Overlapping expression of zebrafish T-brain-1 and eomesodermin during forebrain development

Abstract

T-box transcription factors are important determinants of embryonic cell fate and behaviour. Two T-box genes are expressed in the developing telencephalon of several vertebrate species, including amphibia, birds and mammals. Here we report the cloning of zebrafish T-brain-1 (tbr1) and eomesodermin (eom). As a prelude to genetic studies of neuro-ectodermal fate determination we studied their expression pattern during embryogenesis and early larval development. Eom is expressed in the presumptive telencephalon from around the 4-5 somite stage in bilaterally symmetric groups of cells; the number of positive cells increases dramatically with time and encompasses the entire dorsal telencephalon by the 22 somite stage. Tbr1 is expressed from the 18 somite stage in a subset of eom-expressing cells. By 24 hpf eom and tbr1 are expressed in largely overlapping domains in the dorsal telencephalon, tbr1 is expressed in postmitotic cells whereas eomes is also expressed in proliferative ventricular zone cells. Both genes are also found in a small domain of the diencephalon bordering the telencephalon. A detailed analysis of the expression of tbr1 and eom in the brain of 4 day old larvae shows that the two T-box genes are differentially expressed in various cell populations of the developing brain.

Amphi‐Eomes/Tbr1: an amphioxus cognate of vertebrate Eomesodermin and T‐Brain1 genes whose expression reveals evolutionarily distinct domain in amphioxus development

“…In vertebrates, T-Brain subfamily genes can be subdivided into a Tbr1 class and an Eomesodermin (Tbr2) class. While vertebrate Tbr1 genes are expressed in neuroectodermal tissues only, the expression of Eomesodermin (Tbr2) is observed in developing mesodermal tissue as well as neuroectodermal region (e.g., Mione et al, 2001). Assuming that the vertebrate Tbr1-related genes and Eomesodermin (Tbr2)-related genes appeared only after amphioxus diverged from the lineage leading to the vertebrates, Tbr1 lost its mesodermal expression domain secondarily, whereas the role in mesodermal development was maintained by Eomesodermin (Tbr2).…”

“…Since the expression of T-Brain genes was also observed in vertebrate olfactory placode (Ryan et al, '98;Mione et al, 2001), it is necessary to examine whether the Amphi-Eomes/Tbr1 expression in the pre-oral pit is comparable to that in the vertebrate olfactory placode. The ectodermal placodes of vertebrates each have different ontogenetical processes (for review, see Baker and Bronner-Fraser, 2001), and it might be therefore true in the phylogeny (for review, see Begbie and Graham, 2001).…”

“…Recently, a novel T-box containing gene called T-Brain has been isolated from vertebrates, and the T-Brain subfamily genes are mainly expressed in the developing telencephalon of zebrafish (Yonei-Tamura et al, '99;Mione et al, 2001), frogs (Ryan et al,'98), chicks (Bulfone et al, '99;Puelles et al, 2000), mice (Bulfone et al,'95,'98), and primates (Donoghue and Rakic,'99). In addition to the expression in the forebrain, expression in the olfactory placode and its derivatives was also described in early development of zebrafish (Mione et al, 2001) and Xenopus (Ryan et al,'98).…”

“…In addition to the expression in the forebrain, expression in the olfactory placode and its derivatives was also described in early development of zebrafish (Mione et al, 2001) and Xenopus (Ryan et al,'98). The conserved expression of T-Brain genes in the vertebrate telencephalon implies that T-Brain has a crucial role in its development.…”

An interacting network of T-box genes directs gene expression and fate in the zebrafish mesoderm

“…These findings implicate factors that modify T-box gene function so that individual T-box genes carry out different functions in different regions of their expression domain (12)(13)(14)(15). Because multiple T-box genes are expressed in overlapping patterns in many developmental fields (7,(16)(17)(18)(19), we hypothesized that T-box gene interactions contribute to regionalization of T-box gene function.…”