique molecular architecture that may reflect its function. In addition, we characterized two MyoD genes (Amphi-MyoD1 and Amphi-MyoD2) to gain insight into the genetic circuitry governing the formation of the notochord muscle. One of the MyoD genes (Amphi-MyoD2) is expressed in the central notochord cells, and the coexistence of Amphi-MyoD2 transcripts along with the Amphi-MLP/CRP3 transcripts implies the participation of Amphi-MyoD2 in the myogenic program in the notochord muscle.

Two classic cadherin-related molecules with no cadherin extracellular repeats in the cephalochordate amphioxus: distinct adhesive specificities and possible involvement in the development of multicell-layered structures

“…Three cell types of notochord cells that were denoted as NoD, NoM and NoV cells were observed, consistent with recent work that isolated and examined notochord-specific genes (Suzuki and Satoh, 2000;Urano et al, 2003). The NoD and NoV cells are probably precursors of the Müller's cells, which are found at the dorsal and ventral ends of the amphioxus adult notochord (Conklin, 1932;Ruppert, 1997b;Stach, 1999).…”

“…NoD cells are rather cuboidal in shape and are aligned in a single row just below the ventral midline of the nerve cord (Fig. 8E,G) (Urano et al, 2003). NoV cells are also aligned in a single row on the ventral side of the notochord tissue.…”

“…Three distinct cell types have been described in the early notochord tissue (Conklin, 1932;Stach, 1999;Urano et al, 2003). Here we call these cell types notochord dorsal (NoD), notochord mid (NoM), and notochord ventral (NoV) cells.…”

“…In addition, the patterns of Bb1C and Bb2C localization in the formed notochord are consistent with a previous electron microscopic observation that the Müller's cell precursors are interconnected by adherens junctions (Stach, 1999). Bb1C and Bb2C appear to be required to maintain the multicell-layered organization of the cephalochordate notochord, which somewhat differs from the vertebrate and urochordate notochords in structure and function (Ruppert, 1997a;Ruppert, 1997b;Burighel and Cloney, 1997;Suzuki and Satoh, 2000;Nishino and Satoh, 2001;Urano et al, 2003).…”

Rewiring of an ancestral Tbx1/10-Ebf-Mrf network for pharyngeal muscle specification in distinct embryonic lineages

“…Ebf2 and Ebf3 are also expressed in the somites of chicken embryos, where they might be involved in skeletal development (El-Magd et al, 2013). In Amphioxus, the single homolog of Tbx1 is expressed in the developing branchial arches (Mahadevan et al, 2004), and although neither Ebf nor MyoD expression have been reported in branchial arches of Amphioxus, Tbx1, Ebf and MyoD homologues are all expressed in somitic mesoderm (Mahadevan et al, 2004;Mazet et al, 2004;Schubert et al, 2003;Urano et al, 2003). Taken together, these diverse observations point to deep evolutionary origins for the myogenic functions of Tbx1/10, COE and Mrf orthologs in bilaterians.…”