The Eyes Have It: The Drosophila eye as a model for understanding gene expression regulation

Male a Female Fruit Flies.  Cartoon Via WikiHow

Those tiny little fruit flies that swarm when you pick up a past-its-peak banana at home will almost certainly have red eyes.  In the lab however, fruit fly eyes come in many different varieties.  There are many genes involved in the production of pigments responsible for the eye color in flies and these mutants have been essential to the study of genetic inheritance of traits since the earliest days of fruit fly genetics and Thomas Hunt Morgan. 

Figure ©1965 by Srb et a.;. all text material ©2006 by Steven M. Carr

Mutant flies have been historically named for their phenotypes, how they changed the appearance or behavior of flies.  The white mutation was eventually traced to a gene that is also named white, in honor of its associated mutant phenotype.  This explains the somewhat confusing fact that a functional white gene is necessary for flies to have red eyes (non-functional white gene as found in the white mutant results in white eyes).  This white gene has provided an extremely useful tool for understanding chromatin regulation.  By inserting a functional white gene in different regions of the genome, the ability of the white gene to be expressed can be determined by the appearance of the eyes of the fly.  When the white gene is in a region of the genome with open and accessible chromatin, allowing gene expression, the fly’s eyes are red.  If the white gene is in heterochromatic region, where the gene is inaccessible to transcription factors and the gene expression machinery, the eyes of the fly show variegated red pigment.  Some of the ommatidia, the individual units that make up the compound eye of the fly, will be red, while others will be white, indicating that the gene can be expressed at sometimes but not others due to heterochromatin formation.  This biological representation of gene expression is known as position effect variegation (PEV).

From left to right:  Red eyed, wild-type fly eye; Fly with non-functional white gene; Fly eye exhibiting position effect variegation  (PEV).  

We make use of this system to understand how genome location and DNA sequence can affect how genes are expressed.  The fourth chromosome of Drosophila melanogaster is largely heterochromatic, highly condensed, and yet the genes that found there can be expressed.  What is about those genes that allow them to escape the repressive effects of heterochromatin?  This is the question we are trying to address with the project I will be discussing here.