Meiotic Chromosome Configurations Research Articles

A sexual highly diploidized autotetraploid plant induced from the diploid cytotype of Paspalum chaseanum Parodi

AbstractPaspalum is one of the most common native grass genera of the New World grasslands, and it includes many wild natural forage species and rare domesticated ones. Polyploidy is widespread among the genus. Many species are multiploid, which basically involve two components: a sexual self‐incompatible diploid cytotype and a conspecific apomictic tetraploid, pseudogamous, and self‐compatible cytotype, though other apomictic polyploid levels may occur in the same species. Tetraploid individuals, reproducing exclusively sexually, have never been identified in a natural apomictic tetraploid Paspalum species. Doubling the chromosome number of a 2x cytotype is used to obtain a sexual autotetraploid to carry out sexual × apomictic crosses in breeding programs for plant improvement. After numerous attempts we generated only one autotetraploid plant of P. chaseanum (I4C) which reproduced sexually and was highly self‐incompatible. An unexpected preferential chromosome pairing resulted in the cytological diploidization of I4C. Despite each chromosome having three copies in I4C, meiotic chromosome configurations showed just 9.9% chromosomes forming multivalents (trivalents and quadrivalents), 82.1% bivalents, plus 7.9% univalents. The possible causes of this preferential pairing were analyzed. Although highly self‐incompatible, it was fertile in crosses following intra‐ and interspecific pollinations, but with variable seed set, from over 45% when crossed with an apomictic natural tetraploid strain of P. chaseanum, to 0.9% with a strain of P. wrightii. Thus, a novel autotetraploid P. chaseanum was synthesized with very interesting characteristics: sexual reproduction, high self‐incompatibility, variable cross‐compatibility, and high cytological diploidization. All valuable attributes for breeding programs in a forage grass.

Comparing the effects of a commercial and a prototype vitrification medium on meiotic spindle morphology and survival rate of mouse oocytes.

ObjectiveTo compare oocyte survival and meiotic spindle normality between vitrified-warmed oocytes in a mouse embryo assay using Tvitri-4 or Ingámed vitrification media.MethodsC57BL/6 female mice aged 8-12 weeks were submitted to superovulation with pregnant mare’s serum gonadotropin and human chorionic gonadotropin (hCG) for obtaining of in vivo matured oocytes. The oocytes were randomly distributed into one of the following three groups: CTR - control (fresh oocytes); ING - oocytes vitrified-warmed in a standard commercial kit supplied by Ingámed, and T4 - oocytes vitrified-warmed in the novel prototype Tvitri-4 medium. After warming and recovery culture, oocytes were assessed with respect to survival rate (SR) and both meiotic spindle morphology and chromosome alignment of each oocyte fixed in the sagittal position after immunostaining and analysis by confocal microscopy.ResultsA total of 354 mature oocytes were vitrified in ING (n=178) and T4 (n=176), out of which 299 (85%) survived after warming. Oocyte survival rates were not statistically different (p=0.08) between ING (145/178=81.5%) and T4 (154/176=87.5%). Regarding meiotic normality, there were no significant changes in the proportion of oocytes with normal meiotic spindle morphology and chromosome structure between ING (52,2%) and T4 (63.4%) after warming (RR: 0.95, 95% CI: 0.92-1.607). When the meiotic normality was assessed using the CTR group as a reference in the analysis of relative risk, no significant differences were observed between T4 (63.4%) and CTR (70.5%) (RR: 0.95, 95% CI: 0.72-1.12). On the other hand, the percentage of oocytes retaining normal meiotic spindle morphology and chromosome configuration in ING (52.2%) was lower than in the CTR group (RR: 0.95, 95% CI: 0.57-0.97).ConclusionsThe novel prototype Tvitri-4 medium was efficient in preserve survival rate and meiotic spindle normality of oocytes and, with further verification, may be able to replace commercially available media in future clinical applications.

Varietal variation and chromosome behaviour during meiosis in Solanum tuberosum

Naturally occurring autopolyploid species, such as the autotetraploid potato Solanum tuberosum, face a variety of challenges during meiosis. These include proper pairing, recombination and correct segregation of multiple homologous chromosomes, which can form complex multivalent configurations at metaphase I, and in turn alter allelic segregation ratios through double reduction. Here, we present a reference map of meiotic stages in diploid and tetraploid S. tuberosum using fluorescence in situ hybridisation (FISH) to differentiate individual meiotic chromosomes 1 and 2. A diploid-like behaviour at metaphase I involving bivalent configurations was predominant in all three tetraploid varieties. The crossover frequency per bivalent was significantly reduced in the tetraploids compared with a diploid variety, which likely indicates meiotic adaptation to the autotetraploid state. Nevertheless, bivalents were accompanied by a substantial frequency of multivalents, which varied by variety and by chromosome (7–48%). We identified possible sites of synaptic partner switching, leading to multivalent formation, and found potential defects in the polymerisation and/or maintenance of the synaptonemal complex in tetraploids. These findings demonstrate the rise of S. tuberosum as a model for autotetraploid meiotic recombination research and highlight constraints on meiotic chromosome configurations and chiasma frequencies as an important feature of an evolved autotetraploid meiosis.

Vitrification of In vitro -matured Oocytes: Effects of Meiotic Spindle Morphology on Clinical Outcome

Objective: The meiotic spindle controls chromosome movement and mediates various functions essential for fertilization and early postfertilization events. This study aimed to examine whether vitrification causes meiotic damage in vitro- matured metaphase II (MII) human oocytes, and whether the meiotic spindle morphology influences the subsequent developmental outcomes. Methods: The spindle characteristics of MII human oocytesin vitro matured were studied before and after vitrification using PolScope imaging and immunofluorescence staining. The developmental competence of oocytes was also examined. Results: A total of 419 human MII oocytes were obtained from 593 intracytoplasmic sperm injection cycles at our hospital. Of these oocytes, 54 were used for immunofluorescence staining, whereas the other oocytes were examined by PolScope imaging and classified into three groups according to the meiotic spindle morphology: (A) normal morphology, (B) weak refraction and short meiotic spindle, and (C) no detectable meiotic spindle. The three groups demonstrated statistically significant differences in terms of survival after vitrification. However, differences were not found in terms of oocyte chromosome structure and meiotic spindle morphology on immunofluorescence staining performed before and after vitrification. Oocyte survival, fertilization, and early embryonic development rates were significantly higher in Group A than in Groups B and C with or without vitrification. While vitrification had no effect on these metrics in Group A, Groups B and C demonstrated significantly lower fertilization and cleavage rates after vitrification/warming. Conclusions: Screening for normal meiotic spindle morphology and chromosome configuration before vitrification may increase the yield of healthy viable oocytes for various assisted reproductive technologies.

Testing the translocation hypothesis and Haldane\u2019s rule in Rumex hastatulus

The translocation hypothesis regarding the origin of the XX/XY1Y2 sex chromosome system was tested with reference to the F1 hybrids between two chromosomal races of Rumex hastatulus. The hybrids derived from reciprocal crossing between the Texas (T) race and the North Carolina (NC) race were investigated for the first time with respect to the meiotic chromosome configuration in the pollen mother cells, pollen viability, and sex ratio. A sex chromosome trivalent in the NC × T males and two sex chromosome bivalents in the T × NC males were detected. The observed conjugation patterns confirmed the autosomal origin of the extra chromosome segments occurring in the North Carolina neo-sex chromosomes. Decreased pollen viability was found in the T × NC hybrid in contrast to the NC × T hybrid and the parental forms. Moreover, only in the T × NC hybrid sex ratio was significantly female-biased (1:1.72). Thus, Haldane’s rule for both male fertility and male rarity was shown in this hybrid. According to the authors’ knowledge, R. hastatulus is just the second plant with sex chromosomes in which Haldane’s rule was evidenced.

Meiotic chromosome configurations in oocytes of Cobitis taenia and its polyploid hybrids

Diploid and triploid hybrid females of Cobitis as a rule produce unreduced eggs which mainly develop gynogenetically, but some of the eggs incorporate sperm genome and develop into triploids and tetraploids, respectively. Here, we observed for the first time the meiotic chromosomes in the germinal vesicles (GVs) of mature oocytes of three diploid C. taenia (2n = 48) and 20 allopolyploid females of Cobitis (18 triploid 3n = 74 and 2 tetraploid 4n = 99). The majority of GVs in diploid, triploid and tetraploid females contained 24, 74 and 96 or 99 bivalents, respectively. These results directly indicated premeiotic endomitosis as a mechanism underlying the formation of unreduced eggs in allopolyploid females of Cobitis.

Meiotic behaviour and sperm aneuploidy in an infertile man with a mosaic 45,X/46,XY karyotype

The meiotic behaviour of the germ cells in 45,X/46,XY men has not been extensively studied. This study investigated the meiotic events and sperm aneuploidy in an azoospermic man with a 45,X/46,XY (50/50) mosaic karyotype to better understand the fate of the 45,X cells and the production of chromosomally abnormal spermatozoa. Combining immunofluorescence techniques and fluorescence in-situ hybridization, meiotic recombination, synapsis, meiotic sex chromosome inactivation (MSCI) and configuration were analysed, as well as sperm aneuploidy in the patient and 10 normal, fertile men. Despite the 50:50 somatic mosaicism in the patient, 25% of pachytene cells analysed were 45,X. Furthermore, 63% of pachytene cells were 46,XY with paired sex chromosomes, and 12% were 46,XY with unpaired sex chromosomes, which displayed abnormal MCSI patterns. Although the patient's testicular spermatozoa showed increased aneuploidy, the majority were of normal constitution. The X:Y sperm ratio was significantly increased compared with the controls (P < 0.001), which may indicate that some 45,X cells gave rise to X-bearing spermatozoa. The findings provide insight into the fate of 45,X/46,XY cells in meiosis, supporting the hypothesis that stringent checkpoints ensure the favourable production of spermatozoa with normal chromosomal constitution despite an individual's abnormal karyotype.

Meiotic chromosome configurations in triploid progeny from reciprocal crosses between wild-type diploid and natural tetraploid loach Misgurnus anguillicaudatus in China.

Here, we showed meiotic chromosome configurations prepared from oocyte germinal vesicles and spermatocytes of triploid loaches produced from reciprocal crosses between wild-type diploids (2n = 50) obtained from Dalian, Liaoning Province, China and natural tetraploids (4n = 100) from Chibi, Hubei Province, China. Major meiotic cells in triploids comprised 25 bivalents and 25 univalents, but cells with one to five trivalents were also observed. When three homologous chromosomes bearing nucleolar organizing regions (NOR) were identified with the detection of signals or positive sites by silver staining, chromomycin A3 staining and fluorescence in situ hybridization with a 5.8S + 28S rDNA probe, two third of selected triploid cells gave a configuration including one bivalent with two NORs (association of two homologous chromosomes) and one univalent with one NOR. However, other triploid cells showed three univalent each of which had one NOR, suggesting a failure of synapsis between homologous chromosomes. These results suggested that triploid female and male should produce aneuploid gametes with the theoretical mode at 1.5n (37 or 38 chromosomes).

Confocal Microscopic Analysis of the Spindle and Chromosome Configurations of in vitro-Matured Oocytes from Different Types of Polycystic Ovary Syndrome Patients

Objective: To evaluate the meiotic spindle and chromosome distribution of in vitro-matured oocytes from infertile nonobese or obese women with polycystic ovary syndrome (PCOS) and to compare in vitro maturation (IVM) rates between groups. Design: Prospective study. Setting: Hospital-based IVF center. Patients: A total of 99 patients (26 obese women with PCOS, 25 nonobese women with PCOS, and 48 controls) undergoing stimulated cycles for intracytoplasmic sperm injection had immature oocytes for IVM. Interventions: Immature oocytes (germinal vesicle and metaphase I stages) were collected from obese and nonobese PCOS patients and controlled infertile patients. The meiotic spindle and chromosome configurations in oocytes matured in vitro were studied by confocal microscopy, with fluorescent labeling techniques for visualization of both microtubules and chromosomes. Main Outcome Measures: Meiotic spindle and associated chromosome configurations. Results: There were no significant differences between the different types of PCOS and the control group with respect to IVM rates (61.8, 63.8, and 63.2%, respectively), the percentage of spindle abnormalities in metaphase II oocytes (40.6, 42.9, and 37.5%, respectively) or chromosome abnormalities in metaphase II oocytes (31.2, 34.3, and 33.3%, respectively). Conclusions: In vitro-matured oocytes obtained from stimulated cycles had a high ratio of meiotic abnormalities. The different types of PCOS had the same ratio of meiotic abnormalities.

Preferential bivalent formation in tetraploid male of Pacific oyster Crassostrea gigas Thunberg

Artificially induced tetraploid Pacific oyster, Crassostrea gigas Thunberg, produces more aneuploid gametes than normal diploid one, although they showed a comparable fecundity to diploidy. The meiotic chromosome configuration of 3 tetraploid and 1 tetraploid/triploid mosaic males were analyzed through direct chromosome observation. A majority of metaphase I spermatocytes contained both bivalents and quadrivalents. The chromosome configuration of these males was characterized by preferential formation of bivalents to quadrivalents. Bivalents appeared in all spermatocytes and consisted of 86% of all chromosome aggregates. In comparison, quadrivalents occurred in 91% spermatocytes and consisted of only 12.6% of all chromosome aggregates. The mean bivalent frequency per spermatocyte varied between 14.4 and 17.2; while that of quadrivalents varied between 2.2 and 2.7. Most quadrivalents were tandemly chained (58%) or circled (39%). The total number of chromosome aggregates per spermatocyte ranged from 13 to 20 with an average of 17.6; while 18 (16 bivalents and 2 quadrivalents) was the most frequent. Univalents and trivalents appeared in very low frequency. Aneuploid (hypotetraploid) spermatocytes were observed in a low frequency. The chromosome configuration of in the mosaic individual was similar to that of tetraploid individuals. The percentage of triploid spermatocytes (2%) of the mosaic individual was significantly lower (χ 2 =30, P < 0.01) than that of triploid cells (46%) in its somatic tissue.

Meiotic chromosome configurations in triploid and heteroploid mosaic males of Crassostrea gigas and Crassostrea ariakensis

Reversion from triploids to diploids or heteroploid mosaics may make the revertants recover reproductive ability and lose their aquacultural advantages. Meiotic chromosomes in triploids and mosaics of Crassostrea gigas and Crassostrea ariakensis were studied. Pachytene spermatocytes showed incomplete trivalent formation, varying 64–72% in C. gigas and 62–69% in C. ariakensis. At metaphase I, trivalents, bivalents and univalents occurred in various combinations. Trivalents occurred in all spermatocytes, ranged 6–10 per spermatocyte and averaged 8.53–8.97 in C. gigas and 8.49–8.95 in C. ariakensis. Univalents and bivalents appeared in 59.2–72.1% of the spermatocytes in C. gigas, and ranged 0.77–1.39 and 1.01–1.51 per spermatocyte respectively. In C. ariakensis, they occurred in 57.5–81% of the spermatocytes, and ranged 0.9–1.32 and 1.03–1.35 respectively. The most common trivalent was formed by a bivalent with the third chromosome attaching to its side to form ‘long-tail-cross’ or ‘t’ or ‘frying-pan’-shaped configurations that constituted 65.7% and 59.9% of the trivalents in C. gigas and C. ariakensis respectively. Other kinds of trivalent associations included tandem chains (14.7%, 16.5%), closed circles or triangles (5.3%, 9.8%), convergent ‘T’ or ‘Y’ (5.8%, 7.4%) and unclassified configurations (8.5%, 6.4%). Diploid spermatocytes were not observed from mosaics, although they showed considerable proportions of ‘diploid’ cells in their gills.

Evolution in the Drosophila ananassae species subgroup

Drosophila ananassae and its relatives have many advantages as a model of genetic differentiation and speciation. In this report, we examine evolutionary relationships in the ananassae species subgroup using a multi-locus molecular data set, karyotypes, meiotic chromosome configuration, chromosomal inversions, morphological traits, and patterns of reproductive isolation. We describe several new taxa that are the closest known relatives of D. ananassae. Analysis of Y-chromosomal and mitochondrial haplotypes, shared chromosome arrangements, pre-mating isolation, and hybrid male sterility suggests that these taxa represent a recent evolutionary radiation and may experience substantial gene flow. We discuss possible evolutionary histories of these species and give a formal description of one of them as D. parapallidosa Tobari sp. n. The comparative framework established by this study, combined with the recent sequencing of the D. ananassae genome, will facilitate future studies of reproductive isolation, phenotypic variation, and genome evolution in this lineage.

Chromosomal inversion heterozygosity inPaeonia decomposita(Paeoniaceae)

Abstract Meiosis of Paeonia decomposita Hand.–Mazz., an endangered species endemic to China, was investigated in two natural populations represented by 13 individuals. The meiotic aberrances occurred so frequently that all individuals were found to have a certain amount of bridges, fragments and univalents. Mean meiotic chromosome configurations of the species were 2n = 10 = 0.20 I + 4.90 II. Pairing index ranged from 72.97% to 81.37%. Chromosome bridges, fragments, lagging chromosomes, and unequal segregation were observed at both anaphase I and telophase I. Interestingly, all the individuals bore bridges/fragments (7.69% of PMCs on an average) at ana-phase I, indicating that they were all heterozygotes of paracentric inversion. However, variations were found among individuals in frequency of bridge occurrence and in size of fragments, which indicated that many different inversions existed in this species. The chromosome structure heterozygotes found so frequently in natural populations should have a sel...

Sperm aneuploidy and meiotic sex chromosome configurations in an infertile XYY male

There is little information regarding the behaviour of the extra Y chromosome during meiosis I in men with 47,XYY karyotypes and the segregation of the sex chromosomes in sperm. We applied immunofluorescent and FISH techniques to study the relationship between the sex chromosome configuration in meiotic germ cells and the segregation pattern in sperm, both isolated from semen samples of a 47,XYY infertile man. The sex chromosome configuration of pachytene germ cells was determined by immunostaining pachytene nuclei for synaptonemal complex protein 3 (SCP3) and SCP1. FISH was subsequently performed to identify the sex chromosomes and chromosome 18 in pachytene cells. Dual- and triple-color FISH was performed on sperm to analyse aneuploidy for chromosomes 13, 18, 21, X, and Y. 46,XY/47,XYY mosaic pachytene cells were observed (22.2% vs. 77.8%, respectively). The XYY trivalent, and X+YY configurations were most common. While the majority of sperm were of normal chromosomal constitution, an increase in sex and autosome disomy was observed. The level of germ cell moscaicism and their meiotic sex chromosome configurations may determine sperm aneuploidy rate and fertility status in 47,XYY men. Our approach of immunostaining meiotic cells in the ejaculate is a novel method for investigating spermatogenesis in infertile men.

Evaluation of meiotic spindle and chromosome configurations of in vitro matured human oocytes from PCOS and endometriotic patients submitted to ovarian stimulation: preliminary results

Endometriosis and Policystic Ovarian Syndrome (PCOS) are major causes related to human infertility. According to controversial data both may be related to worse outcomes of assisted reproduction techniques, which could be caused by an impairment of oocyte quality. Thus, we compared meiotic spindle morphology and chromosomal distribution of in vitro matured human oocytes from patients with endometriosis and PCOS submitted to ovarian stimulation for ICSI/embryo transfer.

Confocal microscopic analysis of the spindle and chromosome configurations of human oocytes matured in vitro

To assess the potential effects of in vitro maturation (IVM) of human oocytes on the meiotic spindle and associated chromosome configuration. Prospective study. Hospital-based IVF center. Patients with polycystic ovary syndrome (PCOS) undergoing unstimulated and stimulated cycles of oocyte retrieval. Immature (germinal vesicle and metaphase I) and mature (metaphase II) oocytes were collected from PCOS patients. The meiotic spindle and chromosome configurations in oocytes matured in vitro and in vivo were studied by confocal microscopy, with fluorescent labeling techniques for visualization of both microtubules and chromatin. Meiotic spindle and associated chromosome configurations. Oocytes can develop to the metaphase II stage after IVM. Confocal microscopic observations revealed that the oocytes matured in vitro had a higher frequency of abnormal meiotic spindle and chromosomal alignment morphology than in vivo-matured oocytes. These abnormalities included a partial or total disorganization of the meiotic spindle microtubules. Abnormal chromosome organization included dispersal of chromosomes or chromosomes with an aberrant, less-condensed appearance. The proportions of abnormality in spindle and chromosome configurations in oocytes matured in vitro were 43.7% and 33.3%, respectively, which was significantly higher than in those oocytes matured in vivo (13.6% and 9.1%). In vitro maturation can have deleterious effects on the organization of the meiotic spindle and chromosome alignment of human oocytes. This result suggests one possible explanation for the reduced developmental potential of oocytes matured in vitro compared with those matured in vivo. This is likely a contributing factor to the overall lower clinical outcomes observed after IVM and ET.

Characterization of Amphiploid Hybrids between Bluebunch and Thickspike Wheatgrasses

An amphiploid derivative from hybrids between bluebunch wheatgrass (2n = 2x = 14) [Pseudoroegneria spicata (Pursh) A. Love] and thickspike wheatgrass (2n = 4x = 28) [Elymus lanceolatus (Scribner and Smith) Gould] was developed to compare cytological, morphological, anatomical, and seed characteristics between the triploid F1 hybrids (2n = 3x = 21) and their colchicine induced hexaploid (2n = 6x = 42) derivative (C0). The amphiploid population produced an average of 3.1 seeds spikelet−1 and pollen stainability averaged 85.5%. Seed set under self‐pollination in the amphiploids ranged from &lt;1% to &gt;99% with an average of 50%. Fifty‐eight of the 103 plants studied were hexaploid (2n = 6x = 42). The remaining plants had chromosomes ranging from 2n = 39 to 2n = 44 and were less fertile than the euploids. Meiosis in the euploid plants was regular and typical of a segmental autoallohexaploid St1St1St2St2HH. The most common meiotic chromosome configuration was 21 bivalents. A high frequency of bivalents and a low frequency of quadrivalents indicate that the St genomes of the two parental species have diverged. The amphiploids had thicker culms, more leaves per culm, and longer and wider leaves than either parental species. Multivariate analysis demonstrated that the amphiploids, though morphologically similar to the parental species, was distinct. In general, C0 hybrids were less vigorous than the F1 from which they were derived. Through amphiploidy, genetic introgression between the two species is possible.

RG-007 Influence of maternal age on meiotic spindle and chromosome configuration in oocytes

RG-007 Influence of maternal age on meiotic spindle and chromosome configuration in oocytes

Major chromatin remodeling in the germinal vesicle (GV) of mammalian oocytes is dispensable for global transcriptional silencing but required for centromeric heterochromatin function

Global silencing of transcriptional activity in the oocyte genome occurs just before the resumption of meiosis and is a crucial developmental transition at the culmination of oogenesis. Transcriptionally quiescent oocytes rely on stored maternal transcripts to sustain the completion of meiosis, fertilization, and early embryonic cleavage stages. Thus, the timing of silencing is key for successful embryo development. Yet, the cellular and molecular pathways coordinating dynamic changes in large-scale chromatin structure with the onset of transcriptional repression are poorly understood. Here, oocytes obtained from nucleoplasmin 2 knockout ( Npm2 −/− ) mice were used to investigate the relationship between transcriptional repression and chromatin remodeling in the germinal vesicle (GV) of mammalian oocytes. Although temporally linked, global silencing of transcription and chromatin remodeling in the oocyte genome can be experimentally dissociated and therefore must be regulated through distinct pathways. Detection of centromeric heterochromatin DNA sequences with a mouse pan-centromeric chromosome paint revealed that most centromeres are found in close apposition with the nucleolus in transcriptionally quiescent oocytes and therefore constitute an important component of the perinucleolar heterochromatin rim or karyosphere. Pharmacological inhibition of histone deacetylases (HDACs) with trichostatin A (TSA) revealed that HDACs are essential for large-scale chromatin remodeling in the GV. Importantly, the specialized nuclear architecture acquired upon transcriptional repression is essential for meiotic progression as interference with global deacetylation and partial disruption of the karyosphere resulted in a dramatic increase in the proportion of oocytes exhibiting abnormal meiotic chromosome and spindle configuration. These results indicate that the unique chromatin remodeling mechanism in oocytes may be specifically related to meiotic cell division in female mammals.

Missing oocyte encodes a highly conserved nuclear protein required for the maintenance of the meiotic cycle and oocyte identity in Drosophila.

In Drosophila, a single oocyte develops within a 16-cell germline cyst. Although all 16 cells initiate meiosis and undergo premeiotic S phase, only the oocyte retains its meiotic chromosome configuration and remains in the meiotic cycle. The other 15 cells in the cyst enter the endocycle and develop as polyploid nurse cells. A longstanding goal in the field has been to identify factors that are concentrated or activated in the oocyte, that promote meiotic progression and/or the establishment of the oocyte identity. We present the characterization of the missing oocyte gene, an excellent candidate for a gene directly involved in the differentiation of the oocyte nucleus. The missing oocyte gene encodes a highly conserved protein that preferentially accumulates in pro-oocyte nuclei in early prophase of meiosis I. In missing oocyte mutants, the oocyte enters the endocycle and develops as a polyploid nurse cell. Genetic interaction studies indicate that missing oocyte influences meiotic progression prior to pachytene and may interact with pathways that control DNA metabolism. Our data strongly suggest that the product of the missing oocyte gene acts in the oocyte nucleus to facilitate the execution of the unique cell cycle and developmental programs that produce the mature haploid gamete.