Homeobox Genes DataBase

Drosophila Genes in Development: Gap and Terminal Genes: Torso Pathway

tailless

Function

tailless (tll) is a terminal gene that works and affects development at the terminal (anterior and posterior) ends of the embryo. Without tll, there would be neither brain nor tail development.
In tailless mutants, these gaps are found in the head and the posterior. Therefore, tailless controls terminal genes that result in normal development of head and posterior.
Loss of tll function results in the absence of all protocerebral neuroblasts and loss of all four coherent domains of Fas II expression in the protocerebrum. Also missing is the optic lobe (Younossi-Hartenstein A., 1997).
tll is activated by the torso pathway.
In the anterior domain, tailless represses fushi tarazu, hunchback and deformed, as well ashedgehog, and helps to establish the borders of expression of head gap genes, like orthodenticle.
In the posterior, Tailless acts on the number of genes, among them are the gap and pair-rule genes and POU domain genes pdm1 and pdm2. Tailless also appears to activate T-related gene, a Drosophila Brachyury homolog.

Protein

Two transcription factors, Tailless and Huckebein, constitute the regulatory proteins that are the final products of the terminal system.
The Tailless repressor has an N-terminal DNA binding domain and a C-terminal hormone binding domain. It is identified as a steroid receptor superfamily protein (Pignoni F., 1990). Tailless is an orphan nuclear receptor, meaning that although it possesses a hormone binding domain, no hormone has been identified that can bind to this domain. Although the ligand binding domain is conserved, the effects of mutation are felt most strongly in the DNA binding domain. The ligand binding domain of Tailless and mammalian homolog contains a conserved putative dimerization domain (consisting of seven heptad repeats). There is some evidence that the protein binds DNA as a dimer (Diaz R. J., 1996).

Subcellular location

Nuclear

Expression Pattern

Early tailless expression (blastoderm stage) covers the anlage of the entire brain. Beginning approximately with the onset of gastrulation, an anterior-dorsal region with a high expression level (called HL domain) can be distinguished from a posterior-ventral domain expressing tll at a somewhat lower level. The HL domain coincides with part of the central and anterior protocerebral neurectoderm. The low expression level LL domain covers the remaining part of the protocerebral neuroectoderm (Younossi-Hartenstein A., 1997). Tailless mRNA is found in the blastoderm stage. Initial activation of tll transcription is in two mirror image symmetrical caps at the poles of the embryo. This expression resolves into a posterior cap and an anterior-dorsal stripe. tll is also expressed in brain neuroblasts. Staining in the anterior domain (cap or stripe) is generally weaker than staining in the posterior domain (Pignoni F., 1990).

Dorso-lateral view of tailless expression in a cleavage cycle 14 embryo undergoing cellularization. Anterior is to the left, and individual nuclei appear as small dots. This embryo was stained with antibodies to the Tailless protein and then treated with fluorescently labeled second antibodies.