Allelic Expression of Drosophila Protamines during Spermatogenesis

Abstract

In typical somatic cells, DNA is tightly organized by histones that are necessary for its proper packaging into the nucleus. In sexually-reproducing animals, the haploid product of male meiosis must be further condensed to fit within sperm heads, thus requiring an even greater degree of packaging. This is accomplished in most organisms by replacing histones with protamines, which allows DNA to be compacted into the reduced space. In mammals, protamines are produced after meiosis is complete and are transcribed by the single allele present in the haploid genome that is to be packaged into the sperm head. Here, we present our findings that protamine expression occurs from both alleles in diploid cells, rather than haploid cells, in two species of Drosophila. The differential allelic expression of protamines likely influences the selective pressures that shape their evolution.

Figure 1

D. simulans (a–f) and D. mauritiana (g–l) sperm heads possessing GFP-tagged (a–c, g–i) or RFP-tagged (d–f, j–l) protamine. Sperm containing the protB-GFP transgene only fluoresce green (a, c, g, i) and do not reveal any red light autofluorescence (b, c, h, i). Similarly, sperm containing only the protB-RFP transgene only fluoresce red (e, f, k, l), with no green autofluorescence (d, f, j, l). Images (a–c) and (g–l) were taken at 40x magnification, while images (d–f) were taken at 63x magnification. Bars represent 10 μm.

Figure 2

Transgenic hybrids in D. simulans (a–f) and D. mauritiana (g–l). D. simulans females with the transgene possessing the GFP-tagged protamine mated to D. simulans males with RFP-tagged protamine transgene (a–c), and the reciprocal cross (d–f) fluoresces both red and green (c, f). Similarly, D. mauritiana females with the transgene possessing the GFP-tagged protamine mated to D. mauritiana males with RFP-tagged protamine transgene (g–i), and the reciprocal cross (j–l) also fluoresces both red and green (i, l), thus suggesting that protamine expression occurs during the sperm cell's diploid phase. Images (a–c) were taken at 63x magnification, while images (d–l) were taken at 40x magnification. Bars represent 10 μm.