Allelic Expression of Drosophila Protamines during Spermatogenesis

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. 1. Introduction Spermatogenesis is a highly orchestrated process that, when operating properly, results in functional and motile sperm. The maturation of spermatids into fully functional spermatozoa occurs in the final stages of spermatogenesis, known as spermiogenesis. Here, chromatin reorganization and an increased level of compaction are essential for proper packaging of nuclear material into the sperm heads [1, 2]. This packaging is necessary for proper sperm head morphology, sperm motility, protection against DNA damage, and the ability to penetrate an ovum [3–5]. Unlike somatic cells, where histones serve to condense DNA, most organisms use protamines to properly organize DNA into a more highly condensed state within the sperm head [6]. Protamines increase the ability of DNA to be packed more tightly by organizing the DNA in linear, side-by-side arrays, rather than by induced supercoiling, with further stability achieved through protamine cysteine-cysteine residue interactions [6, 7]. In mammals, transcription of protamines occurs in the haploid genome, after meiosis is complete [8]. Histones are first replaced by transition proteins TP1 and TP2, followed by protamines [9]. To date, it is unclear if the haploid expression of protamines occurs only in mammals, or if this allelic expression is consistent across all sexually-reproducing animals. Extensive studies on the genes that encode for protamines have mostly been performed in vertebrates, particularly in mammalian models (reviewed in [10]). With respect to invertebrates, two genes have been identified and characterized in the fruit fly, Drosophila melanogaster: Mst35Ba and Mst35Bb. These genes encode for Drosophila protamine