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Additionally, otd is involved in development of all eight photoreceptors of adult Drosophila in each of their ~700 individual eye units, known as Ommatidium 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 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.
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.
Regulation
Embryo
In the anterior cap region of the syncytium blastoderm, which has non-segregated Cell nucleus 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. Torso and hunchback expression represses otd expression in the anterior most region of the blastoderm while dorsal expression represses expression in the ventral region. Groucho-independent repression of otd by runt, a pair-rule gene, has also been observed in the anterior and posterior regions of the syncytium and cellular blastoderm.
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 Two alleles oc1 and ocγa1, are caused by lesions downstream of the last exon in otd which showed enhancer activity. Another characteristic of the oc phenotype is the abnormal or deleted bristles in the ocellus region, which relates back to the wild type otd gene’s role in mediating anterior patterning. The same oc phenotype is found in loss-of-function epidermal growth factor receptor (EGFR) alleles and RNAi knockdown pointed (pnt) mutants
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