期刊
DEVELOPMENT
卷 138, 期 8, 页码 1643-1652出版社
COMPANY BIOLOGISTS LTD
DOI: 10.1242/dev.055426
关键词
Motoneuron; Ascidian; Evolution; Spinal cord; BMP; Shh; Nodal; Midline; Dorsal ventral pattern
资金
- Centre National de la Recherche Scientifique
- Universite Pierre et Marie Curie
- Association Francaise contre les Myopathies [11734]
- Agence Nationale de la Recherche [ANR-09-BLAN-0013-01]
- Agence Nationale de la Recherche (ANR) [ANR-09-BLAN-0013] Funding Source: Agence Nationale de la Recherche (ANR)
Ascidians are members of the vertebrate sister group Urochordata. Their larvae exhibit a chordate body plan, which forms by a highly accelerated embryonic strategy involving a fixed cell lineage and small cell numbers. We report a detailed analysis of the specification of three of the five pairs of motoneurons in the ascidian Ciona intestinalis and show that despite well-conserved gene expression patterns and embryological outcomes compared with vertebrates, key signalling molecules have adopted different roles. We employed a combination of cell ablation and gene manipulation to analyse the function of two signalling molecules with key roles in vertebrate motoneuron specification that are known to be expressed equivalently in ascidians: the inducer Sonic hedgehog, produced ventrally by the notochord and floorplate; and the inhibitory BMP2/4, produced on the lateral/dorsal side of the neural plate. Our surprising conclusion is that neither BMP2/4 signalling nor the ventral cell lineages expressing hedgehog play crucial roles in motoneuron formation in Ciona. Furthermore, BMP2/4 overexpression induced ectopic motoneurons, the opposite of its vertebrate role. We suggest that the specification of motoneurons has been modified during ascidian evolution, such that BMP2/4 has adopted a redundant inductive role rather than a repressive role and Nodal, expressed upstream of BMP2/4 in the dorsal neural tube precursors, acts as a motoneuron inducer during normal development. Thus, our results uncover significant differences in the mechanisms used for motoneuron specification within chordates and also highlight the dangers of interpreting equivalent expression patterns as indicative of conserved function in evo-devo studies.
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