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Abstract:
We have isolated a cDNA for the amphioxus ortholog of Chordin. This cDNA codes for a true Chordin ortholog as verified by domain structure comparisons and phylogenetic analysis. The genomic organisation of the amphioxus Chordin resembles that of the vertebrate Chordins more than the Ciona intestinalis or the Drosophila Chordin. The amphioxus Chordin is expressed in the organizer, the notochord and at the neural plate. This neural expression of Chordin, which cannot be found in vertebrates, is proposed to be ancestral for the chordates. Vertebrates have lost this neural-specific expression of Chordin and only use it to induce neural tissue. In vertebrates maternal beta-catenin is necessary for the induction of Chordin. We have used lithium, which is known to upregulate Wnt/β-catenin signaling, to analyze whether Chordin is regulated through the same pathway in amphioxus but also to define the effect that lithium chloride has on the amphioxus embryonic development. I observed that in lithium treated amphioxus embryos the Chordin expression became reduced and localized at the animal pole in an undifferentiated outer layer. Neural differentiation seemed to become delayed by the treatment. The expression analysis via whole mount in situ expression patterns of a number of organiser genes showed that amphioxus possesses an organizer-equivalent structure during gastrulation. However, it seems this region has more of a permissive than an instructive role because organizer-specific genes are expressed before the organiser domain is actually formed. The effect of lithium on the level of expression of organizer-specific genes and those involved in dorsoventral axis specification is discussed.
