BarH1 and BarH2

DEVELOPMENTAL BIOLOGY

Embryonic and Larval

Early expression is found in anterior segments, the labrium, maxilla and procephalic lobes at 5.5 to 6.5 hours into development. Other staining is seen in the mandible and procephalic lobe [Image]. Later, BarH1 and BarH2 are coexpressed in cells of the embryonic central and peripheral nervous systems. Positive cells are found in the procephalic lobe, antenna-maxillary complex, labium, hypopharynx and clypeolabrum [Images]. Expression in the brain is apparent. In each case the Bar proteins are expressed in a subset of neurons. In external sensory organs, their expression is marked in thecogens (glial cells) and neurons late in development (Higashijima, 1992b).

BarH1 and BarH2 are not only specifically coexpressed in the developing eye, but are also functionally required in R1/R6 prephotoreceptors and primary pigment cells in developing ommatidia (see The Drosophila Adult Ommatidium: Illustration and explanation with Quicktime animation). They are also essential for normal lens and pigment cell formation, and for the elimination of excess cells from mature ommatidia (Higashijima, 1992a).

Transient overexpression of BarH1 or BarH2 in the morphogenetic furrow of the developing eye produces a characteristic Bar-like eye malformation. It is suggested that Bar overexpression results in suppression of the anterior progression of the morphogenetic furrow and inhibition of reinitiation of normal ommatidial differentiation (Kojima, 1993).

Mutation of roughex perturbs cell fate determination. Many rux mutant clusters contain multiple boss-expressing cells. In some of these clusters, R8 cells are missing. There is also a reduction in the number of cells expressing bar and Seven-up. This may be due to errors in cell fate determination. Alternatively, the reduced number of cells expressing these markers may reflect cell death. Extensive cell death is seen in rux mutants beginning with the MF and extending to the posterior edge of the disc. In rux mutant discs, neuronal differentiation is delayed by approximately 6 hours of development (Thomas, 1994).

The simplest external sensory organ (es) found in the thorax and abdomen consists of a neuron and a set of two support cells. The glial cell (or thecogen) forms a sheath around the tip of a dendrite, whereas the outer support cells, the trichogen and tormogen, secrete cuticule structures. It is the thecogen cell (a glial type) that specifically expresses BarH1. The es neurons express both BarH1 and BarH2. These cells also produce Prospero and Cut, but not under control of Bar (Higashijima, 1992b).

Effects of Mutation or Deletion

Simultaneous deletion of Bar genes leads to irregularly fused, bulging ommatidia, greatly differing in the number of rhabdomeres. Thus Bar genes are required for proper eye morphogenesis (Higashijima, 1992a and Kojima, 1993).

A new segment polarity gene of Drosophila melanogaster, oroshigane (oro) was identified as a dominant enhancer of Bar (B). The B1 allele of the Bar locus is associated with a tandem duplication of the division 16A of the X chromosome and causes the overexpression of the Bar (B) gene. decapentaplegic expression in the morphogenetic furrow is abolished in the B background, and therefore morphogenetic furrow progression prematurely ceases resulting in the characteristic bar-shapped compound eye. hedgehog expression also fades 8 hours after heat induction of the Bar protein, indicating that hh is another target for inhibition by B. Since the Bar protein is required for R1, R6 and primary pigment cell differentiion behind the furrow, the overexpressed B protein apparently interferes with furrow progression by inhibiting dpp expression in the furrow and hedgehog expression just behind the furrow. Overexpression of the Bar protein has little deleterious effect on differentiation of photoreceptor clusters that have already started to develop behind the morphogenetic furrow. Failure of morphogenetic furrow progression likely triggers programmed cell death in the undifferentiated cells ahead of the morphogenetic furrow (Epps, 1997).

oro is a recessive embryonic lethal, and homozygous oro embryos show variable substitution of denticles for naked cuticle. These patterns are distinctly similar to those of hedgehog and wingless mutant embryos, which indicates that oro functions in determining embryonic segment polarity. oro works downstream of hedgehog but upstream of dpp to enhance the Bar phenotypes. Although dpp expression is reduced in oro heterozygotes, hh expression remains the same as that found in wild-type discs. Evidence that oro function is involved in Hh signal transduction during embryogenesis is provided by its genetic interactions with the segment polarity genes patched and fused. ptcIN is a dominant suppressor of the oro embryonic lethal phenotype, suggesting a close and dose-dependent relationship between oro and ptc in Hh signal transduction. oro function is also required in imaginal development. The oro1 allele significantly reduces decapentaplegic (but not hh) expression in the eye imaginal disc. oro enhances the fused1 wing phenotype in a dominant manner. Based upon the interactions of oro with hh, ptc, and fu, it is proposed that the oro gene plays important roles in Hh signal transduction (Epps, 1997).

In the developing Drosophila eye, BarH1 and BarH2, paired homeobox genes expressed in R1/R6 outer photoreceptors and primary pigment cells, are essential for normal eye morphogenesis. BarH1 was ectopically expressed under the control of the sevenless enhancer (sev-BarH1). The sev enhancer drives gene expression strongly, not only in R7 precursors but also in R3/R5 and cone cell precursors. Evidence is presented that sev-BarH1 causes two types of cone cell transformation: transformation of anterior/posterior cone cells into outer photoreceptors and transformation of equatorial/polar cone cells into primary pigment cells. The ectopic primary pigment cells are partially similar in morphology to cone cells. sev-BarH1 represses the endogenous expression of the rough homeobox gene in R3/R4 photoreceptors, while the BarH2 homeobox gene is activated by sev-BarH1 in an appreciable fraction of extra outer photoreceptors. In primary pigment cells generated by cone cell transformation, the expression of cut, a homeobox gene specific to cone cells, is completely replaced with that of Bar homeobox genes. Extra outer photoreceptor formation is either suppressed or enhanced, respectively, by reducing the activity of Ras/MAPK signaling or by dosage reduction of yan, a negative regulator of the pathway, suggesting interactions between Bar homeobox genes (cell fate determinants) and Ras/MAPK signaling in eye development. It is concluded that cone cell precursors may adopt four different cell fates: an outer photoreceptor fate, a primary pigment cell fate, a cone cell fate, or the fate of disappearance f from ommatidia (X-cell fate). Cone cell precursors appear to be divided into two subgroups with respect to sensitivity to sev-BarH1: either anterior/posterior or equatorial/polar cone cell precursors. sev-BarH1 causes transformation of a fraction of anterior/posterior cone cells into outer photoreceptors partially expressing R1/R6-specific genes and also causes the transformation of a fraction of equatorial/polar cone cells into primary pigment cells; this suggests that BarH1 serves as a determinant of R1/R6 and primary pigment cell fates in normal eye development (Hayashi, 1998).

The progression of the morphogenetic furrow in the developing Drosophila eye is an early metamorphic, ecdysteroid-dependent event. Although Ecdysone receptor-encoded nuclear receptor isoforms are the only known ecdysteroid receptors, it has been shown that the Ecdysone receptor gene is not required for furrow function. DHR78, which encodes another candidate ecdysteroid receptor, is also not required. In contrast, zinc finger-containing isoforms encoded by the early ecdysone response gene Broad-complex regulate furrow progression and photoreceptor specification. br-encoded Broad-complex subfunctions are required for furrow progression and proper R8 specification, and are antagonized by other subfunctions of Broad-complex. There is a switch from Broad complex Z2 to Z1 zinc-finger isoform expression at the furrow that requires Z2 expression and responds to Hedgehog signals. These results suggest that a novel hormone transduction hierarchy involving an uncharacterized receptor operates in the eye disc (Brennan, 2001).

Bar is a dominant mutation causing premature arrest of the furrow, which results in the deep anterior nick in the adult eye. Since Bar has the dominant effect of stopping the furrow early, one might expect loss-of-function mutations at other loci that normally act to promote furrow progression to be genetic enhancers of Bar and loss-of-function mutations in genes that normally antagonize the furrow to act as genetic suppressors of Bar. Thus, genetic interactions between BR-C sub loci and Bar were examined. Mutants defective for different BR-C subfunctions display unexpected heterogeneity in their genetic interactions with Bar, suggesting that the role of the BR-C in the regulation of furrow function might be complex. BR-C has several recessive lethal complementation groups that correspond to mutations that remove the function of all or individual zinc finger-containing isoforms subgroups. npr1 mutations lack all function, whereas rbp, br and 2Bc mutant groups correspond to the loss of Z1-, Z2-, and Z3-containing isoforms, respectively. Both npr1/Bar and br/Bar eyes are significantly smaller than +/Bar, indicating a dominant enhancement of the Bar furrow-stop phenotype, consistent with the earlier reports that the BR-C is required for furrow progression. However, br/Bar eyes are smaller than npr1/Bar, suggesting that the BR-C might encode isoforms that act antagonistically during furrow progression, so that the effect of losing isoforms that positively regulate furrow progression is more severe than losing all isoforms. This idea is supported by the observation that rbp/Bar and 2Bc/Bar eyes are larger than +/Bar, suppressing the phenotype, and possibly representing furrow-antagonistic functions of rbp- or br-encoded BR-C isoforms (Brennan, 2001).

Hemizygous males of all BR-C mutant groups survived through the third instar: the eye discs of these males displayed defects consistent with the genetic interactions with Bar. npr1/Y discs show ommatidial disorganization, and signs of furrow failure, including mature ommatidial clusters at the furrow. br/Y discs show a much more dramatic failure of furrow progression, as well as ommatidial disorganization. rbp/Y and 2Bc/Y discs do not show any defects. While rbp does interact strongly with Bar it shows no eye disc defect alone -- it may be that rbp is a redundant function (Brennan, 2001).

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date revised: 10 July 2019

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