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In Drosophila, male and female gametes begin development when a stem cell divides to produce a cyst precursor. Subsequently, four special divisions give rise to a cluster of 16 interconnected cystocytes that develop into a single egg or 64 sperm. We identified and characterized a gene, bag-of-marbles (bam), that disrupts cyst formation in both sexes. An apparent null mutation causes abnormal cysts to form containing an excess number of cells that cannot differentiate into gametes. bam function resides within a simple 2.2-kb transcription unit encoding a single 442-amino-acid protein that shows similarity to the product of the ovarian tumor gene. The specific expression of bam RNA within female cystoblasts suggested that it might be involved in the specific cell-cycle alterations that occur during cystocyte divisions.

Mammalian gametogenesis is regulated through complex interactions between germ and somatic cells. To investigate the mechanism underlying the differentiation of functional gametes, some genes specifically expressed during gametogenesis have been isolated and characterized. In a search for further examples of such genes, we have isolated from a newborn mouse testis cDNA library, a clone corresponding to mouse inhibin alpha-subunit. Although it is known that the inhibin alpha-subunit molecule is abundantly produced in ovarian follicle and in testicular Sertoli cells, the spatial and temporal patterns of expression of this gene remain to be elucidated. In this study, the patterns of expression of inhibin alpha-subunit mRNA during mouse gametogenesis were examined by RNA blot, cytoplasmic dot and in situ hybridization techniques. In the testis, the concentration of inhibin alpha-subunit mRNA increased from about 16 dpc (days post coitum), peaked at birth and then gradually decreased, paralleling testicular development. Inhibin alpha-subunit mRNA was localized in Sertoli cells of wild type as well as W/Wv testes. In adult testis, mRNA was restricted to the perinuclear cytoplasm of Sertoli cells. Inhibin alpha-subunit mRNA was expressed in follicle cells of adult ovary more abundantly than in adult testis. Analysis of expression during folliculogenesis showed that the accumulation of this mRNA began in preantrum follicles and the level of expression reached a maximum in Graafian follicles.

Mitotic chromosomes of the European water frogs Rana ridibunda and Rana lessonae, the parental species of Rana esculenta, differ significantly in their centromeric regions: when C-banded or when made fluorescent, the centromeres of R. ridibunda (and of ridibunda chromosomes in R. esculenta) are visible as a conspicuous dark granule or as a conspicuous fluorescent spot; the centromeres of R. lessonae (and of the lessonae chromosomes in R. esculenta) are inconspicuous or not fluorescent. Lampbrush chromosomes of these three taxa are described in detail for the first time; those of R. ridibunda and R. lessonae differ significantly in morphostructural characters such as conspicuousness of centromeres and number, form, and location of giant loops as well as in chiasma frequency. Chromosomes of the two parental species can thus be distinguished when present in lampbrush complements of hybrids. Reproduction in both sexes of natural R. esculenta lineages is hemiclonal: only the unrecombined genome of one parental species, usually R. ridibunda, is transmitted to haploid gametes (hybridogenesis). In 18 hybrids from natural populations of Poland, somatic tissues had allodiploid complements with chromosomes from each parental species. In contrast, spermatocytes I of five males and oocytes I of seven of eight females (221 of 222 oocytes) were autodiploid and contained only R. ridibunda chromosomes that formed n bivalents. These 12 hybrids thus were hybridogenetic. A single female hybrid had oocytes I (33 of 34) with genomes of both parental species; they showed various disturbances including tetraploidy, reduced number of chiasmata, and incomplete synapsis resulting in univalents. This individual thus was not hybridogenetic. The irregular lampbrush patterns indicate that such hybrids will have severely reduced fertility and most of their successful gametes will result in allotriploid progeny.

The Drosophila gonadal (gdl) gene is differentially expressed in the male and female germ lines. In males, expression in the gdlM mode results in a 1200-/1500-nucleotide RNA pair, whereas in females, expression in the gdlF mode results in a 1000-/1300-nucleotide RNA pair. Since the two expression modes are a result of alternative promoter usage, the sex-specific transcripts differ at their 5' ends. These sequence differences affect the coding capacity of the gene. A common open reading frame (ORF) of 193 codons (ORF193) is present in all four gdl transcripts; a consequence of the additional sequences at the 5' end of the gdlM transcripts is the presence of an additional ORF of 39 codons (ORF39). Translation of gdlF and gdlM cRNAs in a reticulocyte lysate reveals that these transcripts can serve as monocistronic and bicistronic mRNAs in vitro. An analysis of germ-line transformants harboring gdl-lacZ gene fusions provides information on gdl gene expression during gametogenesis. The fusion genes are transcribed and translated in the germ line; beta-galactosidase activity is detected in premeiotic and postmeiotic spermatogenic stages in males, and in nurse cells and oocytes of developing egg chambers in females. Both gdlM ORFs are used because transformant lines expressing the lacZ gene, fused in frame with either ORF39 or ORF193, are positive for beta-galactosidase activity in the testes. These studies also reveal that separable transcription control elements are responsible for gdl expression in the male and female germ lines.

The averaged incidence of chromosome abnormalities from samples of infertile males has been estimated to be approximately 5%, of which 4% are sex chromosome abnormalities and 1% autosomal abnormalities. Variations in the frequencies among different samples are probably due to ascertainment bias. The autosomal abnormalities consist mainly of balanced translocations which are found with a frequency of nine per thousand. Most translocations are Robertsonian ones and most of them are familial. The reasons why a balanced translocation interferes with the normal meiotic process are discussed. The effects on female fertility of numerical and structural aberrations of the X chromosome are discussed with special attention to the deficiencies of the short and long arm of the X chromosome. Several of these deficiencies are secondary to X/autosome translocations transmitted by the mother of the probands. It is concluded that the function of the abnormal X chromosome rather than the loss of specific segments is probably correlated with the lack of gamete production.

We have analyzed the mechanism of sex determination in the germ line of Drosophila by manipulating three parameters: (1) the ratio of X-chromosomes to sets of autosomes (X:A); (2) the state of activity of the gene Sex-lethal (Sxl), and (3) the sex of the gonadal soma. To this end, animals with a ratio of 2X:2A and 2X:3A were sexually transformed into pseudomales by mutations at the sex-determining genes Sxl (Sex-lethal), tra (transformer), tra-2 (transformer-2), or dsx (double-sex). Animals with the karyotype 2X;3A were also transformed into pseudofemales by the constitutive mutation SxlM1. The sexual phenotype of the gonads and of the germ cells was assessed by phase-contrast microscopy. Confirming the conclusions of Steinmann-Zwicky et al. (Cell 57, 157, 1989), we found that all three parameters affect sex determination in germ cells. In contrast to the soma in which sex determination is completely cell-autonomous, sex determination in the germ line has a non-autonomous component inasmuch as the sex of the soma can influence the sexual pathway of the germ cells. Somatic induction has a clear effect on 2X;2A germ cells that carry a Sxl+ allele. These cells, which form eggs in an ovary, can enter spermatogenesis in testes. Mutations that cause partial loss of function or gain of function of Sxl thwart somatic induction and, independently of the sex of the soma, dictate spermatogenesis or oogenesis, respectively. Somatic induction has a much weaker effect on 2X;3A germ cells. This ratio is essentially a male signal for germ cells which consistently enter spermatogenesis in testes, even when they carry SxlM1. In a female soma, however, SxlM1 enables the 2X;3A germ cells to form almost normal eggs. Our results show that sex determination in the germ line is more complex than in the soma. They provide further evidence that the state of Sxl, the key gene for sex determination and dosage compensation in the soma, also determines the sex of the germ cells, and that, in the germ line, the state of activity of Sxl is regulated not only by the X:A ratio, but also by somatic inductive stimuli.

We have analyzed a line of transgenic mice derived from injection of a cloned human interferon cDNA. This line manifests total male sterility of males carrying the human sequence, while male littermates not harboring the foreign DNA are fertile. All females are fertile. Karyotypes of transgenic animals show 2:12 translocation. The microinjected sequence maps to one of the translocation chromosomes composed of a large portion of chromosome 12 to which has been translocated a segment of chromosome 2. Analysis of the sterile males reveals significant abnormalities of spermatogenesis and faulty chromosome synapsis that involves the translocation chromosomes. These findings show that transfer of foreign DNA into mouse embryos may lead to chromatin breakage and infertility of transgenic animals.