Effects of Heterochromatin Protein 1 Duplications on Drosophila melanogaster

Perry Morocco and Sarah Elgin

Biology Department, Washington University in St. Louis

 

     Chromatin in eukaryotes can be either euchromatic, or heterochromatic. Euchromatin contains most actively transcribed genes, and is loosely packaged, whereas heterochromatin contains a much lower gene density, does not go through recombination, and is denser. Heterochromatin protein 1 (HP1) is part of the cell machinery that silences genes, and forms heterochromatin. Drosophila melanogaster stocks with an HP1 gene duplication were used to investigate the effects of excessive HP1 on the fly’s development. Interestingly, no larvae with the male phenotype appeared among the larvae with four copies of HP1. Adult male flies with the duplication emerged from the presumably all female larvae though, indicating that males possessed the female phenotype in the larval stage. Considering that larvae are sexed according to the size of their genital disc, with males having a large clear spot on their sides due to their testes, it was deduced that increased levels of HP1 interfered with proper male development. To check the fertility of these flies, both males and females with four copies of HP1 were crossed with wildtype flies, with no offspring emerging from either cross. To discover whether the flies with four copies of HP1 were sterile because of improper development or poor health, males have been dissected to look for testes, with no, or severely reduced testes found. Immunofluorescent staining of chromosomes from individuals with four copies of HP1 and wildtype flies has revealed brighter HP1 antibody staining in the D. melanogaster chromosomes with the duplications. The effects of HP1 on the Y chromosome, which is primarily heterochromatic, were investigated through the use of P element reporter containing white, the gene responsible for the red pigment in fly eyes. If the insert lands in a site that is heterochromatic, or close to heterochromatin, there is a greater chance that the white gene will be silenced, resulting in a variegating phenotype. Thus, the amount of red pigment in fly eyes is a measure of how heterochromatic the insertion site is on the Y. Individuals with three copies of the HP1 gene, flies with a mutation in the HP1 gene, and wildtype flies were crossed with three lines with the P reporter in different locations on the Y. Pigment assays of the F₁ generation were then performed, and showed that individuals with the HP1 gene duplication possess the lowest amount of red pigment, and the flies with the HP1 gene mutation possess the most. As the Y chromosome only contains genes responsible for male gametogenesis, it can be hypothesized that excessive HP1 leads to increased silencing on the Y chromosome, resulting in D. melanogaster males lacking proper testes.