**3. Allele-specific effects of the** *sbr* **gene including the male infertility**

Mutations of the house-keeping genes are characterized by a broad range of pleiotropic effects. It is no surprise that mutations of the *Dm nxf1* gene lead to both male and female sterility and increase the frequency of the development malformations (FlyBase [35]). That may be the result of disruption of the NXF1 general functions. Allele-specific effects of mutations in the *Dm nxf1* gene suggest that its products may have specialized functions. The organ-specific products of *sbr* can provide such specialization [22–24]. In addition, the source of specialized functions may be the presence of functionally significant sequences in the SBR protein, responsible for interaction with the certain partners. As a result, SBR may be involved in other processes than the nuclear-cytoplasmic export of mRNAs [36, 37]. A mutant product that disrupts the system of macromolecular interactions can have a dominant negative effect, in this case heterozygosity at the null allele will be better than the presence of a mutant protein along with the normal product [38]. Among the mutant alleles of the *sbr (Dm nxf1)* gene with the dominant effects are the formation of three-pole spindles during the first meiotic division in *sbr<sup>5</sup> /+* females [36] and sterility of *sbr<sup>12</sup>/ Dp (1;Y)y + v +* males. *Dp (1;Y)y + v +* is the duplication of the segment of the X chromosome with the *sbr+*, y + and v + alleles, translocated on the Y chromosome. The duplication compensates a lethal effect of the different alleles of the *sbr* gene localized on X-chromosome [39]. *sbr10* is the thermosensitive allele with a block of the heat shock protein synthesis under heat shock (HS) [40]. It is a recessive feature of the *sbr10* allele [38]. Adult males carrying the *sbr10* allele are thermosensitive and die from 37°C, 1 h [8]. HS increased the number of abnormalities, of not only paternal but also maternal sex chromosome sets, in the offspring of the *sbr10* males. Meiocytes were the thermosensitive stage for this unexpected effect [8].

It was interesting to know what feature is characterized the SBR protein distribution in the future gonads of *D. melanogaster* males on the pupal stage. On a pre-pupa stage of development, the future gonad in *D. melanogaster* males is spherical with two poles with dividing cells (**Figure 3**). One pole produces germ-line cells and

*Animal Models in Medicine and Biology*

**154**

**Figure 4.**

**Figure 3.**

*Spermatids at the elongation or individualization stages in* D. melanogaster *males (Alexa Fluor 546). Spermatids at the elongation stage: the SBR protein moves onto one side of the spermatid nuclear. The SBR protein leaves the condensed nuclei and is translocated in large granules into the formed spermatozoon tails at* 

*Prepupal testis immunofluorescence using anti-SBR (Alexa 488 goat anti-mouse – A, B) and DAPI (A′, B′). A. Most part of the future gonad is represented of growing spermatocytes, primary spermatocytes, and meiocytes. Initially, the shape of the nucleus is round (A, thin arrow); then the shape becomes irregular (A, short arrow). B. Cyst of 16 meiocytes with the cytoplasmic localization of the SBR protein. Bivalents* 

*partially condensed (arrows). Asterisk denotes the stem cells zone. Scale bar, 75 μm.*

*the final stage of spermatid elongation before individualization. 63× magnification.*

another—somatic cells of a future gonad. Both poles are quite poor SBR protein. It is as a rule characteristic for SBR in a zone of dividing cells in different tissues. Most part of a future gonad is represented by cysts with 16 spermatocytes. Cysts from 16 meiocytes are outstanding by the highest content of the cytoplasmic SBR protein (**Figure 3B**).

As the growing spermatocytes are characterized by the high level of transcriptional activity, the long-lived RNAs seem to be the most appropriate candidate for the role of factors that, along with the spermatozoon, enter an ovum during fertilization and can participate in chromosome disjunction. Due to the process of translation, a small number of these molecules are sufficient for the regulation of chromosome segregation. This is particularly important during the first hours of embryonal development of *D. melanogaster*, as it takes place in the absence of transcriptional activity of the zygotic genome [41]. It has been demonstrated that along with the spermatozoon, paternal RNAs that may also play a role in fertilization, as well as zygotic and embryonic development, also enter the oocyte [42, 43]. High level of the SBR protein in cytoplasm shows that this protein may play a crucial role in biogenesis of the long-lived RNAs in *D. melanogaster*. The significance of SBR in forming the meiotic spindle in female *D. melanogaster* [36] and for the mitotic divisions in early embryos [44] leave unknown the molecular partners of the SBR protein involved in these processes.

One more target of *sbr<sup>12</sup>* mutant alleles is the sperm flagellum [26, 45, 46]. The sperm axoneme as a main component of a flagellum defining mobility of a spermatozoon enters the ovum during fertilization [47, 48]. Axoneme is a derivative of a spermatid centroiole, which becomes the basis for the paternal centrosome formation, providing fusion of pronuclei and division of embryonic nuclei [49].

## **4. The centrosome for male meiosis and building of the axoneme**

The centrosome plays a special role in spermatogenesis. It determines the polarity of the stem cells, maintaining contact with the hub and enabling the asymmetric division of stem cells [50]. Male spermatocytes of the wild type contain two centrosomes each. Each centrosome has two orthogonal centrioles that are 10 times larger than those in other cells [51]. Thus, during meiosis I, each of two centrosomes contains a pair of duplicated centrioles.

No centriole duplication occurs before meiosis II. Before chromatid segregation in secondary spermatocytes, each pair of centrioles divides into two single centrioles. Prior to chromatid segregation during meiosis II, one of the centrioles migrates to the opposite pole of a cell. As a result, each spermatozoon inherits only one centriole, which becomes the basal body forming the axoneme [1, 52]. Paternal centrosome participates in the formation of astral microtubules for moving male and female pronuclei toward each other [49]. The role of centrosome RNAs in the asymmetry distribution of cytoplasmic determinants among daughter cells [53] makes plausible the hypothesis that centrosome may be a carrier of the paternal factors, affecting the embryo development.

The axoneme growth during sperm maturation needs translation of the longlived mRNAs coding the components of axonemal complex. Morphological defects of axoneme are a characteristic dominant manifestation of *sbr12* mutant allele (**Figure 5**) [26, 27].

Translational control is crucial for morphogenical events that take place in the absence of transcription during spermiogenesis [26, 54]. The transcripts encoding proteins required for post-meiotic processes including spermiogenesis are almost all

**157**

cellular membrane.

**Figure 5.**

**feature of male generative cells**

*Spermatogenesis in* Drosophila melanogaster: *Key Features and the Role of the NXF1…*

abundantly transcribed in spermatocytes [5]. In *D. melanogaster*, elongation of the flagellar axoneme does not require the ubiquitous process of intraflagellar transport [55, 56]. The axoneme growth and spermatid elongation can be carried out at the expense of translation of mRNAs encoding tubulin subunits and other axonemal proteins within the ciliary pocket [25]. The genetic analysis of male fertility has identified numerous genes involved in spermiogenesis control [25, 57]. Spermatid individualization process depends on genes involved in RNAs metabolism [58]. In *sbr12/Dp (1;Y)y + v +* males, the morphology of mitochondrial derivatives and cytokinesis are defective in the elongated spermatids (**Figure 5**). Mitochondria morphogenesis depends on translation efficiency [54], also as a creation of new

*(B) Morphological defects at the spermatid elongation stage in* D. melanogaster sbr12/Dp (1; Y) y + v + *males. Spermatids are often not subdivided into individual cells, and there are anomalies in axoneme structure (long arrow) and in mitochondrial derivatives (short arrow) (published in Mamon et al. [27]). Scale bar, 200 nm.*

*Electron microscopy images of a cross section of 64 cell cysts. (A) Control (males of* Oregon R*).* 

**5. Incomplete cytokinesis providing a cellular communication as a** 

are the main components supporting of equality of each cell in the syncytium.

Syncytial development during the formation of male generative cells is conservative and is characteristic for different animals [59, 60]. It is assumed that the link between the cells in the syncytium is important primarily for synchronizing differentiation processes [59]. The cytoskeleton and the molecules providing a cellular communication

The cytokinesis of spermatogenic cells is characterized by the formation of ring canals linking the cells that have undergone meiosis or mitosis [61]. During cell division, a contractile ring, made of actin and myosin II filaments, assembles as a result of the interaction between the plus ends of the microtubules and the cell cortex [62]. The contractile ring is double-sided. The microtubules whose minus ends directed toward centrosome on one pole of the dividing cell bind to one side of the contractile ring by their plus ends. The microtubules that have their minus ends turned toward centrosome on the other pole of the dividing cell bind to the opposite side of the contractile ring. Thus, the ring attached equatorially to the membrane of the dividing cell forms an actin-myosin cleavage furrow [60]. The ingression of the cleavage furrow is accompanied by the growth of the daughter cell membranes.

*DOI: http://dx.doi.org/10.5772/intechopen.90917*

*Spermatogenesis in* Drosophila melanogaster: *Key Features and the Role of the NXF1… DOI: http://dx.doi.org/10.5772/intechopen.90917*

#### **Figure 5.**

*Animal Models in Medicine and Biology*

protein involved in these processes.

contains a pair of duplicated centrioles.

factors, affecting the embryo development.

(**Figure 3B**).

another—somatic cells of a future gonad. Both poles are quite poor SBR protein. It is as a rule characteristic for SBR in a zone of dividing cells in different tissues. Most part of a future gonad is represented by cysts with 16 spermatocytes. Cysts from 16 meiocytes are outstanding by the highest content of the cytoplasmic SBR protein

As the growing spermatocytes are characterized by the high level of transcriptional activity, the long-lived RNAs seem to be the most appropriate candidate for the role of factors that, along with the spermatozoon, enter an ovum during fertilization and can participate in chromosome disjunction. Due to the process of translation, a small number of these molecules are sufficient for the regulation of chromosome segregation. This is particularly important during the first hours of embryonal development of *D. melanogaster*, as it takes place in the absence of transcriptional activity of the zygotic genome [41]. It has been demonstrated that along with the spermatozoon, paternal RNAs that may also play a role in fertilization, as well as zygotic and embryonic development, also enter the oocyte [42, 43]. High level of the SBR protein in cytoplasm shows that this protein may play a crucial role in biogenesis of the long-lived RNAs in *D. melanogaster*. The significance of SBR in forming the meiotic spindle in female *D. melanogaster* [36] and for the mitotic divisions in early embryos [44] leave unknown the molecular partners of the SBR

One more target of *sbr<sup>12</sup>* mutant alleles is the sperm flagellum [26, 45, 46]. The sperm axoneme as a main component of a flagellum defining mobility of a spermatozoon enters the ovum during fertilization [47, 48]. Axoneme is a derivative of a spermatid centroiole, which becomes the basis for the paternal centrosome forma-

tion, providing fusion of pronuclei and division of embryonic nuclei [49].

**4. The centrosome for male meiosis and building of the axoneme**

The centrosome plays a special role in spermatogenesis. It determines the polarity of the stem cells, maintaining contact with the hub and enabling the asymmetric division of stem cells [50]. Male spermatocytes of the wild type contain two centrosomes each. Each centrosome has two orthogonal centrioles that are 10 times larger than those in other cells [51]. Thus, during meiosis I, each of two centrosomes

No centriole duplication occurs before meiosis II. Before chromatid segregation in secondary spermatocytes, each pair of centrioles divides into two single centrioles. Prior to chromatid segregation during meiosis II, one of the centrioles migrates to the opposite pole of a cell. As a result, each spermatozoon inherits only one centriole, which becomes the basal body forming the axoneme [1, 52]. Paternal centrosome participates in the formation of astral microtubules for moving male and female pronuclei toward each other [49]. The role of centrosome RNAs in the asymmetry distribution of cytoplasmic determinants among daughter cells [53] makes plausible the hypothesis that centrosome may be a carrier of the paternal

The axoneme growth during sperm maturation needs translation of the longlived mRNAs coding the components of axonemal complex. Morphological defects of axoneme are a characteristic dominant manifestation of *sbr12* mutant allele

Translational control is crucial for morphogenical events that take place in the absence of transcription during spermiogenesis [26, 54]. The transcripts encoding proteins required for post-meiotic processes including spermiogenesis are almost all

**156**

(**Figure 5**) [26, 27].

*Electron microscopy images of a cross section of 64 cell cysts. (A) Control (males of* Oregon R*). (B) Morphological defects at the spermatid elongation stage in* D. melanogaster sbr12/Dp (1; Y) y + v + *males. Spermatids are often not subdivided into individual cells, and there are anomalies in axoneme structure (long arrow) and in mitochondrial derivatives (short arrow) (published in Mamon et al. [27]). Scale bar, 200 nm.*

abundantly transcribed in spermatocytes [5]. In *D. melanogaster*, elongation of the flagellar axoneme does not require the ubiquitous process of intraflagellar transport [55, 56]. The axoneme growth and spermatid elongation can be carried out at the expense of translation of mRNAs encoding tubulin subunits and other axonemal proteins within the ciliary pocket [25]. The genetic analysis of male fertility has identified numerous genes involved in spermiogenesis control [25, 57]. Spermatid individualization process depends on genes involved in RNAs metabolism [58].

In *sbr12/Dp (1;Y)y + v +* males, the morphology of mitochondrial derivatives and cytokinesis are defective in the elongated spermatids (**Figure 5**). Mitochondria morphogenesis depends on translation efficiency [54], also as a creation of new cellular membrane.
