In embryos of an invertebrate chordate, Ciona intestinalis, two transcription factors, Foxa.a and Zic-r.b, are required for specification of the brain and the notochord, which are derived from distinct cell lineages. In the brain lineage, Foxa.a and Zic-r.b are expressed with no temporal overlap. In the notochord lineage, Foxa.a and Zic-r.b are expressed simultaneously. In the present study, we found that the temporally non-overlapping expression of Foxa.a and Zic-r.b in the brain lineage was regulated by three repressors: Prdm1-r.a (formerly called BZ1), Prdm1-r.b (BZ2) and Hes.a. In morphant embryos of these three repressor genes, Foxa.a expression was not terminated at the normal time, and Zic-r.b was precociously expressed. Consequently, Foxa.a and Zic-r.b were expressed simultaneously, which led to ectopic activation of Brachyury and its downstream pathways for notochord differentiation. Thus, temporal controls by transcriptional repressors are essential for specification of the two distinct fates of brain and notochord by Foxa.a and Zic-r.b. Such a mechanism might enable the repeated use of a limited repertoire of transcription factors in developmental gene regulatory networks.
The authors declare no competing or financial interests.
T.I. and Y.S. designed the study and wrote the paper. T.I. performed the experiments.
This research was supported by a Grant-in-Aid from the Japan Society for the Promotion of Science (JSPS) (15J01153 to T.I.) and the CREST program of the Japan Science and Technology Agency to Y.S.
Supplementary information available online at http://dev.biologists.org/lookup/doi/10.1242/dev.142174.supplemental
- Received July 12, 2016.
- Accepted November 14, 2016.