**1. Introduction**

Spermatogenesis is an evolutionary conservative process. In testes, the developing germline cells divide synchronously and with incomplete cytokinesis remaining interconnected by cytoplasmic bridges. Therefore, the germline cells develop as a syncytium and only become separated from each other at the end of spermatogenesis during spermatid individualization. In *Drosophila melanogaster*, the stem cell daughter, the gonialblast, undergoes four rounds of synchronous mitotic divisions to produce 16 precursor cells, called spermatogonia [1–3]. The spermatogonia then become spermatocytes and grow 25 times in size [1, 4]. During the growth phase, the majority of the transcripts for sperm differentiation are expressed and potentially stored until their protein activity is required [5–7]. Many of the transcribed genes are important for meiosis and post meiotic stages [6] or as are suggested to be reserved up to sperm maturing and delivered into the egg at fertilization [8, 9]. After the growth phase, the spermatocytes divide twice by meiosis and then differentiate into 64 haploid spermatid interconnected in a syncytium. At sperm individualization, most unneeded products are pushed to the end of the spermatozoon and an

actin structure, termed the investment cone, forms a new membrane [10, 11]. Each cell becomes independent and turns into a mobile spermatozoon. Development of a mature spermatozoon with the change of histones on protamines in nucleus, the building of an acrosome and a tail, the forming of cellular membrane, occurs without transcription owing to translation of the long-lived mRNAs. Such mRNAs are keeping as a part of the RNP-complexes localized in the different cellular compartments. The testis-specific isoform of the Dm NXF1 (SBR) protein can be an important component of these RNP-complexes.
