The Plan

Here's a flowchart illustrating the question we're asking and approach we're taking.

How do genes on the fourth chromosome of Drosophila melanogaster get expressed?

The fourth chromosome of Drosophila melanogaster, also referred to as the dot chromosome and the Muller F element, is tiny compared to the other chromosomes that comprise the Drosophila genome.  Another defining characteristic of the dot chromosome is its heterochromatic nature.  This highly condensed chromosome does not experience crossing-over, also known as genetic recombination, the process by which homologous chromosomes exchange portions of their genetic material during prophase I of meiosis. 

Drosophila Chromosomes.  Can you find the fourth chromosome?  Now you know why its also called the dot chromosome.  Adapted from Berkeley Drosophila Genome Project by G.M. Rubin.  

And yet the ~80 genes on the fourth chromosome are transcribed with both temporal and spatial regulation.  Despite a non-permissive transcriptional environment, fourth chromosome genes are turned on and off in much the same way as euchromatic genes elsewhere in the genome.  How can this be?  We postulate that genes on the fourth chromosome have evolved a mechanism that allows them to be expressed appropriately despite their inhospitable locale.  This is the question we aim to address with the experiments I’ll share here. 

Polytene chromosome preparation with chromosome arms labeled.  Right: Protein localization of HP1, a protein known to be associated with heterochromatic DNA.  Notice the fourth chromosome has a strong HP1 signal across the entirety of the chromosome.  Adapted from James and Elgin. Mol Cell Biol. 1986 Nov; 6(11):3862-72

Thus far, sequence analysis of fourth chromosome genes has not uncovered any obvious DNA elements that would account for the ability of all fourth chromosome genes to be expressed in a heterochromatic domain.  To explore this question, we are taking a gene from the fourth chromosome and putting it in other areas of the fourth to see if its expression pattern is maintained.  Making use of genetic tools in Drosophila we will explore what characteristics of fourth chromosome genes allow them to be expressed appropriately.