Differential expression pattern of two Brachyury genes in amphioxus embryos

Cephalochordate amphioxus contain two Brachyury genes (AmphiBra1 and AmphiBra2). Using probes from the highly conserved coding regions, a summation of their expression profiles in amphioxus embryos have been investigated by several previous studies. However, their respective expression patterns have not been determined up to date. We here address this issue using both qRT-PCR and in situ hybridization methods (with probes from the divergent untranslated regions). qRT-PCR detected a very low maternal expression for AmphiBra2, but not for AmphiBra1. Zygotic expression of both genes are activated around early gastrula stage and change in a similar pattern at subsequent stages. However, compared to AmphiBra1, the expression level of AmphiBra2 is much higher in all examined stages of embryos; in some extreme cases an over fifty-times difference is observed. In situ hybridization and embryonic sections reveal that while AmphiBra2 is highly expressed in the blastopore, the tail bud and the notochord, AmphiBra1 is weakly transcribed only in the notochord. Our results show that the two Brachyury genes, resulted from a lineage-specific duplication in amphioxus, have evolved different embryonic expression profiles.