Evolutionarily conserved roles of foxg1a in the developing subpallium of zebrafish embryos.

The vertebrate telencephalic lobes consist of the pallium (dorsal) and subpallium (ventral). The subpallium gives rise to the basal ganglia, encompassing the pallidum and striatum. The development of this region is believed to depend on Foxg1/Foxg1a functions in both mice and zebrafish. This study aims to elucidate the genetic regulatory network controlled by foxg1a in subpallium development using zebrafish as a model. The expression gradient of foxg1a within the developing telencephalon was examined semi-quantitatively in initial investigations. Utilizing the CRISPR/Cas9 technique, we subsequently established a foxg1a mutant line and observed the resultant phenotypes. Morphological assessment revealed that foxg1a mutants exhibit a thin telencephalon together with a misshapen preoptic area (POA). Notably, accumulation of apoptotic cells was identified in this region. In mutants at 24 h postfertilization, the expression of pallium markers expanded ventrally, while that of subpallium markers was markedly suppressed. Concurrently, the expression of fgf8a, vax2, and six3b was shifted ventrally, causing anomalous expression in regions typical of POA formation in wild-type embryos. Consequently, the foxg1a mutation led to expansion of the pallium and disrupted the subpallium and POA. This highlights a pivotal role of foxg1a in directing the dorsoventral patterning of the telencephalon, particularly in subpallium differentiation, mirroring observations in mice. Additionally, reduced expression of neural progenitor maintenance genes was detected in mutants, suggesting the necessity of foxg1a in preserving neural progenitors. Collectively, these findings underscore evolutionarily conserved functions of foxg1 in the development of the subpallium in vertebrate embryos.

Retinoic acid-dependent establishment of positional information in the hindbrain was conserved during vertebrate evolution.

Zebrafish hoxb1b is expressed during epiboly in the posterior neural plate, with its anterior boundary at the prospective r4 region providing a positional cue for hindbrain formation. A similar function and expression is known for Hoxa1 in mice, suggesting a shared regulatory mechanism for hindbrain patterning in vertebrate embryos. To understand the evolution of the regulatory mechanisms of key genes in patterning of the central nervous system, we examined how hoxb1b transcription is regulated in zebrafish embryos and compared the regulatory mechanisms between mammals and teleosts that have undergone an additional genome duplication. By promoter analysis, we found that the expression of the reporter gene recapitulated hoxb1b expression when driven in transgenic embryos by a combination of the upstream 8.0-kb DNA and downstream 4.6-kb DNA. Furthermore, reporter expression expanded anteriorly when transgenic embryos were exposed to retinoic acid (RA) or LiCl, or injected with fgf3/8 mRNA, implicating the flanking DNA examined here in the responsiveness of hoxb1b to posteriorizing signals. We further identified at least two functional RA responsive elements in the downstream DNA that were shown to be major regulators of early hoxb1b expression during gastrulation, while the upstream DNA, which harbors repetitive sequences with apparent similarity to the autoregulatory sequence of mouse Hoxb1, contributed only to later hoxb1b expression, during somitogenesis. Possible implications in vertebrate evolution are discussed based on these findings.