Involvement of Y chromosomal loci in the synthesis of Drosophila hydei sperm proteins

Androcam, a Drosophila calmodulin-related protein, is expressed specifically in the testis and decorates loop kl-3 of the Y chromosome

The Drosophila genome encodes a protein that is 68% identical to Drosophila calmodulin (Cam). We show here that this Cam-related gene is specifically expressed in the germ-line of the testis, leading to the name Androcam (Acam). Early in spermatogenesis Acam accumulates on one of the chromatin loops of the Y chromosome, kl-3. This association with kl-3 may indicate an RNA processing-related role for Acam and/or could reflect an unusual storage/assembly function hypothesized for the Y loops. After meiosis Acam is detectable in developing sperm tail cytoplasm, where at least some of the protein is not tightly associated with tubulin. Late in spermiogenesis, some Acam staining overlaps the periphery of the investment cones, actin-containing structures hypothesized to support the motor function for cytoplasmic stripping of the tail. Acam cannot be detected in mature sperm by immunolocalization, but immunoblotting established that Acam is present in sperm stored in mated females, suggesting epitope masking during final maturation. Proteins more related to Acam than Cam are present in the testes of other Drosophila species and a mammalian species, the mouse.

Association of RB97D, an RRM protein required for male fertility, with a Y chromosome lampbrush loop in Drosophila spermatocytes

The Y chromosome of Drosophila melanogaster, which is required only for male fertility, contains six loci that are essential for spermatogenesis. In primary spermatocytes, three of these loci form large lampbrush loops containing RNA transcripts and associated proteins. The identities and functions of these Y chromosome loop-binding proteins are largely unknown. This report demonstrates that the RB97D protein, which is essential for spermatogenesis, bound to a specific lampbrush loop. RB97D contains two copies of a well-characterized RNA binding domain, the RNA recognition motif, followed by a proline-glutamine rich domain. Immunohistochemical and immunofluorescence experiments showed that in the testis, RB97D was found only in primary spermatocyte nuclei and associated with the C loop from the ks-1 fertility locus in an RNAse-sensitive manner. The anti-RB97D antibodies also bound a single Y chromosome loop in D. hydei, suggesting that the protein and its loop-binding function have been evolutionarily conserved. These results demonstrate that the proteins that bind lampbrush loops can be essential for fertility. Since RB97D was present only premeiotically, its function is likely to be directly related to the metabolism of the C loop transcripts.

Minor-myosin, a novel myosin isoform synthesized preferentially in Drosophila testis is encoded by the muscle myosin heavy chain gene

Searching for structural proteins involved in spermatogenesis of Drosophila, we found a novel myosin isoform in the testis of Drosophila hydei and D. melanogaster. The transcript encoding this isoform, which we called 'minor-myosin', initiates within the intron between exons 12 and 13 of the muscle myosin heavy chain (mMHC) gene. Minor-myosin contains a common myosin tail but no ordinary myosin head domain. Instead, it has a short N-terminal domain which displays similarity with the N-termini of certain myosin light chain proteins. Western blots with male germ line mutants showed that the novel mMHC isoform is synthesized in the male germ cells, mainly postmeiotically. However, minor-myosin is not testis-specific, as it is expressed at a low level in the fly carcasses. The possible functions of the myosin isoform in the male germ line are discussed.

Laminin in the male germ cells of Drosophila

To study genes that may be crucial for the male germ cell development of Drosophila we screened a cDNA expression library with a polyclonal antiserum against testis proteins of Drosophila hydei. We identified a cDNA fragment that exhibited a complete sequence similarity with the cDNA of the laminin B2 chain, an important component of the extracellular matrix. Transcripts of laminin B2 were detected in the RNA of male germ cells with the polymerase chain reaction and by in situ hybridization. We studied the reaction of different polyclonal antibodies including those against a Drosophila laminin B2-lac fusion protein, the entire Drosophila laminin complex, or against the mouse laminin complex and against laminin A and B1 chains with specific structures in developing male germ cells of Drosophila. Antigenic sites against laminin B2 were found in the lampbrush loops in primary spermatocyte nuclei, in nuclei of spermatids, and in heads of spermatozoa. The axonemes of elongating spermatids react with antibodies against the Drosophila laminin B1, B2 and laminin A chains. The possible biological functions of the laminin in the male germ cells of Drosophila are discussed.

Drosophila spermatogenesis as a model system

Spermatogenesis is very similar throughout the animal kingdom and is probably based on very old evolutionarily principles. Drosophila can serve as a suitable model system to understand the underlying processes. The molecular and ultrastructural data obtained for Drosophila germ cell development can be applied to understanding spermatogenesis in other organisms, including humans. Various methods used in studies of Drosophila spermatogenesis are presented together with observations which exemplify this conclusion.

Genetic and cytogenetic analysis of the "Th-Ps" region of the Y chromosome of Drosophila hydei: evidence for dual functions of the lampbrush loop-forming fertility genes?

Two competing hypotheses have been proposed for the function of the Y chromosomal fertility factors in Drosophila, which form giant lampbrush loops during the primary spermatocyte stage. The first hypothesis suggests a conventional coding function, the second proposes an unconventional gene function mediated through protein binding by nascent transcripts. Therefore, we studied the genetics and cytogenetics of the two Y chromosomal fertility genes A and C of Drosophila hydei (which form the lampbrush loops threads and pseudonucleolus) in order to test the validity of these different hypotheses. Both lampbrush loops bind specific proteins, which are recognized by different antisera. Absence of either of the lampbrush loops does not interfere with the synthesis of the antigens but completely prevents the binding of the particular antigen to other lampbrush loops. Absence of the loops also does not interfere with the postmeiotic presence and localization of the particular antigen. Deletion (or inactivation) of either of the lampbrush loops threads or pseudonucleolus causes sterility of the male flies as do other male-sterile alleles of both fertility genes, which do not affect the morphology of the lampbrush loops. The phenotypic effects of these mutations on sperm morphogenesis are identical for all various male-sterile alleles of each of the fertility genes A and C, regardless of whether a particular allele leaves the loop intact, modifies that loop, or deletes (or inactivates) the loop completely. Finally, the isolation of fertile Y chromosomal mutations which modify the morphology of the lampbrush loops demonstrates that it is possible to uncouple loop morphology and genetic function. These findings do not support the hypothesis that the binding of proteins to a lampbrush loop has a substantial impact on spermiogenesis.

Evolution of Y chromosomal lampbrush loop DNA sequences of Drosophila

The evolutionary conservation of Y chromosomal DNA sequences of Drosophila hydei in different species of the genus Drosophila was studied by in situ hybridization and on genomic DNA blots of restriction enzyme digested DNA. We demonstrated that Y specific DNA sequences, which form major parts of lampbrush loops related to the male fertility genes, are only retained in a few closely related species during evolution. Other Y chromosomal DNA sequences, also present in lampbrush loops but with homology to autosomal and X chromosomal locations, were found in distant species. We propose a model for the evolution of the Y chromosomal lampbrush loops.

Isolation of Y-chromosomal repetitive DNA sequences of Drosophila hydei via enrichment of chromosome-specific sequences by heterogeneous hybridization between female and male DNA

Male or female DNA of Drosophila hydei was sheared by sonication, denatured, reannealed to different C0t-values and fractionated by hydroxyapatite. The highly repetitive, moderately repetitive and unique fractions of female DNA were denatured again and coupled via diazotization or cyanogen bromide to macroporous Sephacryl S-500. Enrichment of Y-chromosomal sequences was achieved by cycling each of the different fractions of male DNA under optimized hybridization conditions over a column with a manifold excess of immobilized female DNA of the corresponding complexity. Thereby, Y-chromosomal sequences of D. hydei could be enriched about 100-fold for highly and moderately repetitive DNA and about 10-fold for unique DNA. When a library of male D. hydei DNA was screened with Y-enriched highly repetitive DNA, more than 98% of all hybridizing phages contained inserts of repetitive Y-chromosomal DNA of at least four different sequence families.

Lampbrush loop-specific protein of Drosophila hydei

By immunofluorescence techniques and protein blotting experiments we have shown that an antiserum specifically reacts with a Mr 80,000 protein (the "Ps protein") in the lampbrush loop " pseudonucleolus " in spermatocyte nuclei of Drosophila hydei. Comparative studies of X/Y and X/0 testes indicate that the gene encoding the Ps protein is not located on the Y chromosome but on an autosome or the X chromosome. The Ps protein is tissue specific. It is likely to be a rather conserved protein since the antigenic determinant recognized by the antiserum could be detected in the spermatocyte nuclei of a number of other Drosophila species. For those species with prominent Y chromosomal lampbrush loops, it could be shown that the cross-reaction is, as in D. hydei, associated with a specific Y chromosomal loop.