New methodologic approaches and perspectives in experimental cytogenetics of embryonic mammalian development

When using heterozygotic carries of reciprocal or Robertsonian translocations and new methods permitting to identify homologous chromosomes in the karyotype, it is possible to obtain mouse embryos with trisomy and monosomy of any of the autosomes. This allows to analyze the effect of excess or deficiency of genetic material concerning definite linkage groups on morphogenetic processes. The cleavage and formation of blastocyst are not controlled by the zygote chromosomes; beginning from the late blastocyst stage in Muridae all or almost all autosomes display the functional activity. At these stages embryos with monosomy are eliminated whereas the embryos with autosome trisomy survive till later stages. No dependence was found between the time of embryonic death and the size of autosome in excess. The importance of data obtained by means of new methods of cytogenetic analysis is discussed.     

Chromosomal control of early embryonic development in mice I. Experiments on embryos with autosomal monosomy

The effects of autosomal monosomy on early embryonic development were studied in mice with Robertsonian and reciprocal (non-Robertsonian) translocations. It was found that monosomy for autosomes 1, 2, 3, 5, 6, 16, 17 and 19 had substantially different effects on preimplantation development and survival of mouse embryos. Monosomy for autosomes 1, 3, 6, 16 and 19 does not affect cleavage, compaction and blastulation and in some cases is compatible with implantation. Most of these embryos, however, die as early blastocysts (Ms 3, 6 or 19) and some of them are eliminated at early postimplantation stages (Ms 1 or 16). The embryos with monosomy for autosomes 2, 5 or 17 can be identified during cleavage owing to the reduced blastomere number and pathological changes in the nuclei. Most of these monosomies do not survive beyond the morula stage. The results indicate that differential genetic activity of autosomes in mice becomes already evident in very early embryonic development. A hypothetical mechanism for homologous autosome activation at the onset of embryonic development in mice is suggested.     

Role of various portions of chromosome 17 in the early embryogenesis of the laboratory mouse

Peculiarities of the early development of mice embryos with structural aberrations of chromosome 17 were studied in mice heterozygous for reciprocal translocations T (16; 17) 43H and T (9; 17) 138Ca. Deficiency for the distal part of chromosome 17 corresponding to the E-region (Df 17 E1-E5), as well as deficiency for its most proximal part, the AB region (Df 17 A1-A-3, B) carrying all genes of complex T-locus, does not block cleavage, blastulation and implantation but severely affects immediate postimplantation development and causes embryonic death during early neurulation. Deficiency for the middle part of chromosome 17, including Giemsa-positive band 17C and the most of Giemsa-negative band D, becomes evident just after a few cleavage divisions and all these embryos die at the morula stage (8-16 blastomeres). It has been concluded that genes of the CD region in chromosome 17 are of major importance for genetic control of the early development in laboratory mice. The possible causes for early embryonic death are discussed in connection with the genetic map of chromosome 17 and a hypothesis of mutual activation of homologous autosomes and their segments at initial stages of embryogenesis is suggested.     

Cytogenetic and histologic analyses of spontaneous abortions

Summary In a study of spontaneous abortions the correlations between karyotype (166 cases), anamnestic data, and macroscopic and histologic findings in placentas (107 cases) and embryos (73 cases) were analyzed. The main results were: 1. The rate of chromosomal aberrations was 39%. Trisomies predominated (60%), followed by monosomy X (20%), triploidies (14%), and structural aberrations (6%). 2. In trisomies a clear prevalence of female sex constitution (2:1) was observed. In normal karyotypes a slight prevalence of females was seen (1.2:1). 3. With increasing maternal age, more trisomies were found in the abortions. 4. Women whose index abortion had a normal karyotype had a history of fewer births but more abortions. 5. Trisomies of acrocentric chromosomes were mainly chorionic sacs with an embryo, while trisomies of the other autosomes resulted in intact empty sacs. 6. The average developmental stage of the embryos was 5 weeks, with a mean gestational age of 14 weeks. Gross malformations were found in 58% of the embryos.     

XY pair associates with the synaptonemal complex of autosomal male-sterile translocations in pachytene spermatocytes of the mouse (Mus musculus)

Analysis of the chromosome behaviour at pachytene has been performed by means of the silver staining technique visualizing the synaptonemal complexes (SCs) in male mice heterozygous for the male-sterile translocations T(5;12)31H, T(16;17)43H and T(7;19)145H, respectively. The T(9;17)138Ca male heterozygotes and T43H/T43H homozygous males were used as fertile controls. The sterile mice displayed a high frequency (about 60%) of pachytene spermatocytes with autosomal translocation configuration located in close vicinity of the XY pair. The dense round body (XAB), normally located near the X-chromosome axis in fertile males, exhibited abnormal affinity to translocation configuration in the sterile translocation heterozygotes. The incomplete synapsis of autosomes involved in translocation configuration was observed in more than 70% of the pachytene spermatocytes with the male-sterile translocations but in less than 20% of the cells from T138Ca fertile male.s. A hypothesis relating the spermatogenic arrest of carriers of male-sterile rearrangements to the presumed interference with X chromosome inactivation in male meiosis is discussed.     

Contribution of chromosomal imbalance to sperm selection and pre-implantation loss in translocation-heterozygous Chinese hamsters

Chinese hamster stocks with various structurally abnormal chromosomes have been produced by X irradiation. Among these stocks, 18 with various reciprocal translocations were used to investigate the participation of unbalanced gametes in fertilization and the development of unbalanced embryos. Among males as well as females heterozygous for the same translocation, there is no difference in the frequency of each disjunctional class. The participation of chromosomally unbalanced gametes in fertilization was investigated by chromosomal analysis of meiotic cells in heterozygotes for the 18 reciprocal translocations and pronuclei of fertilized ova obtained from crossing these heterozygotes. Compared with the expected frequencies from MII scoring, the frequencies of male pronuclei having a common deficiency of chromosome 1 (1q17-->1q42) or chromosome 3 (3p23-->3q31) decreased significantly in one-cell embryos. However, the frequencies of male pronuclei with other abnormalities were all consistent with those expected from MII scoring. In contrast, the frequencies of female pronuclei with any karyotype including the same abnormalities as those decreased in male pronuclei from the translocation heterozygotes were all consistent with those estimated from MII scoring. These results revealed clearly that most gametes with nullisomies as well as disomies for any chromosomal segments may participate in fertilization, whereas only male gametes nullisomic for certain segments of chromosomes 1 and 3 failed to participate in fertilization. The zygotic selection of chromosomal imbalance was also investigated by direct chromosomal and morphological analyses of preimplantation embryos from crosses between karyotypically normal females and male heterozygotes from the 18 stocks with various reciprocal translocations. These analyses revealed that some embryos were arrested in development at the two-cell stage. The karyotype of these two-cell embryos had a common deficiency in a segment of chromosome 1 or chromosome 2. Embryos with partial monosomy including chromosomes 1, 3, 4 and 5 showed arrested development at four- to eight-cell stages. Among day 4 embryos, some chromosomally unbalanced embryos, mainly with a deficiency of segments of chromosomes 1p, 1q, 2q, 5q, 7q and 8, had fewer blastomeres than karyotypically normal and balanced embryos. The homology between Chinese hamster and mouse chromosomes relating to abnormal embryogenesis at early stages has been partially confirmed from reported maps of chromosomes. The Chinese hamster is useful for further cytogenetic studies during the stages of meiosis and early embryogenesis.     

Contribution of magnetic resonance imaging to the knowledge of CNS malformations related to chromosomal aberrations

Summary Twelve patients presenting with various clinicopathological syndromes related to chromosomal diseases have been evaluated using magnetic resonance imaging. They include patients with trisomy 21, trisomy 18, trisomy 13, 4p-syndrome, 5p-syndrome, and 7p-syndrome. In all these patients karyotype studies were performed demonstrating the chromosomal aberrations. All patients were examined using magnetic resonance imaging to evaluate the head and neck malformations which may be specifically associated with their chromosomal anomaly. We were particularly interested in brain abnormalities and the morphological findings correlated with some pathologic anatomical findings. A review of the literature on neuropathological data is reported and compared with the in vivo anatomical results obtained using this highly anatomical non-ionising and non-invasive investigative procedure. Particular interest is paid to trisomy 21 in which all recognizable stereotyped morphological skull and brain malformations are depicted with magnetic resonance and some other malformations demonstrated such as the excessive forward bending and ascension of the brainstem which correlated well with a simian cephalic organization.     

Analysis of meiotic segregation in a man heterozygous for two reciprocal translocations using the hamster in vitro penetration system.

Sperm chromosomal complements of a man heterozygous for two reciprocal translocations and exhibiting the karyotype 46,XY,t(5;11) (p13;q23.2),t(7;14)(q11;q24.1) were analyzed following in vitro fusion with golden hamster zona-free eggs (the hamster in vitro penetration [HIP] system). Products of alternate, adjacent 1, and 3:1 segregation at meiosis I of both translocation quadrivalents were recovered, and the analysis of their output, which was dissimilar between the two translocations, permitted prediction of probable sites of chiasma formation in the chromosomes involved in the translocation. These data, which comprise the first reported analysis of the products of two translocations in a single individual (hence, in a common genetic background), emphasize the uniqueness in genetic behavior of individual translocations; they further demonstrate the usefulness of the HIP system to carry out such studies.     

Chromosome homologies between tsessebe (Damaliscus lunatus) and Chinese muntjac (Muntiacus reevesi) facilitate tracing the evolutionary history of Damaliscus (Bovidae, Antilopinae, Alcelaphini)

Genome-wide homologies between the tsessebe (Damaliscus lunatus, 2n = 36) and Chinese muntjac (Muntiacus reevesi, 2n = 46) have been established by cross-species painting with Chinese muntjac chromosome paints. Twenty-two autosomal painting probes detected 35 orthologous segments in the tsessebe. Hybridization results confirmed that: (i) D. lunatus carries the (9;14) reciprocal translocation that has been proposed to be a derived chromosomal landmark shared by all species of the Antilopinae; (ii) the karyotype of D. lunatus can be derived almost exclusively from the bovid ancestral karyotype through 12 Robertsonian translocations involving 24 ancestral acrocentric autosomes; (iii) in addition to the Rb fusions, pericentric heterochromatic amplification has shaped the morphology of several of the D. lunatus chromosomes. Integrated analysis of these and published cytogenetic data on pecorans has allowed us to accurately discern the karyotype history of Damaliscus (D. lunatus; D. pygargus, 2n = 38; D. hunteri, 2n = 44). The phylogenomic relationships of 3 species reflected by specific chromosomal rearrangements were consistent with published phylogenies based on morphology, suggesting that chromosomal rearrangements have played an important role in speciation within the Alcelaphini, and that karyotype characters are valuable phylogenetic markers in this group.     

Further insights into the ancestral murine karyotype: the contribution of the Otomys-Mus comparison using chromosome painting

The African vlei rat, Otomys irroratus, comprises several distinct chromosomal races that may be grouped into two major cytogenetic clades. Recognition of these clades is underpinned by a complex chromosomal rearrangement involving three different autosomes in the unfused state. We have used unidirectional fluorescence in situ hybridization (FISH) of mouse chromosome-specific painting probes to molecularly define the components of this rearrangement as well as to establish the chromosomal homologies between the mouse and the vlei rat genomes. This has allowed for the detection of 41 autosomal segments of conserved synteny. Nine mouse chromosomes were conserved in toto (MMU3, 4, 6, 7, 11, 12, 14, 18, 19) with a further seven (MMU2, 5, 8, 9, 10, 13, 16) showing homology to two discrete regions in the vlei rat genome. Two mouse autosomes (MMU15, 17) correspond to three regions in O. irroratus with MMU1 being the most fragmented showing five sites of hybridization in this species. By mapping these data to published sequence-based phylogenies we are able to confirm most of the published putative ancestral murine chromosomal states. Our data further indicate that MMU15a+ MMU13b+MMU10b+MMU17b was present in the murine ancestral karyotype suggesting an ancestral 2n = 52 rather than the 2n = 54 previously postulated.     

Noninvasive Detection of Fetal Subchromosome Abnormalities via Deep Sequencing of Maternal Plasma

The purpose of this study was to determine the deep sequencing and analytic conditions needed to detect fetal subchromosome abnormalities across the genome from a maternal blood sample. Cell-free (cf) DNA was isolated from the plasma of 11 pregnant women carrying fetuses with subchromosomal duplications and deletions, translocations, mosaicism, and trisomy 20 diagnosed by metaphase karyotype. Massively parallel sequencing (MPS) was performed with 25-mer tags at approximately 10⁹ tags per sample and mapped to reference human genome assembly hg19. Tags were counted and normalized to fixed genome bin sizes of 1 Mb or 100 kb to detect statistically distinct copy-number changes compared to the reference. All seven cases of microdeletions, duplications, translocations, and the trisomy 20 were detected blindly by MPS, including a microdeletion as small as 300 kb. In two of these cases in which the metaphase karyotype showed additional material of unknown origin, MPS identified both the translocation breakpoint and the chromosomal origin of the additional material. In the four mosaic cases, the subchromosomal abnormality was not demonstrated by MPS. This work shows that in nonmosaic cases, it is possible to obtain a fetal molecular karyotype by MPS of maternal plasma cfDNA that is equivalent to a chromosome microarray and in some cases is better than a metaphase karyotype. This approach combines the advantage of enhanced fetal genomic resolution with the improved safety of a noninvasive maternal blood test.     

Satellited Y chromosomes: Structure,origin, and clinical significance

Summary Three cases of inherited satellited Y chromosomes (Yqs) were analysed using several cytogenetic techniques. The cytogenetic data of the 14 cases of Yqs chromosomes described to date were reviewed. All Yqs chromosomes carry an active nucleolus organizer region (NOR) in their long arm and must have developed from translocations involving the short arms of the acrocentric autosomes. The structure of the heterochromatic satellite region in the Yqs chromosomes shows conspicuous inter-familial differences; this permits the reconstruction of the translocations from which the various Yqs were derived. Some causal factors leading to the development of Yqs chromosomes are considered: the specific localization of the four satellite DNAs and highly methylated DNA sequences in the karyotype, and some new experimental data on the spatial arrangement of heterochromatic regions in interphase nuclei. These provide distinct evidence for a preferential involvement of the autosomes 15 and 22 in the translocations with the Y heterochromatin. All clinical reports documenting Yqs males born with malformations were reviewed. It appears that the presence of an extra NOR and NOR-associated heterochromatin in the Yqs chromosomes does not cause any phenotypic abnormalities (as long as the Y euchromatin is intact). The possibility that a Yqs chromosome predisposes to non-disjunction and/or to translocations of other chromosomes is discussed.     

Teratogenic effect of D-amphetamine sulphate: histodifferentiation and electrocardiogram pattern of mouse embryonic heart

Pregnant albino mice (ICR random-bred strain) received daily injections of 50 or 100 mg/kg body weight of D-amphetamine sulphate between days 9 and 11 of gestation. Parallel control animals were injected with saline solution. Treated mice were sacrificed on day 15 or 19 of gestation. The embryos were examined for gross malformations and direct embryonic ECG recordings were made; they were weighed, and their hearts were carefully dissected. Microscopic sections of the heart were stained with hematoxylin and eosin. It was found that high doses of D-amphetamine raised the incidence of mortality in the treated pregnant mice up to 40%. The resorption rate in the survivors was high (up to 58%) following the high dose of the drug. Up to 15% of the embryos from the treated groups showed gross malformations, including skeletal and eye malformations and exencephaly. The electrocardiogram (ECG) recordings in most 19-day-old embryos from both treated groups showed a pattern with prolonged Q-T interval (PQT), similar to that of control embryos in the intermediate developmental stages (days 14-16 of gestation). The ECG of control embryos (from day 17 on) resembled that of prenatal fetuses. Microscopically, the hearts of treated embryos showed a large number of undifferentiated cardiac myoblasts. It can be inferred that high doses of D-amphetamine affect embryonic development generally and delay the histodifferentiation of the myocardium, resulting in incomplete maturation of the cardiac muscles, thus leading to the immature ECG pattern, with PQT intervals.     

Chromosomal control of early mammalian development

A review of recent studies on mammalian embryos, mostly mice, with chromosomal aberrations. Morphological, biochemical and cytological studies on mice with polyploidy, aneuploidy and some structural aberrations are discussed. Some types of chromosomal aberrations, especially monosomy for individual chromosomes (2, 5, 7, or 17), are already evident during early cleavage and are inevitably lethal by the morula stage. A direct relationship exists between the duration of survival and chromosome aberrations (trisomy and monosomy) for every chromosome. Differential gene activity of the mouse autosomes becomes evident already at the very early developmental stages. Some feasible causes of the early death of embryos with autosomal monosomy are discussed and a hypothetical mechanism for the activation of homologous autosomes at the early developmental stages is proposed. Perspectives of future studies in cytogenetics of mammalian development are outlined.     

Transmission of chromosomal abnormalities: participation of chromosomally unbalanced gametes in fertilization and early development of unbalanced embryos in the Chinese hamster

Using 14 Chinese hamster stocks with various reciprocal translocations, chromosomally unbalanced gametes were produced and used to investigate the participation of the unbalanced gametes in fertilization and the development of unbalanced embryos. The selection of chromosomally abnormal gametes during fertilization was investigated by the chromosomal analysis of meiotic cells in heterozygotes for the 14 reciprocal translocations and pronuclei of fertilized ova obtained from crossing these heterozygotes. Compared with the expected frequencies from meiotic metaphase II (MII) scoring, the frequencies of male pronuclei having commonly a deficiency of chromosome 1 (q14-->q42) or chromosome 3 (p23-->q31) in one-cell embryos decreased significantly. However, the frequencies of male pronuclei with other abnormalities were all consistent with those expected from MII scoring. In contrast, the frequencies of female pronuclei with any karyotype including the same ones, as those decreased in male pronuclei from the translocation heterozygotes were all consistent with those estimated from MII scoring. These results suggest that gametes with nullisomies as well as disomies for any chromosomal segments may mostly participate in fertilization, whereas some sperm nullisomic for the specific segments of chromosomes 1 and 3 may fail to fertilize. On the other hand, the zygotic selection of chromosomal imbalance was investigated by direct analyses of pre-implantation embryos from crosses between chromosomally normal females and male heterozygotes from the 14 stocks with various reciprocal translocations. The chromosomal and morphological analysis revealed that some embryos were arrested in development at the two-cell stage and their common abnormality was partial monosomy for chromosome 1 or 2. Embryos with partial monosomy including chromosomes 1, 3 and 4 showed arrested development at four-eight-cell stages. Among day 4 embryos, some chromosomally unbalanced embryos, mainly with a deficiency of other segments, such as chromosomes 1p, 2q, 5q and 8, had fewer blastomeres than karyotypically normal and balanced embryos. The homology between the mouse and the Chinese hamster chromosomes relating to the developmental abnormalities at early stages was partially confirmed.     

Synapsis and chiasma distribution in mice heterozygous for translocations in chromosomes 16 and 17

Electron microscopic analysis of synaptonemal complexes and analysis of chiasmata distribution in male mice heterozygous for Robertsonian translocation T(16; 17)7Bnr - (Rb7), for synaptonemal reciprocal translocation T(16;17)43H - (T43), in double heterozygotes for these translocations and in males with partial trisomy of the proximal region of chromosome 17 was carried out. Synaptic disturbances around the breakpoints of the translocations, such as asynapsis of homologous regions of partners and non-homologous synapsis of centromeric regions of acrocentric chromosomes, were revealed. Synaptic regularity in the proximal part of the chromosome 17 appeared to be affected by no t12 haplotype. Good coincidence between sizes of mitotic chromosomes and corresponding lateral elements of synaptonemal complexes was found for all chromosomes, with the exception of Rb7 in trisomics. In the latter karyotype, the proximal part of chromosome 17 involved in Robertsonian fusion seems to be shortened in the course of zygotene and never synapted with homologous segment of neither the acrocentric chromosome 17 nor large product of reciprocal translocation. Drastic increase in chiasmata frequency in the proximal part of chromosome 17 was revealed in heterozygotes for T43H and in trisomics, as compared with the double heterozygotes Rb7/T43. The latter finding was explained by the existence of two independent pairing segments in the former karyotypes.     

Chromosomal expression and localization of aphidicolin-induced fragile sites in the standard karyotype of river buffalo (Bubalus bubalis)

The present study reports on the chromosomal expression and localization of aphidicolin-induced fragile sites in the standard karyotype of river buffalo (Bubalus bubalis, 2n = 50) with the aim of establishing a 'fragile site map' of the species. Totally, 400 aphidicolin-induced breakages were analyzed from eight young and clinically healthy animals, four males and four females; these breakages were localized in 106 RBG-negative chromosome bands or at the band-interband regions. The number of breakages per chromosome did not vary statistically 'among' the animals investigated but the differences among individual chromosomes were highly significant thus indicating that the chromosomal distribution of the breakages is not random and appears only partially related to chromosome length. Fragile sites were statistically determined as those chromosomal bands showing three or more breakages. In the river buffalo karyotype, 51 fragile sites were detected and localized on the standardized ideogram of the species. The most fragile bands were as follows: 9q213 with 24 breakages out of 400; 19q21 with 16, 17q21 and inacXq24 with 15, 15q23 with 13 and 13q23 with 12 breaks, respectively. Previous gene mapping analysis in this species has revealed that the closest loci to these fragile sites contain genes such as RASA1 and CAST (9q214), NPR3 and C9 (19q19), PLP and BTK (Xq24-q25), OarCP09 (15q24), and EDNRB (13q22) whose mutations are responsible for severe phenotypic malformations and immunodeficiency in humans as well as in mice and meat quality in pigs. Further cytogenetic and molecular studies are needed to fully exploit the biological significance of the fragile sites in karyotype evolution of domestic animals and their relationships with productive and reproductive efficiency of livestock.     

Structural rearrangements of chromosomes in the domestic chicken: experimental production by X-irradiation of spermatozoa

In order to produce chicks heterozygous for structural aberrations of chromosomes, 67 hens were inseminated with semen that had been exposed to 1200 R of X-rays. A sample of 204 chicks was hatched and survived. Among these, 18 (8.9%) contained rearrangements comprising 19 translocations and one pericentric inversion. All 10 males and eight females heterozygous for rearrangements were fertile and transmitted these rearrangements to approximately half their hatched progeny. Each of the major chromosomes of the chicken karyotype, except number 6, was involved in one or more of the translocations. The pericentric inversion was of a segment of chromosome number 2.     

Mitosis in embryonic trisomy 1 mouse eye

Trisomy 1 embryos consistently show eye defects (e.g., aphakia, microphakia, retention of lens stalk). To determine if the plane of division of mitotic figures is abnormal in the eyes of these animals, trisomic embryos (9.5 through 12 gestational days) were produced from mice doubly heterozygous for Robertsonian translocation chromosomes [Rb(1.3)/Rb(1.10)]. To accommodate for the known delay in trisomic embryo development, animals were grouped according to stages of eye development rather than to gestational age. Serial sections were evaluated without knowledge of karyotype for orientation of mitotic figures (parallel, perpendicular, oblique) in lens, optic cup, and diencephalon. Location of mitotic figures was scored as apical (nearest the lumen), middle, and basal. Numerous anomalies were noted in trisomic eye development. No difference was found between orientation of mitotic figures in the lens and optic cup of trisomy 1 and normal embryos. Location of mitotic figures in trisomy 1 lens was significantly different from that of normal littermates. The data confirm earlier studies that trisomy 1 affects the eye, and they tend to corroborate evidence that this trisomy affects the lens more than it affects the optic cup.     

A chromosomal analysis of some water beetle species recently transferred from Agabus Leach to Ilybius Erichson, with particular reference to the variation in chromosome number shown by I. montanus Stephens (Coleoptera: Dytiscidae)

The karyotypes of seven Ilybius species are described and illustrated. All except I. wasastjernae have a basic karyotype of 34 autosomes plus sex chromosomes which are X0 ( male symbol ), XX ( female symbol ), with the X chromosome among the largest in the nucleus. This karyotype appears to be the norm for Ilybius and supports the transfer of the species concerned from Agabus to Ilybius. I. wasastjernae has 36 autosomes and the X chromosome is the smallest in the nucleus and its karyotype is unlike any other known karyotype in either Ilybius or Agabus. In most of the species studied no intraspecific variation has been detected. Exceptions are I. chalconatus, where there is one inversion polymorphism in one of the autosomes, and I. montanus whose autosome number has been found to vary from 29 to 34. Such variation is highly unusual among Coleoptera. The variation results from fusion-fission polymorphisms involving three different pairs of autosomes. In each case the fusions may be homozygous, heterozygous or absent. All populations investigated were polymorphic for some of the fusions, but only one (La Salceda, Spain) included individuals lacking all fusions. The frequencies of fused and unfused chromosomes were analysed in three English populations. In only one case was there a departure from the values expected from the Hardy-Weinberg equilibrium, and this population also showed a significant difference from the other two. Meiosis in males heterozygous for fusions involves the production of trivalents in first division, but results in the production of abundant sperm, with no evidence of chromosomal abnormalities in second metaphase, or of degenerating cells as a result of failed meiosis. The three fusions sites are consistent in all the populations studied, and it is concluded that these fusions represent unique historical events rather than current chromosomal instability.