XXY Females Research Articles

Ribosomal DNA Instability as a Potential Cause of Karyotype Evolution.

Karyotype refers to the configuration of the genome into a set of chromosomes. The karyotype difference between species is expected to impede various biological processes, such as chromosome segregation and meiotic chromosome pairing, potentially contributing to incompatibility. Karyotypes can rapidly change between closely related species and even among populations of the same species. However, the forces driving karyotype evolution are poorly understood. Here we describe a unique karyotype of a Drosophila melanogaster strain isolated from the Seychelles archipelago. This strain has lost the ribosomal DNA (rDNA) locus on the X chromosome. Because the Y chromosome is the only other rDNA-bearing chromosome, all females carry at least one Y chromosome as the source of rDNA. Interestingly, we found that the strain also carries a truncated Y chromosome (YS) that is stably maintained in the population despite its inability to support male fertility. Our modeling and cytological analysis suggest that the Y chromosome has a larger negative impact on female fitness than the YS chromosome. Moreover, we generated an independent strain that lacks X rDNA and has a karyotype of XXY females and XY males. This strain quickly evolved multiple karyotypes: two new truncated Y chromosomes (similar to YS), as well as two independent X chromosome fusions that contain the Y-derived rDNA fragment, eliminating females' dependence on the Y chromosome. Considering that Robertsonian fusions frequently occur at rDNA loci in humans, we propose that rDNA loci instability may be one of driving forces of karyotype evolution.

Drosophila melanogaster Y Chromosome Genes Affect Male Sensitivity to Microbial Infections.

Simple SummaryThis manuscript focuses on the protein-encoding genes of the Drosophila melanogaster Y chromosome and their role in immunity. Since their discovery, these genes have mainly been studied for their role(s) in male fertility, such as spermatogenesis. Two previously published papers have linked the polymorphism within the Y chromosome and immunity. Currently, there is no research article to our knowledge that has studied the effect of the individual genes of the Y chromosome on fly immunity. Here, we aim to address the lack of immunity-related knowledge of Y chromosome genes by determining the effect of many of these genes on male sensitivity to microbial infections. We challenged loss-of-function gene mutants with bacterial and fungal pathogens orally and observed any significant fly survival changes. We discovered several genes affecting male sensitivity to both bacterial and fungal infections. While most of the Y chromosome genes were found to reduce sensitivity, we found one gene increases sensitivity to fungal infection. Because several genes were found to increase male sensitivity to microbial infections, inhibitors for such genes could be introduced in areas where fruit flies are pests. Amid declining bee populations, Diptera pollinators could be protected from fungal infections with Y-gene inhibitors.The genders of Drosophila melanogaster vary in their sensitivities to microbial pathogens. While many of the immunity-related genes are located on the X chromosome, the polymorphisms within the Y chromosome were also shown to affect the immunity of flies. In this study, we investigated the necessity of individual genes on the Y chromosome (Y-genes) for male sensitivity to microbes. We identified several Y-genes whose genetic inactivation either increases or decreases the sensitivity of males to gastrointestinal infections with fungal Saccharomyces cerevisiae and bacterial Serratia liquefaciens. Specifically, the loss of function mutations in fly kl-5 and Ppr-Y Y-genes lead to increased and decreased sensitivity of males to fungal challenge, respectively, compared to female sensitivity. In contrast, mutations in Drosophila Pp1-Y1, kl-5, kl-3, Ppr-Y, CCY, and FDY Y-genes lead to increased sensitivity of males to bacterial infection, compared to females. Moreover, while these Y-genes are necessary, the Y chromosome is not sufficient for the sensitivity of males to microbes, since the sensitivity of XXY females to fungal and bacterial challenges was not different from the sensitivity of wild-type female flies, compared to males. This study assigns a new immunity-related function to numerous Y-genes in D. melanogaster.

The Y chromosome may contribute to sex-specific ageing in Drosophila.

Heterochromatin suppresses repetitive DNA, and a loss of heterochromatin has been observed in aged cells of several species, including humans and Drosophila. Males often contain substantially more heterochromatic DNA than females, due to the presence of a large, repeat-rich Y chromosome, and male flies generally have shorter average life spans than females. Here we show that repetitive DNA becomes de-repressed more rapidly in old male flies relative to females, and repeats on the Y chromosome are disproportionally mis-expressed during aging. This is associated with a loss of heterochromatin at repetitive elements during aging in male flies, and a general loss of repressive chromatin in aged males away from pericentromeric regions and the Y. By generating flies with different sex chromosome karyotypes (XXY females; X0 and XYY males), we show that repeat de-repression and average lifespan is correlated with the number of Y chromosomes. This suggests that sex-specific chromatin differences may contribute to sex-specific aging in flies.

The Drosophila Y Chromosome Affects Heterochromatin Integrity Genome-Wide.

The Drosophila Y chromosome is gene poor and mainly consists of silenced, repetitive DNA. Nonetheless, the Y influences expression of hundreds of genes genome-wide, possibly by sequestering key components of the heterochromatin machinery away from other positions in the genome. To test the influence of the Y chromosome on the genome-wide chromatin landscape, we assayed the genomic distribution of histone modifications associated with gene activation (H3K4me3) or heterochromatin (H3K9me2 and H3K9me3) in fruit flies with varying sex chromosome complements (X0, XY, and XYY males; XX and XXY females). Consistent with the general deficiency of active chromatin modifications on the Y, we find that Y gene dose has little influence on the genomic distribution of H3K4me3. In contrast, both the presence and the number of Y chromosomes strongly influence genome-wide enrichment patterns of repressive chromatin modifications. Highly repetitive regions such as the pericentromeres, the dot, and the Y chromosome (if present) are enriched for heterochromatic modifications in wildtype males and females, and even more strongly in X0 flies. In contrast, the additional Y chromosome in XYY males and XXY females diminishes the heterochromatic signal in these normally silenced, repeat-rich regions, which is accompanied by an increase in expression of Y-linked repeats. We find hundreds of genes that are expressed differentially between individuals with aberrant sex chromosome karyotypes, many of which also show sex-biased expression in wildtype Drosophila. Thus, Y chromosomes influence heterochromatin integrity genome-wide, and differences in the chromatin landscape of males and females may also contribute to sex-biased gene expression and sexual dimorphisms.

Behavior of aberrant chromosome configurations in Drosophila melanogaster female meiosis I.

One essential role of the first meiotic division is to reduce chromosome number by half. Although this is normally accomplished by segregating homologous chromosomes from each other, it is possible for a genome to have one or more chromosomes that lack a homolog (such as compound chromosomes), or have chromosomes with multiple potential homologs (such as in XXY females). These configurations complete meiosis but engage in unusual segregation patterns. In Drosophila melanogaster females carrying two compound chromosomes, the compounds can accurately segregate from each other, a process known as heterologous segregation. Similarly, in XXY females, when the X chromosomes fail to cross over, they often undergo secondary nondisjunction, where both Xs segregate away from the Y. Although both of these processes have been known for decades, the orientation mechanisms involved are poorly understood. Taking advantage of the recent discovery of chromosome congression in female meiosis I, we have examined a number of different aberrant chromosome configurations. We show that these genotypes complete congression normally, with their chromosomes bioriented at metaphase I arrest at the same rates that they segregate, indicating that orientation must be established during prometaphase I before congression. We also show that monovalent chromosomes can move out on the prometaphase I spindle, but the dot 4 chromosomes appear required for this movement. Finally, we show that, similar to achiasmate chromosomes, heterologous chromosomes can be connected by chromatin threads, suggesting a mechanism for how heterochromatic homology establishes these unusual biorientation patterns.

Natural variation of the Y chromosome suppresses sex ratio distortion and modulates testis-specific gene expression in Drosophila simulans

X-linked sex-ratio distorters that disrupt spermatogenesis can cause a deficiency in functional Y-bearing sperm and a female-biased sex ratio. Y-linked modifiers that restore a normal sex ratio might be abundant and favored when a X-linked distorter is present. Here we investigated natural variation of Y-linked suppressors of sex-ratio in the Winters systems and the ability of these chromosomes to modulate gene expression in Drosophila simulans. Seventy-eight Y chromosomes of worldwide origin were assayed for their resistance to the X-linked sex-ratio distorter gene Dox. Y chromosome diversity caused males to sire ∼63% to ∼98% female progeny. Genome-wide gene expression analysis revealed hundreds of genes differentially expressed between isogenic males with sensitive (high sex ratio) and resistant (low sex ratio) Y chromosomes from the same population. Although the expression of about 75% of all testis-specific genes remained unchanged across Y chromosomes, a subset of post-meiotic genes was upregulated by resistant Y chromosomes. Conversely, a set of accessory gland-specific genes and mitochondrial genes were downregulated in males with resistant Y chromosomes. The D. simulans Y chromosome also modulated gene expression in XXY females in which the Y-linked protein-coding genes are not transcribed. The data suggest that the Y chromosome might exert its regulatory functions through epigenetic mechanisms that do not require the expression of protein-coding genes. The gene network that modulates sex ratio distortion by the Y chromosome is poorly understood, other than that it might include interactions with mitochondria and enriched for genes expressed in post-meiotic stages of spermatogenesis.

Chromosomal Variants in Klinefelter Syndrome

Klinefelter syndrome (KS) describes the phenotype of the most common sex chromosome abnormality in humans and occurs in one of every 600 newborn males. The typical symptoms are a tall stature, narrow shoulders, broad hips, sparse body hair, gynecomastia, small testes, absent spermatogenesis, normal to moderately reduced Leydig cell function, increased secretion of follicle-stimulating hormone, androgen deficiency, and normal to slightly decreased verbal intelligence. Apart from that, amongst others, osteoporosis, varicose veins, thromboembolic disease, or diabetes mellitus are observed. Some of the typical features can be very weakly pronounced so that the affected men often receive the diagnosis only at the adulthood by their infertility. With a frequency of 4%, KS is described to be the most common genetic reason for male infertility. The most widespread karyotype in affected patients is 47,XXY. Apart from that, various other karyotypes have been described, including 46,XX in males, 47,XXY in females, 47,XX,der(Y), 47,X,der(X),Y, or other numeric sex chromosome abnormalities (48,XXXY, 48,XXYY, and 49,XXXXY). The focus of this review was to abstract the different phenotypes, which come about by the various karyotypes and to compare them to those with a ‘normal’ KS karyotype. For that the patients have been divided into 6 different groups: Klinefelter patients with an additional isochromosome Xq, with additional rearrangements on 1 of the 2 X chromosomes or accordingly on the Y chromosome, as well as XX males and true hermaphrodites, 47,XXY females and Klinefelter patients with other numeric sex chromosome abnormalities. In the latter, an almost linear increase in height and developmental delay was observed. Men with an additional isochromosome Xq show infertility and other minor features of ‘normal’ KS but not an increased height. Aside from the infertility, in male patients with other der(X) as well as der(Y) rearrangements and in XXY women no specific phenotype is recognizable amongst others due to the small number of cases. The phenotype of XX males depends on the presence of SRY (sex-determining region Y) and the level of X inactivation at which SRY-negative patients are generally rarely observed.

The effects of number of Y chromosomes on mating behaviour and bristle number in Drosophila melanogaster

The effects of the dosage of Y chromosomes on some metrical characters in Drosophila melanogaster have been investigated. XO and XYY males were found to be considerably inferior than XY males in regard to mating activity, mating capacity and mating competition ability, while XXY females did not differ from XX females in the studied mating characteristics. Similarly, deviation in the Y chromosome number affected sternopleural bristle number significantly in males but not in females as well as carrying with it an increased developmental instability with respect to intraindividual bristle frequencies. The effect of the differences in mating activity among males with different numbers of Y chromosomes is a strong stabilizing selection for the XY/XX sets of sex chromosomes.

Sequence of meiotic events in female Drosophila melanogaster. Temperature-sensitive periods of autosomal crossing-over and secondary nondisjunction of X chromosomes.

In autosomal translocation/inversion bearing XXY females of Drosophila melanogaster the autosomal crossing-over frequency is very sensitive to heat shock in an early meiotic stage which is coincidental with premeiotic DNA synthesis. The increase in autosomal crossing-over caused an increase in X chromosome nondisjunction in the same oocytes. Following this stage a heat sensitive period more directly affecting nondisjunction occurs. At the end of this period there is a minor heat sensitive stage of crossing-over. It is suggested that the sequence of meiotic events is as follows: 1) Determination of preconditions of crossing-over, 2) non-homologous and homologous chromosome pairing, 3) final determination of crossing-over.

Genetics of inbred Drosophila melanogaster

In two inbred lines of Drosophila melanogaster, marker genes had been induced (y, wa, ex, v and f in Oregon, code “2”; and w in Karsnäs, code “K”). As well as the two original lines, two derived lines with mutual exchange of cytoplasm and genome were obtained by more than 40 generations of back-crossing of reciprocal hybrids to males from the paternal line. Both XX and XXBsYy + females were available from the four lines. Isogenic hybrids with mothers from the two original and the two derived lines were tested for recombination in the y-cx-v-f regions. Simultaneous comparisons were made between hybrids from XX mothers and both XX and XXY daughters of XXY mothers. There was no difference between the two groups of XX females. In the distal region, y-ex, there was in all tests an increasing frequency of recombination over successive broods with the pattern influenced by the maternal genotype, the origin of the cytoplasm and the presence of an extra Y-chromosome. The median region, cx-v, showed a tendency towards decreasing recombination, with some influence exerted by the cytoplasm. The sub-proximal region, v-f, showed a slight tendency towards decreasing frequencies, in early broods related to the maternal genotype. The XXY females had lower frequencies than XX females. – It was shown that the Y-chromosome in XXY hybrids had, in all four combinations, a tendency to be included in the same nucleus as the centromere of the “K”-line more often than randomly. The similarity in the increasing frequency of recombination in the distal region, y-ex, in regular and secondary non-disjunction exceptionals as well as the decreasing frequencies of both recombination in the v-f region in XXY females and secondary non-disjunction are discussed.

Recombination in the region between the marker forked and the centromere in inbred Drosophila melanogaster XX and XXY females

Recombination in the proximal X chromosome interval (forked-centromere) has been studied in hybrids from two inbred lines of Drosophila melanogaster. Attached Ysy+ arms or the proximal region of the Dp (1:1) scV1y+ were used as markers of the centromere. A derived Y chromosome (BsYK) induced a reduction of the recombination frequency in only one of five comparative experiments of XXBsYK and XX females. In two of the tests there was a higher frequency of recombination in the presence of the BsYK. This is not consistent with a competitive preexchange XX and XY association, which according to a generally accepted idea should result in an inevitable decrease in recombination. This is considered as further evidence for non-random Y distribution at regular X disjunction being determined after exchange.

Genetics of inbred Drosophila melanogaster. VI. Crossing-over in secondary non-disjunction exceptionals.

Although Bridges (1916, Genetics 1: 1–52 and 107–183) found recombinants among exceptional XXY females in tests of secondary non-disjunction, he misinterpreted their origin and claimed that true secondary non-disjunction resulted in XX eggs with non-crossover chromosomes. Preliminary tests of inter-strain crosses of inbred lines indicated different frequencies of recombinant exceptional females in reciprocal crosses. Large scale tests confirmed this observation. However, they indicated that, as judged by recombination in the most distal end (y-w interval), there was presumably nearly the same frequency of chromosome pairing in XX females as in exceptional females from XXY mothers. In the more proximal regions there was a decreasing proportion of recombinants among exceptional females as compared to regular females. The decrease was more pronounced in one direction of crosses than in the reciprocal indicating a maternal influence. The use of inbred lines made it possible to compare the frequencies of recombinant X-chromosomes in females of primary and secondary non-disjunctional origin in the same genetic and cytoplasmic background. It was shown that the absolute frequency of recombinants was much higher in secondary non-disjunction than in primary. Some complex phenodeviants among the exceptional females revealed that their recombination could occur between euchromatic parts of the X-chromosomes and between X and either Y-arm indicating trivalent formation, however, of type different from that suggested by Cooper (1948, Proc. Nat. Acad. Sci. 34,5: 179–187).

Genetics of inbred Drosophila melanogaster. VII. Evidence of normal chromosome pairing in the distal ends of X-chromosomes in secondary non-disjunction.

In tests of secondary non-disjunction in hybrids from two inbred lines (+w ++ H– K/y wa ex vf – 2IBS Yy+) the majority of exceptional females are phenotypically w/wa. 1252 such females were tested individually to reveal whether recombination had occurred or not. 30 of these females were recombinants, and 13 of them had recombined in the y-w region. A subdivision based on the brooding scheme indicated that in the 1–5th day broods the frequency of recombination in the y-w region was 0.49 % and in the 11–14th day broods 2.09 %. (The corresponding frequencies from XX mothers of the same inter-strain cross were 0.88 and 2.59 %, respectively.) It is concluded that these results are in accordance with a former analysis of phenotypic deviants among exceptionals, thereby indicating that recombination at the most distal end of the X-chromosome is very similar in XXY and XX females. The results also suggest near normal pairing in the most distal end of the X-chromosomes of XXY females prior to non-disjunction.

Genetics of inbred Drosophila melanogaster

In two inbred lines of Drosophila melanogaster, marker genes had been induced (y, wa, ex, v and f in Oregon, code “2”; and w in Karsnas, code “K”). As well as the two original lines, two derived lines with mutual exchange of cytoplasm and genome were obtained by more than 40 generations of back-crossing of reciprocal hybrids to males from the paternal line. Both XX and XXBsYy + females were available from the four lines. Isogenic hybrids with mothers from the two original and the two derived lines were tested for recombination in the y-cx-v-f regions. Simultaneous comparisons were made between hybrids from XX mothers and both XX and XXY daughters of XXY mothers. There was no difference between the two groups of XX females. In the distal region, y-ex, there was in all tests an increasing frequency of recombination over successive broods with the pattern influenced by the maternal genotype, the origin of the cytoplasm and the presence of an extra Y-chromosome. The median region, cx-v, showed a tendency towards decreasing recombination, with some influence exerted by the cytoplasm. The sub-proximal region, v-f, showed a slight tendency towards decreasing frequencies, in early broods related to the maternal genotype. The XXY females had lower frequencies than XX females. – It was shown that the Y-chromosome in XXY hybrids had, in all four combinations, a tendency to be included in the same nucleus as the centromere of the “K”-line more often than randomly. The similarity in the increasing frequency of recombination in the distal region, y-ex, in regular and secondary non-disjunction exceptionals as well as the decreasing frequencies of both recombination in the v-f region in XXY females and secondary non-disjunction are discussed.

Genetics of inbred Drosophila melanogaster XII. Effects of an X-chromosome inversion on primary and secondary non-disjunction

Luning., K. G., Lake, S. and Linde, M. 1984. Genetics of inbred Drosophila melanogaster XII. Effects of an X-chromosome inversion on primary and secondary non-disjunction.—-Hereditas 100: 247–257. Lund, Sweden. ISSN 0018–0661. Received August 1, 1983 In an inbred line of Drosophila melanogaster. Oregon (code 2), an inversion covering the proximal half of the X-chromosome was induced (symbol: In(l) Lu-l y wd cxvf-2). It has been shown that inversion homozygotes have low frequencies of primary nun-disjunction. in the same manner as equilinear females with regular gene order of the 2–line. Intra-strain structural heterozygotes gave an excess of XO exceptionals but also a 5–fold increase of true primary non-disjunction (XXY) as compared to structural hornozygotes. Inter-strain crosses with mothers from line “K” revealed an influence both by the maternal genome and by the origin of the cytoplasm. Tests of secondary non-disjunction in structural heterozygous XXY females from both intra- and inter-strain crosses indicated highly complicated relations. When the mothers were of “2” origin the first brood showed lower frequencies of XXY exceptionals than in successive broods. whereas when the mother was of “K” origin there was a decrease after the 3rd brood (7th day). The “K”-line showed a dominance effect which was manifested by higher frequencies of XXY exceptionals. In regular daughters the Y-chromosome followed preferentially the proximal part of the X-chromosome of “K”. origin. There was no preference in the Y-chromosome distribution in intra-strain hybrids. There was a pronounced low frequency of recombinants among exceptionals. This indicates that the high frequencies of exceptionals from inversion heterozygous XXY females occur by another mechanism than in equilinear XXY females with the same genetic background.

Recombination in the X-chromosome in hybrid females with and without a marked centromere from three inbred lines of Drosophila melanogaster.

Hybrid females of three inbred lines of Drosophila melanogaster, with and without BSYL attached to the marker line centromere, were produced by reciprocal crosses. The recombination frequency was scored along the X-chromosome. In XX females one inter-line cross showed differences between reciprocal crosses in all regions examined, indicating maternal influence, while the other inter- and intra-line crosses showed no obvious effects. Only the inter-line cross manifesting effects in XX females showed maternal effects in XX-YL females. Of special interest was the brood pattern of the f-centromere region. Concerning the effects of the YL arm, the recombination in the y-cx region seemed to increase in early broods and decrease in late broods. In the v-f region there were decreases in both early and late broods, while in the cx-v region the YL-effects were varying. These results are discussed in the light of recombination in XXY females from former tests, and the brood pattern of the f-centromere is compared with findings of higher frequencies of non-disjunction in early broods compared with later ones. The spatial location of the chromosomes in the nucleus during meiosis is discussed as a possible explanation for the discrepancy between crosses.

The mechanism of secondary nondisjunction in Drosophila melanogaster females.

Bridges (1916) observed that X chromosome nondisjunction was much more frequent in XXY females than it was in genetically normal XX females. In addition, virtually all cases of X nondisjunction in XXY females were due to XX <--> Y segregational events in oocytes in which the two X chromosomes had failed to undergo crossing over. He referred to these XX <--> Y segregation events as "secondary nondisjunction." Cooper (1948) proposed that secondary nondisjunction results from the formation of an X-Y-X trivalent, such that the Y chromosome directs the segregation of two achiasmate X chromosomes to opposite poles on the first meiotic spindle. Using in situ hybridization to X and YL chromosomal satellite sequences, we demonstrate that XX <--> Y segregations are indeed presaged by physical associations of the X and Y chromosomal heterochromatin. The physical colocalization of the three sex chromosomes is observed in virtually all oocytes in early prophase and maintained at high frequency until midprophase in all genotypes examined. Although these XXY associations are usually dissolved by late prophase in oocytes that undergo X chromosomal crossing over, they are maintained throughout prophase in oocytes with nonexchange X chromosomes. The persistence of such XXY associations in the absence of exchange presumably facilitates the segregation of the two X chromosomes and the Y chromosome to opposite poles on the developing meiotic spindle. Moreover, the observation that XXY pairings are dissolved at the end of pachytene in oocytes that do undergo X chromosomal crossing over demonstrates that exchanges can alter heterochromatic (and thus presumably centromeric) associations during meiotic prophase.

PRE NATAL TESTOSTERONE LEVELS IN XXY AND XYY MALES

It has been postulated that behavioural differences between normal males and those with an additional X or Y chromosome may be related to hormonal variations pre or post natally. We have previously demonstrated that XXY males have a testosterone surge in the neonatal period (1) and that during childhood salivary testosterone levels are slightly higher than in control males (2).The pre-natal hormone status is now investigated using amniotic fluid obtained at ante natal diagnosis between 16 and 20 weeks gestation age in a collaborative project between the Departments of Medical Genetics of Guy's Hospital, London, Oxford, Glasgow and Edinburgh.The (geometric) mean testosterone levels and ranges (p mol/L) were XY 433.5, range 165-1027 (n=25); XYY 500.0, range 224-1092 (n=15); XXY 419.8 range 87-1021 (n=17); XX 146.9, range 41-474 (n=13).After log transformation, using the t test no significant differences were found between XY, XYY and XXY males but females had lower testosterone levels (p<0.001). These findings give no support to the hypothesis that pre-natal testosterone levels contribute to later behavioural characteristics.1 Ratcliffe SG. Proc Roy Soc Med, 1976, 69, 189-191.2 Ratcliffe SG. Butler GE, Jones M. BDOAS, 26, 1-44.

Tdy-negative XY, XXY and XYY female mice: breeding data and synaptonemal complex analysis.

In this paper we have compared the breeding performance of Tdy-negative XY, XXY and XYY females to assess the relative importance of the lack of a second X chromosome compared with the presence of a Y chromosome, in reducing fertility. The XY females were of poor fertility, although five of twelve produced at least one offspring. The XXY females had larger, more frequent litters, and a longer reproductive lifespan, implicating the lack of a second X as the major cause of the poor fertility of XY females. Nevertheless, XYY females appeared to be more seriously affected than the XY females, suggesting that the presence of the Y may be a contributory factor. Pachytene analysis demonstrated that the Y is a very inefficient pairing partner for the X during female meiosis. In XY females only 11% of pachytene cells had the X and Y paired; in XXY females the two X chromosomes paired and the Y was almost always a univalent, while in XYY females the X paired with a Y in only 15% of pachytene cells. The presence of unpaired sex chromosomes has previously been implicated as a cause of oocyte loss during pachytene, and the proportion of cells with unsynapsed sex chromosomes decreased as pachytene proceeded, suggesting that they were progressively eliminated. Significantly, protection against elimination was afforded not only by synapsis between sex chromosomes, but also by self-synapsis if a sex chromosome remained as a univalent. It is concluded that sex chromosome univalence leading to pachytene oocyte failure is responsible for the postnatal oocyte deficiency seen in XY females. A separate study has shown that XXY females have a similar level of oocyte deficiency. It is suggested that the presence of a second X chromosome improves the fertility of XXY females, compared with XY females, by improving oocyte quality and by eliminating the production of lethal XY and OY zygotes. The genotype frequencies for the offspring of XY and XYY females differed from those predicted from the pachytene data. The XY females showed a marked deficiency of XO offspring compared with XXY and XYY aneuploid offspring, whereas the XXY females had fewer than expected XXY and XYY aneuploid offspring.

Dependence on genic balance for synaptonemal complex formation in Drosophila melanogaster.

Electron microscopic examination of gonads of Drosophila melanogaster with different genotypes, including a metafemale 3X;2A and an intersex XXY;3A have revealed that the formation of synaptonemal complexes is controlled by the genic balance, i.e., the ratio of X chromosomes to autosomes. The Y chromosome is not involved in the genetic control of the formation of precursors of the central element of synaptonemal complexes in males, nor does it disturb their formation in XXY females. Hyperploidy for sections 1-3A and 18A-20 of the X chromosome does not lead to the appearance of synaptonemal complexes in males and does not interfere with their formation in females. Females hyperploid for extensive regions of the X chromosome (sections 1-11A, 11A-20, and 8C-20) are fertile and show apparently normal formation of synaptonemal complexes. Hyperploidy for sections 8C-11A of the X results in a sharp decrease in the viability of females, in abnormal differentiation of ovary cells, and in the lack of synaptonemal complexes. These data suggest a possible important role for the sections 8C-11A in the genic balance controlling the formation of synaptonemal complexes in D. melanogaster. The lack of synaptonemal complexes in hypoploid females may be the result of abnormal cell differentiation in gonads.