Orthodenticle

Function

During embryonic Drosophila development, otd is required for the head and ventral midline to develop correctly. In the larval stage, otd is expressed in specific sac-like epithelial structures known as imaginal discs that later give rise to external structures of the head and thorax. Particularly, otd is required for the development of the dorsal region of the adult Drosophila head, that forms from the fusion of two eye-antennal discs. Distribution of the otd protein occurs along a concentration of the imaginal disc primordia for these head structures such that different levels of otd expression are required for differential mediolateral subdomains to be established. In Drosophila, the function of otd as an essential factor in anterior pattern formation has been replaced by Bicoid, another homeodomain protein

Additionally, otd is involved in development of all eight photoreceptors of adult Drosophila in each of their ~700 individual eye units, known as ommatidia and their proximal-distal distribution in the eye. The two types of photoreceptors, outer (OPRs) and inner (IPRs), are distinguished through their function and anatomy such that OPRs have six neurons that respond to dim light conditions and are important for motion detection, while IPRs have two neurons that differentiate colour. OPRs possess light-gathering apical surfaces called rhabdomeres, and otd is required for rhabdomere morphogenesis. Rapid expression of otd in all OPRs and IPRs is found following neuronal cell specification in the late larval stage and persists through photoreceptor differentiation. In later development of pupation through to adulthood, otd activates light detecting rhodopsin proteins Rh3 and Rh5 in IPRs while repressing Rh6 in OPRs.

Regulation

Embryo

In the anterior cap region of the syncytial blastoderm, which has non-segregated nuclei along the periphery of the blastoderm, bicoid in low concentrations is sufficient to activate otd expression through an enhancer located upstream of the otd gene. Later, in the cellular blastoderm, which has all nuclei along the periphery separated in individual cells with their own cell membrane, otd expression is limited to a dorsal and anterior band due to repression of bicoid by torso, hunchback, and dorsal.

Ocelliless

Ocelliless (oc) is a general mutation in the otd gene locus in Drosophila, which results in a loss of ocelli: the three photoreceptor organs located on the anterior region of Drosophila heads which relates back to the wild type otd gene’s role in mediating anterior patterning.

The effect of oc on ocelli can be complemented by otduvi, another hypomorphic allele of the otd gene. otduvi/oc heterozygotes shows no ocellar phenotype or photoreceptor (R cell) phenotype, a phenotype that is related with otduvi allele.

The Ocelliless mutant phenotype is not limited to loss of ocelli only. It has a role in disrupting oogenesis, where the inner and outer layer of the chorion in oc mutant eggs separate from one another leaving space for debris that is not seen in wild type otd, and disrupting reproductive capabilities, where female flies homozygous for the oc mutation are sterile.

References

References

1. (July 1990). "The orthodenticle gene encodes a novel homeo domain protein involved in the development of the Drosophila nervous system and ocellar visual structures". Genes & Development.
2. (April 1998). "Equivalence of the fly orthodenticle gene and the human OTX genes in embryonic brain development of Drosophila". Development.
3. (November 1995). "Pattern formation in Drosophila head development: the role of the orthodenticle homeobox gene". Development.
4. (May 2018). "A feed-forward relay integrates the regulatory activities of Bicoid and Orthodenticle via sequential binding to suboptimal sites". Genes & Development.
5. (November 2012). "Separable transcriptional regulatory domains within Otd control photoreceptor terminal differentiation events". Developmental Biology.
6. (January 1996). "orthodenticle Is Required for Photoreceptor Cell Development in the Drosophila Eye". Developmental Biology.
7. (May 1996). "orthodenticle regulation during embryonic head development in Drosophila". Mechanisms of Development.
8. (September 1998). "Targeting gene e expression to the head: The drosophila orthodenticle gene is a direct target of the bicoid gene". Development.
9. (October 2022). "How one becomes many: Blastoderm cellularization in Drosophila melanogaster". BioEssays.
10. (December 1998). "Pair-rule gene runt restricts orthodenticle expression to the presumptive head of the drosophila embryo". Developmental Genetics.
11. (May 2009). "Wingless and Hedgehog signaling pathways regulate orthodenticle and eyes absent during ocelli development in Drosophila". Developmental Biology.
12. (May 2022). "Development of the ocellar visual system in Drosophila melanogaster". The FEBS Journal.
13. (December 1974). "Oogenesis in the ocelliless mutant of Drosophila melanogaster Meigen (Diptera: Drosophilidae)". International Journal of Insect Morphology and Embryology.