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Drosophila melanogaster or the common fruit fly has been used to study many key aspects of eukaryotic genetics throughout modern experiments. They have been used in experiments to help identify the structure of proteins and to help understand many important functions such as development, behavioral patterns, sleeping conditions, and physiological responses (Zhu 2004). Drosophila melanogaster is a tiny fly which is roughly 3 millimeters in length and is visible to the naked eye. They are commonly found on the skin or surface of fruits such as bananas and grapes. This species frequently reproduce and are able to create a new generation in under two weeks time. Each generation has the potential to produce hundreds of flies as their progeny. Drosophila melanogaster are inexpensive and are easy to care for and maintain.
The development of Drosophila melanogaster occurs in four stages egg, larvae, pupae and adult (C.J. 2006). The genome of Drosophila melanogaster is fully sequenced and consists of about 180 million base pairs with an estimated 14,000 genes total. (Zhu 2004) Geneticists use this information and are able to narrow down the chromosomal location of different genes responsible for any particular phenotype, like eye color. Thousand s of diverse mutations in Drosophila have also been identified and mapped, including mutations that affect behavior and learning (Schlenke 2002). At the genetic level, more is identified about Drosophila species than any other multi cellular organism.
The eye of Drosophila melanogaster consists of approximately 800 individual unit eyes which are known as ommatidia. These ommatidia are arranged in a hexagonal lattice pattern to form each eye of the fly. The eye is also composed of eight photoreceptor neurons, four cone cells, two primary pigment cells, secondary and tertiary pigment cells as well as bristle cells (Bonini 1997).
The mutant allele echinus (ec) which was identified by Calvin Bridges in 1918 is a recessive X-chromosome linked allele that affects the eyes of Drosophila melanogaster. (Copeland 2007) This allele causes programmed cell death of the interommatidial cells to be disrupted causing an overall disorganization of the ommatidia formation. It gives the eyes a rough appearance; extra primary pigment cells as well as cone cells can be present in the eyes (Copeland 2005).
Phenotypic characteristics of mutations in the gene
The echinus allele causes visible changes in the eyes of D. melanogaster. A general explanation of the phenotype would be that there is an overall disorganization of the interommatidial cells that cause many orientation defects in the eyes of Drosophila melanogaster giving the eyes a rough appearance. Interommatidal cells are secondary and tertiary pigment cells that surround the ommatidia and pattern the cellular arrangement. The echinus allele implies that there is a precise orientation of ommatidia in eyes of a normal adult that is varied by this mutation. In a normal fly the ommatidia are structures that point precisely in two different directions, dorsally in the dorsal half and then ventrally in the ventral half of the eye (Montrasio 2007). During eye development the larval eye there are clusters of ommatidial behind the morphogenetic furrow that undergo two rotations of 45 degrees each to end up in a final position of 90 degrees from their original arrangement. The rotation is clockwise in the dorsal half and counterclockwise in the ventral half. The rotation of the ommatidia clusters is completed in the late larval stages before they become pupae. Echinus mutants display ommatidia with an extra primary cell or cone cell (Montrasio 2007) Eyes of a homozygous with the echinus mutation display rotation defects with around 15% of the ommatidia disorganized near more than a 20 degrees rotation of compared to the location of a normal wild type adult eye. (Montrasio 2007)
Molecular characteristics of the gene and gene product
Mutations in genes affect the normal developmental process. The Echinus allele encodes protein with homology to ubiquitin-specific proteases. Then those proteins cleave ubiquitin-conjugated proteins at the ubiquitin C-terminus resulting in the sorting and cell death of interommatidial cells in the fly eye. Cell death only occurs after rearrangement is complete (Copeland 2007). During eye development, cell death plays a key role in achieving patterned structure of the flyââ‚¬â„¢s eye. Interommatidial cells start off ordered in rows of two to three between the ommatidia in the eyes of the fly. During the later stages of pupae life apoptosis occurs to removed excess interommatidial cells. Apoptosis is a type of cell death implemented by caspases also known as cysteine proteases. The echinus locus encodes multiple splices like two proteins which lack residues that are considered vital for deubiquitination activity. Ubiquitous expression in the echinus eye of that lack the residues vital for ubiquitin protease activity. Most mutations that affect ommatidial rotation display the first strangely rotated clusters at the very first stages of rotation. The rotation defects appear to be even more prominent than in the adult flies. The echinus mutation was thought to have a role in both cell death signaling and cell sorting. However echinus did not have genetic interactions with known death regulators, which supports the idea that echinus functions mainly to regulate cell sorting. The allele is expressed during the pupae development due to the decreased amount of apoptosis in the retinal development of the eyes. Interactions between echinus, enabled, wingless, and expanded have been observed (Bosnet 2008). Not much is known about the mutation echinusââ‚¬â„¢ mechanism of how it works or what proteins it targets to control the lattice organization in the eyes of Drosophila melanogaster (Copeland 2007). More genetics research will be necessary to fully understand the role of the echinus mutation in the eyes of the flies with this mutation.
The fly retina undergoes a complex process of cell sorting and programmed cell death throughout the larval and pupae phases of life. It helps to mold the flyââ‚¬â„¢s tissue into its specific arrangement of cells. During the late pupae stage the ommatidial cells are sorted in a monolayer then becomes more complex as a fully developed adult eye. In the adult eye small changes in the number and location of cells in the eye can be visible.
The mutation echinus has been known to reduce the automatic cell death in the developing Drosophila retina. Giving rise to the idea that this gene has an important role in the process. The Drosophila melanogaster retina is used as a model for studying planned cell death, cell differentiation and cell-cell communication. The identification of echinus and distinguishing its function will lead to a better understanding of the growth of this complex system. The understanding of faulty programmed cell death is an important contributor to the growth and development of diseases such as cancer, a better understanding of the role of echinus in this process may also provide insights into these diseases. The echinus locus encodes multiple splice homologous proteins to proteases. During normal eye development the interommatidial cells for in a side by side arrangement that becomes a lattice formation one cell wide. After the cell sorting process excess cells are then removed from the lattice. The echinus mutation disrupts the cell death process however it does not affect the cell sorting process that occurs before that (Bosnet 2008). The echinus mutation acts as an important element in a the death signaling pathway. It functions mainly to regulate cell sorting but its failure to do so leads to cell continued existence like interommatidial cells that are unable to successfully transmit or receive death signals (Copeland 2007). The echinus allele will contribute to a better understanding of cell to cell communication as well as the process of apoptosis. Drosophila melanogaster has once again been able to demonstrate why it is a model organism for understanding different genetic processes.
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