EM investigations of surface spread synaptonemal complexes in a human male carrier of a pericentric inversion inv(13)(p12q14): the role of heterosynapsis for spermatocyte survival

Prdm9 deficiency of rat oocytes causes synapsis among non-homologous chromosomes and aneuploidy

Aneuploidy (abnormal chromosome number) accompanies reduced ovarian function in humans and mice, but the reasons behind this concomitance remain underexplored. Some variants in the human gene encoding histone-3-lysine-4,36-trimethyltransferase PRDM9 are associated with aneuploidy, and other variants with ovarian function reduced by premature ovarian failure (POF), but no link between POF and aneuploidy has been revealed. SHR/OlaIpcv rat females lacking PRDM9 manifest POF-a reduced follicle number, litter size, and reproductive age. Here, we explored this model to test how POF relates to oocyte euploidy. The mutant rat females displayed increased oocyte aneuploidy and embryonic death of their offspring compared to controls. Because rat PRDM9 positions meiotic DNA breaks, we investigated the repair of these breaks. Fertile control rodents carry pachytene oocytes with synapsed homologous chromosomes and repaired breaks, while sterile Prdm9-deficient mice carry pachytene-like oocytes with many persisting breaks and asynapsed chromosomes. However, most PRDM9-lacking rat oocytes displayed a few persisting breaks and non-homologous synapsis (NHS). HORMAD2 protein serves as a barrier to sister-chromatid repair and a signal for the synapsis and DNA repair checkpoints. NHS but not asynapsis was associated with HORMAD2 levels similar to the levels on rat pachytene chromosomes with homologous synapsis. NHS was accompanied by crossing-over decreased below the minimum that is essential for euploidy. We argue that the increased mutant rat aneuploidy is due to NHS, which allows some oocytes to pass meiotic checkpoints without one crossing-over per chromosomal pair, leading to segregation errors, and thereby NHS links POF to aneuploidy.

Synapsis in grasshopper bivalents heterozygous for centric shifts

Analysis of surface-spread synaptonemal complexes of zygotene and pachytene spermatocytes was carried out on centric-shift heterozygotes of grasshoppers. These rearrangements affected the M7 chromosome in Chorthippus vagans and the M6 and S8 chromosomes in Chorthippus apricarius. The shifts in the latter two chromosomes were also associated with C-heterochromatin variations between homologous chromosomes. Rearranged chromosomes proceeded directly to heterosynapsis without an apparent intervening homosynaptic phase in M7 bivalents of Ch. vagans and M6 bivalents of Ch. apricarius. In the latter case, axial equalization of the heterochromatin polymorphism was also achieved. On the other hand, asynapsis of the intercentromeric regions throughout pachytene was the rule in the centric shift involving the S8 chromosome of Ch. apricarius. In the three cases analysed, the production of unbalanced gametes in the heterozygotes is precluded either by the lack of chiasma formation in heterosynapsed rearranged segments or by the lack of pairing between such segments. Chiasmata were limited to the homologous regions of the heteromorphic bivalents.

Impact of pericentric inversion of Chromosome 9 [inv (9) (p11q12)] on infertility

BACKGROUND

One of the frequent occurrences in chromosome rearrangements is pericentric inversion of the Chromosome 9; inv (9) (p11q12), which is consider to be the variant of normal karyotype. Although it seems not to correlate with abnormal phenotypes, there have been many controversial reports indicating that it may lead to abnormal clinical conditions such as infertility. The incidence is found to be about 1.98% in the general population.

MATERIALS AND METHODS

We investigated the karyotypes of 300 infertile couples (600 individuals) being referred to our infertility clinic using standard GTG banding for karyotype preparation.

RESULTS

The chromosomal analysis revealed a total of 15 (2.5%) inversions, among these, 14 male patients were inversion 9 carriers (4.69%) while one female patient was affected (0.33%). The incidence of inversion 9 in male patients is significantly higher than that of normal population and even than that of female patients (P< 0.05).

CONCLUSIONS

This result suggests that inversion 9 may often cause infertility in men due to spermatogenic disturbances, which are arisen by the loops or acentric fragments formed in meiosis.

Synapsis and recombination in inversion heterozygotes

Inversion heterozygotes are expected to suffer from reduced fertility and a high incidence of chromosomally unbalanced gametes due to recombination within the inverted region. Non-homologous synapsis of the inverted regions can prevent recombination there and diminish the deleterious effects of inversion heterozygosity. The choice between non-homologous and homologous synapsis depends on the size of inversion, its genetic content, its location in relation to the centromere and telomere, and genetic background. In addition, there is a class of inversions in which homologous synapsis is gradually replaced by non-homologous synapsis during meiotic progression. This process is called synaptic adjustment. The degree of synaptic adjustment depends critically on the presence and location of the COs (crossovers) within the inversion loop. Only bivalents without COs within the loop and those with COs in the middle of the inversion can be completely adjusted and became linear.

Chromosomal abnormalities, meiotic behavior and fertility in domestic animals

Since the advent of the surface microspreading technique for synaptonemal complex analysis, increasing interest in describing the synapsis patterns of chromosome abnormalities associated with fertility of domestic animals has been noticed during the past three decades. In spite of the number of scientific reports describing the occurrence of structural chromosome abnormalities, their meiotic behavior and gametic products, little is known in domestic animal species about the functional effects of such chromosome aberrations in the germ cell line of carriers. However, some interesting facts gained from recent and previous studies on the meiotic behavior of chromosome abnormalities of domestic animals permit us to discuss, in the frame of recent knowledge emerging from mouse and human investigations, the possible mechanism implicated in the well known association between meiotic disruption and chromosome pairing failure. New cytogenetic techniques, based on molecular and immunofluorescent analyses, are allowing a better description of meiotic processes, including gamete production. The present communication reviews the knowledge of the meiotic consequences of chromosome abnormalities in domestic animals.

Chromosome segregation in an infertile man carrying a unique pericentric inversion, inv(21)(p12q22.3), analysed using fluorescence in situ hybridization on sperm nuclei: significance for clinical genetics. A case report

We report the case of a 40-year-old patient referred to our centre after 3 years of infertility. Karyotyping with the aid of fluorescence in situ hybridization (FISH) analysis showed a unique pericentric inversion of chromosome 21:46,XY,inv(21)(p12q22.3). This type of intrachromosomal structural rearrangement can lead to chromosome imbalance in offspring by producing unbalanced gametes if an odd number of crossover events occur within the inverted segment. Therefore, partial trisomy/monosomy with clinical consequences can be observed in the progeny of carriers. Semen samples from the inversion carrier were analysed by FISH using a combination of probes [a subtelomeric 21q probe and a locus-specific Down's syndrome critical region (DSCR) probe] to evaluate the proportion of recombinant chromosomes. Sperm-FISH analysis of 3400 spermatozoa revealed a 67.4% rate of balanced chromosomes (normal or inverted). The frequencies of recombinant chromosomes with duplication of the long arm and deletion of the short arm, and vice versa, were 11.2 and 21.4%, respectively. The risk for the couple of conceiving a child with an unbalanced chromosome 21 is estimated to be around 32%. This case study shows the utility of sperm-FISH analysis in the genetic counselling of a pericentric inversion in a male carrier to assess the frequency of recombinant chromosomes and therefore evaluate the probability of having a normal conception.

Altered patterns of meiotic recombination in human fetal oocytes with asynapsis and/or synaptonemal complex fragmentation at pachytene

Meiotic recombination was analysed in human fetal oocytes to determine whether recombination errors are associated with abnormal chromosome synapsis. Immunostaining was used to identify the synaptonemal complex (SC, the meiosis-specific proteinaceous structure that binds homologous chromosomes) and the DNA mismatch repair protein, MLH1, that locates recombination foci. It was found that 57.1-74.2% of zygotene oocytes showed fragmentation and/or defective chromosome synapsis. Fewer such abnormal cells occurred at pachytene (15.8-28.9%). MLH1 foci were present from zygotene to diplotene in both normal and abnormal oocytes. However, the proportions of oocytes having MLH1 foci, and mean numbers of foci per oocyte, were both lower in abnormal oocytes. Oocytes with fragmented SC had more foci than those with synaptic anomalies. Analysis of chromosomes 13, 18, 21 and X by fluorescence in-situ hybridization (FISH) did not implicate particular chromosomes in recombination deficiency. These observations indicate that recombination is disturbed in oocytes with SC fragmentation and/or synaptic abnormalities during meiotic prophase I. Such disturbances might be a risk factor for selection of fetal oocytes for atresia, as occurs for homologous chromosome pairing. Recombination errors may potentially increase the risk of abnormal chromosome segregation in oocytes that survive and contribute to the reserve in the mature ovary.

Sperm studies in heterozygote inversion carriers: a review

The risk of producing unbalanced gametes in heterozygous inversion carriers mostly depends on the occurrence of recombination events within the inverted segment. Recombination determines the possibility of producing chromosomes with duplications/deficiencies (pericentric inversions) or with duplications/deficiencies which furthermore appear as dicentric and acentric fragments (paracentric inversions). In this work, a general description of the close relationship between the occurrence of crossovers in pericentric and paracentric inversions and the final segregation outcome is presented. After this introduction, a compilation of inversion segregation data and interchromosomal effect results from previously published sperm studies have been reviewed. Segregation results indicate a great heterogeneity in the percentage of unbalanced gametes, from 0 to 37.38%. The size of the inverted segments and their proportion in the chromosome are two parameters closely related with the incidence of recombination (P < 0.0001; using a quadratic model and Pearson's correlation test). These results suggest that the production of a significant level of unbalanced gametes would require a minimum inversion size of 100 Mbp and the inversion of at least 50% of the chromosome. Interchromosomal effects are seldom observed in chromosomal inversions. Finally, implications of the meiotic behavior of the inversions in the progeny of the carriers and the incorporation of sperm FISH segregation analysis for reproductive genetic counseling are discussed.

Meiotic abnormalities in infertile males

Meiotic anomalies, as reviewed here, are synaptic chromosome abnormalities, limited to germ cells that cannot be detected through the study of the karyotype. Although the importance of synaptic errors has been underestimated for many years, their presence is related to many cases of human male infertility. Synaptic anomalies can be studied by immunostaining of synaptonemal complexes (SCs), but in this case their frequency is probably underestimated due to the phenomenon of synaptic adjustment. They can also be studied in classic meiotic preparations, which, from a clinical point of view, is still the best approach, especially if multiplex fluorescence in situ hybridization is at hand to solve difficult cases. Sperm chromosome FISH studies also provide indirect evidence of their presence. Synaptic anomalies can affect the rate of recombination of all bivalents, produce achiasmate small univalents, partially achiasmate medium-sized or large bivalents, or affect all bivalents in the cell. The frequency is variable, interindividually and intraindividually. The baseline incidence of synaptic anomalies is 6-8%, which may be increased to 17.6% in males with a severe oligozoospermia, and to 27% in normozoospermic males with one or more previous IVF failures. The clinical consequences are the production of abnormal spermatozoa that will produce a higher number of chromosomally abnormal embryos. The indications for a meiotic study in testicular biopsy are provided.

Unusual segregation products in sperm from a pericentric inversion 17 heterozygote

Chromosome segregation and interchromosomal effect were studied in spermatozoa from a carrier of a pericentric chromosome 17 inversion, 46,XY,inv(17)(p13.1q25.3). Sperm chromosome segregation, lymphocytes of the inversion carrier, and cells from his offspring were analysed by multicolour fluorescence in situ hybridization. The frequency of balanced sperm was 73%. An unusual segregation of recombinants was observed, viz. deletion of the p arm (14.6%) or duplication of the p arm with the presence of one q arm (8.4%), instead of the expected recombinants, viz. duplication of one arm with deletion of the other and vice versa. These unusual recombinants were explained by the position of the 17q breakpoint, which was between the q arm telomere-associated repeats and the unique q subtelomere region. The offspring of the donor were found to have a 17p deletion including the Miller-Dieker critical region, similar to the most frequent recombinant sperm class. The disomy frequency was significantly increased for chromosome 17 compared with other autosomes, suggesting that pairing and recombination of the inversion may predispose to non-disjunction. There was no significant difference between the frequencies of aneuploidy for chromosomes 13, 21, X and Y in the chromosome inversion heterozygote compared with controls. Thus, this unique pericentric inversion of chromosome 17 produces unusual recombinant products; no evidence was apparent of an interchromosomal effect in any of the tested chromosomes.

Analysis of chromosome segregation in sperm from a chromosome 2 inversion heterozygote and assessment of an interchromosomal effect

Using fluorescence in situ hybridization (FISH) analysis, the chromosome segregation of a pericentric inversion of chromosome 2 was studied in spermatozoa. An interchromosomal effect (ICE) was also determined for chromosomes 13, 21, X, and Y. This chromosome inversion included more than 2/3 of the total length of the chromosome and the breaks points were in G-light bands. The frequency of non-recombinant sperm was 55.9%, and that of recombinant sperm was 34.5% (with a 1:1 ratio of duplication of the p arm and deletion of the q arm and vice versa). There was a significantly increased frequency of disomy for chromosome 2 (0.6%) compared to the other autosomes, suggesting that pairing and recombination of the inversion may predispose to nondisjunction. There was no significant difference between the frequencies of aneuploidy for chromosomes 13, 21, X, and Y for the chromosome inversion heterozygote compared to control donors. Thus we did not find evidence for an ICE.

Increased incidence of meiotic anomalies in oligoasthenozoospermic males preselected for intracytoplasmic sperm injection

PURPOSE

Based on data from the literature, to detect the possible presence of an increased frequency of meiotic anomalies in oligoasthenozoospermic (OA) patients preselected for intracytoplasmic sperm injection.

METHODS

Meiotic studies in as many successive patients with a clinical indication for a diagnostic testicular biopsy as needed to complete at least 100 cases with a severe OA (motile sperm concentration < or = 1.5 x 10(6)/ml).

RESULTS

An increased incidence of meiotic anomalies was found in 102 patients with a severe OA (17.6%) compared to the mean for 105 patients with other etiologies in the series (5.7%) or the mean for patients reviewed in the literature (6.5%).

CONCLUSIONS

Patients with a severe OA have a higher incidence of synaptic anomalies. This may result in the malsegregation of chromosomes at meiosis I, producing abnormal sperm, and could explain the high incidence of sterility and some cases of abortion (in two thirds of the couples with abortions the husband had meiotic anomalies) in this group.

Screening for abnormalities of chromosomes X, Y, and 18 and for diploidy in spermatozoa from infertile men participating in an in vitro fertilization-intracytoplasmic sperm injection program

OBJECTIVE

To evaluate the frequency of disomy (for chromosomes X, Y, and 18) and of diploidy in the spermatozoa of infertile men undergoing intracytoplasmic sperm injection (ICSI).

DESIGN

Prospective analysis of sperm nuclei by fluorescence in situ hybridization (FISH).

SETTING

University-affiliated IVF-ICSI program.

PATIENT(S)

Semen samples from 19 patients participating in an IVF-ICSI program.

INTERVENTION(S)

Semen samples were analyzed and prepared for FISH.

MAIN OUTCOME MEASURE(S)

Semen parameters were evaluated. The frequency of disomy for chromosomes X, Y, and 18 and the frequency of diploidy were analyzed by FISH.

RESULT(S)

A total of 9,373 spermatozoa from 19 infertile patients were analyzed and compared with spermatozoa from a control group of 5 healthy men. No differences in the frequency of disomy 18 were found, but statistically significant differences in the incidence of sex chromosome disomy and of diploidy were observed.

CONCLUSION(S)

The study of sperm nuclei by FISH is useful to improve genetic counseling in infertile patients selected for ICSI.

Genetic analysis of meiotic recombination in humans by use of sperm typing: reduced recombination within a heterozygous paracentric inversion of chromosome 9q32-q34.3

To investigate patterns of genetic recombination within a heterozygous paracentric inversion of chromosome 9 (46XY inv[9] [q32q34.3]), we performed sperm typing using a series of polymorphic microsatellite markers spanning the inversion region. For comparison, two donors with cytogenetically normal chromosomes 9, one of whom was heterozygous for a pericentric chromosome 2 inversion (46XY inv[2] [p11q13]), were also tested. Linkage analysis was performed by use of the multilocus linkage-analysis program SPERM, and also CRI-MAP, which was adapted for sperm-typing data. Analysis of the controls generated a marker order in agreement with previously published data and revealed no significant interchromosomal effects of the inv(2) on recombination on chromosome 9. FISH employing cosmids containing appropriate chromosome 9 markers was used to localize the inversion breakpoint of inv(9). Analysis of inv(9) sperm was performed by use of a set of microsatellite markers that mapped centromeric to, telomeric to, and within the inversion breakpoints. Three distinct patterns of recombination across the region were observed. Proximal to the centromeric breakpoint, recombination was similar to normal levels. Distal to the telomeric breakpoint, there was an increase in recombination found in the inversion patient. Finally, within the inversion, recombination was dramatically reduced, but several apparent double recombinants were found. A putative model explaining these data is proposed.

Combined immunocytogenetic and molecular cytogenetic analysis of meiosis I oocytes from normal human females

The microspread oocytes of three fetuses, two of 16 weeks gestation and one of 15 weeks gestation, were labelled with a combination of anti-lateral element antiserum and a human centromere labelling auto-immune serum. The anti-lateral element serum was found to label both asynapsed axial elements and synapsed lateral elements strongly. Nuclei were found from leptotene to diplotene in all three fetuses. The use of the human auto-immune serum led to the observation of 'staggered centromeres' and 'centromeric associations' as well as tightly clustered centromeres in 'stellar nuclei'. Nuclei displaying various aberrant features were detected. The use of antibody-labelled microspread oocytes as substrates for fluorescence in situ hybridisation (FISH) was found to be reliably successful only with repetitive (centromeric and telomeric) probes.

Case report of rec(7)dup(7q)inv(7)(p22q22) and a review of the recombinants resulting from parental pericentric inversions on any chromosomes

We report a rare case of duplication for 7q22 --> 7qter and deletion for 7p22 --> 7pter, resulting from a meiotic recombination of a paternal pericentric inversion, inv(7)(p22q22). The newborn boy had the 7q trisomy syndrome. In addition, the diagnosis of chondrodysplasia punctata was made from lumbar and hand X-ray films taken soon after birth. Only two cases of rec(7)dup(7q), both in a single family, have been reported previously. We review 133 offspring with recombinations resulting from pericentric inversions on any chromosomes reported between 1981 and 1995. Of the 133 cases, 110 had a long-arm duplication and short-arm deletion, while only 23 had a short-arm duplication and long-arm deletion. In 85 of the 133 cases, the mother was an inversion carrier (five carriers had two affected offspring), and in 46, the carrier was a father (one carrier had three affected offspring). Kaiser [Hum Genet 1984;68:1-47] reviewed 63 offspring with recombinations derived from a parental pericentric inversion reported between 1972 and 1981. In both surveys, recombinations resulting from pericentric inversions of chromosomes 1, 12, 19, and Y were not found.

Pericentric inversion of chromosome 4 giving rise to dup(4p) and dup(4q) recombinants within a single kindred

Theoretically, every pericentric inversion can give rise, during meiosis, to 2 alternate recombinant chromosomes. One of these will have a duplication of short arm material and deletion of long arm material (dup p), and the other, a duplication of a long arm material and deletion of short arm material (dup q). However, most published cases have been limited to a single recombinant type occurring within a given kindred. Here we document a large pericentric inversion of chromosome 4 which gave rise, within 2 generations of a kindred, to both dup p and dup q recombinants. The family was ascertained by the birth of a baby girl with multiple congenital anomalies suggestive of Wolf-Hirschhorn syndrome, and was found to have a dup 4q recombinant. Subsequent studies of her father and of her 27-year-old mentally retarded aunt showed a balanced inv(4) (p15.32q35) and a dup 4p recombinant, respectively. Given that: (a) the balanced inversion involves approximately 87% of the length of chromosome 4; (b) the predicted meiotic pairing would be homosynapsis with loop formation; (c) the size of the segments distal to the breakpoints of the inversion are of similar and relatively small size; and (d) both recombinants are compatible with life, then the risk for recurrence of a recombinant in this family is high. Genetic counseling addressed these issues, and to date, both chronic villus sampling (CVS) and amniocentesis have been provided for prenatal diagnosis.

Synaptic interrelationships between the segments of the heteromorphic bivalent in double heterozygotes for paracentric inversions in chromosome 1 of the house mouse

Electron microscopic analysis of synaptonemal complexes in double heterozygotes for the partially overlapping inversions In(1)1Rk and In(1)12Rk in chromosome 1 of the house mouse was carried out. A great variety of synaptic configurations with complicated combinations of homologously and non-homologously paired segments was observed. Analysis of these configurations revealed at least five independent pairing regions in chromosome 1. Interrelationships between these regions with respect to their pairing ability were estimated. Pairings in the distal non-inverted segment and in inversions inhibit each other, while pairing in either inverted segment facilitates synapsis in the other. In other words, pairing initiations in different parts of the same bivalent are not independent events.

Infertility in human males with autosomal translocations: meiotic study of a 14;22 Robertsonian translocation

Pachytene analysis was undertaken in a male patient heterozygous for a 14q22q Robertsonian translocation. The relatively low rate of XY autosome association led us to examine the relationships existing between the chromosomes involved in the translocation, the rate of XY-autosome association and the degree of spermatogenic failure. Cytogenetic investigations in infertile men and the results of the meiotic studies suggest a direct correlation between the frequency of XY-autosome association at pachytene and the degree of spermatogenic failure. Whether associations arise as a consequence or cause of germ cell failure is still not certain.

Synapsis in single and double heterozygotes for partially overlapping inversions in chromosome 1 of the house mouse

Electron microscopic (EM) analysis of synaptonemal complexes (SC) in single and double heterozygotes for the partially overlapping inversions In(1)1Icg, In(1)1Rk and In(1)12Rk in chromosome 1 of the house mouse reveals that synapsis and synaptic adjustment are dependent on the size and location of the inversions and interaction between the latter. Is(1)1Icg contains insertions of the inverted repeats Is(HSR;1C5)1Icg and Is(HSR;1D)2Icg and an inverted euchromatic region. Synaptic adjustment of the D-loops by shortening of the asynapsed segments of the lateral elements belonging to the insertions occurs at the late zytogene to early pachytene stage. Synaptic adjustment of the inversion loops takes place at early to late pachytene. A delay in adjustment was found in the double heterozygotes In(1)1Icg/In(1)1Rk and In(1)1Icg/In(1)12Rk. A correspondence between the lifespan of asynapsis in inverted regions and the probability of association of XY and heteromorphic bivalents was revealed.

Correlation between chromosomal breakpoint positions and synaptic behaviour in human males heterozygous for a pericentric inversion

The examination of synaptic data and localization of chromosomal breakpoints in a review of human pericentric inversions suggest that synaptic and recombination behaviour in rearranged chromosomes during meiosis can be predicted by determining the subband in which the breakpoint is located. According to this hypothesis, it can be postulated that loops in pericentric inversions are routinely formed only in cases when both breaks occur in G-light bands, with the genetic consequences of crossing-over. In other cases, heterosynapsis is accomplished without previous homosynapsis, thereby minimizing the production of unbalanced gametes.

Chromosomally derived sterile mice have a 'fertile' active XY chromatin conformation but no XY body

We have previously shown that the sex chromosome bivalent of normal, fertile male mice possesses extensive regions of potentially active chromatin, even though, as has been shown by others, certain X-linked genes, and perhaps most of the X chromosome, become inactivated during pachytene. The male meiosis of a fertile (2;11) translocation carrier mouse, a chromosomally derived sterile (11; 19) translocation carrier and that of normal mice is compared. In situ nick translation shows a similar DNase I sensitivity pattern in the sex chromosomes of all examined mice. The X chromosome has four regions of potentially active chromatin conformation, two at the ends of the chromosome and two interstitial ones, coinciding with flexures which become prominent towards late pachytene. The Y chromosome is almost uniformly sensitive to DNase I. The similarity of chromatin conformation patterns in fertile and sterile mice is compatible with the hypothesis that unscheduled transcription of particular genes, possibly included in the active conformation regions, occurs in mice which become sterile. In the sterile (11;19) translocation carrier, a vast majority of all pachytenes are "associated": usually one unpaired segment of chromosome 19 is in end-to-end contact with the X chromosome. The tips of both unpaired segments of chromosome 19 have a thickened axis and display a peculiar chromatin appearance, similar to the modification of the centromeric tip of the X chromosome. Telomeric unpairedness of certain chromosome segments seems to be conducive to autosome-X chromosome association. We suggest that compartmentalization of the nucleus into an autosome mass and a fully developed, protruding, metabolically quiescent XY body, is a precondition for the normal progressing of meiosis. In the associated cells, the autosomal quadrivalent anchors the XY bivalent among the autosomes; as a consequence no XY body is formed. This interference with the course of compartmentalization leads to the abolishment of inactivation of part or all of the potentially active genes and results in meiotic arrest, and hence in sterility.

Spreading synaptonemal complexes from Zea mays. I. No synaptic adjustment of inversion loops during pachytene

Four different inversion heterozygotes of maize were examined for the occurrence of synaptic adjustment. Three substages of pachytene were identified in synaptonemal complex (SC) spreads using side-by-side comparisons of chromosome squashes with two-dimensional spreads of SCs. In SC spreads, inversion loop frequency did not change substantially from early through late pachytene for any of the four inversion heterozygotes examined. In addition, the position and size of the inversion loops remained essentially constant throughout pachytene. These results indicate that synaptic adjustment of inversion loops does not occur during pachytene in Zea mays.