Arginine-rich nucleoprotein transition occurs in the two size classes of spermatozoa of Drosophila subobscura males

The normal male of Drosophila subobscura displays polymegaly, which is the presence of two sizes of spermatozoa in the same testis. It is still unknown whether both kinds of sperm are able to fertilize the egg. An indicator of normal functioning of Drosophila spermatozoa is the replacement of the somatic histones by sperm-specific arginine-rich nucleoproteins during spermiogenesis. The appearance of these arginine-rich nucleoproteins in the two kinds of sperm was investigated using the fluorescent dye sulfoflavine, which stains basic proteins at pH 8. In the spherical nuclei of early spermatids of Drosophila subobscura the somatic histones fluoresced strongly, but fluorescence could not be detected in later stages when the spermatid nuclei were elongating. After elongation, however, the nuclei of both kinds of sperm, long and short, fluoresced brightly again, due to the presence of sperm-specific arginine-rich nucleoproteins. Half of the cysts of both types contained spermatid nuclei with aberrant fluorescent pattern including 5–9% of both cyst types which do not undergo histone transition at all. These results indicate that both sperm types may be functional.