Genetic analysis of mitochondrial rho-mutability in Saccharomyces. IV. Relation between spontaneous rho-mutability and mitotic stability in disomic Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae the disomy for chromosome XIV resembles the previously described disomy for chromosome IV in that it leads to a significant decrease in spontaneous rho- mutability. The nuclear srm1 mutation, reducing spontaneous rho- mutability, diminishes significantly the mitotic disome stability. So, the mechanisms of spontaneous rho- mutagenesis and mitotic disome stability seem to compete for the function affected by the srm1 mutation.     

A New Property of the Maize B Chromosome

TB-9Sb is a translocation between the B chromosome and chromosome 9 in maize. Certain deletions of B chromatin from the translocation cause a sharp decrease in B-9 transmission compared to the rate for standard TB-9Sb. The deletions remove components of a B chromosome genetic system that serves to suppress meiotic loss in the female. At least two distinct B-chromosome regions suppress meiotic loss: one on the B-9 and one on 9-B. The system operates by stabilizing univalent B-type chromosomes. It allows the univalents to migrate to one pole in meiosis, despite the absence of a pairing partner. The findings reported here are the first evidence for genetic control of meiotic loss by a B chromosome. However, it is proposed that the practice of suppressing meiotic loss is common to the B chromosomes of all species. The need to suppress meiotic loss results from the fact that B chromosomes are frequently unpaired in meiosis and subject to very high frequencies of loss. B chromosomes may utilize one or more of the following methods to suppress meiotic loss: (a) regular migration of univalent B's to one pole in meiosis, (b) enhanced recombination between B chromosomes and (c) mitotic nondisjunction.     

Chromosomes of caryophyllidean cestodes: Diploidy, triploidy, and parthenogenesis in Glaridacris catostomi

Mitotic and meiotic chromosomes were studied in Giemsa post-stained Feulgen squashes of testicular, ovarian, and vitelline cells. G. catostomi is 2n = 20 with eight pairs of metacentrics and two pairs of submetacentrics in its karyotype. The fundamental number of chromosome arms is FN = 40. At mitotic metaphase the mean total chromosome length (TCL) of the complement was 111·4 μm; the largest chromosomes were 8 μm (7% TCL) and the smallest 3 μm (3% TCL). Karyotype variation was not found among five diploid populations from New York State and Canada. Meiotic activity was abundant in spermatogenesis which proceeds as usual with sperm developing after two meiotic divisions. Chiasma frequency at diakinesis was 37/cell; as many as five chiasmata were observed in a single bivalent. A triploid ‘race’ (3n = 30) of G. catostomi was discovered in Bozenkill Creek near Albany, New York. Karyotype analysis confirms the triploid nature of these variants. Spermatogenesis in triploids is abnormal to the point of complete failure. The presence of an extra set of chromosomes has resulted in a breakdown of meiosis with subsequent sterility of the male system. Eggs remain unfertilized and parthenogenetic reproduction is presumed to occur. The possible origins of this unusual condition and its evolutionary implications are discussed. Seventeen figures, two tables of measurements, and a system of chromosome nomenclature supplement the paper.     

Genomic in situ hybridization analysis of a trigenomic hybrid involving Solanum and Lycopersicon species.

A 4x potato (+) tomato fusion hybrid (2n = 4x = 48) was successfully backcrossed with a diploid Lycopersicon pennellii (2n = 2x = 24). Genomic in situ hybridization (GISH) on somatic and meiotic chromosomes confirmed that the progenies were triploids (2n = 3x = 36) and possessed three different genomes: potato, tomato, and L. pennellii. Therefore, they have been called trigenomic hybrids. Total genomic probes of both Lycopersicon species were found to hybridize mutually, whereas the potato genome was clearly differentiated. During metaphase I, bivalents were formed predominantly between tomato and L. pennellii chromosomes and the univalents of potato chromosomes were most common. Trivalents in all cases included homoeologous chromosomes of potato, tomato, and L. pennellii. However, the triploids were totally sterile as determined from extensive crossing. On chromosome doubling of triploids by shoot regeneration from callus, hexaploids (2n = 6x = 72) were obtained. Despite exhibiting clear allohexaploid behaviour by forming 36 bivalents at meiosis, these were also completely sterile like their triploid counterparts. In spite of this drawback, the prospects of chromosome pairing between potato L. pennellii and Solanum genomes does open the possibilities for bringing the two genera close.     

Acrocentric chromosome disomy is increased in spermatozoa from fathers of Turner syndrome patients

The aim of the present study was to investigate whether there was an increase of aneuploidy in the sperm from fathers of Turner syndrome patients of paternal origin who, in a previous study, showed an elevated incidence of XY meiotic nondisjunction. Sperm disomy frequencies for chromosomes 4, 13, 18, 21 and 22 were assessed by fluorescence in situ hybridisation in four of these individuals. As a group, the Turner syndrome fathers showed a general increase in disomy frequencies for chromosomes 13, 21 and 22, with a statistically significant increase in disomy frequencies for chromosomes 13 and 22 in one of the fathers and for chromosome 21 in two of them. Data from a previous work carried out by us in two fathers of Down syndrome patients of paternal origin also revealed increased sperm disomy frequencies for chromosomes 13, 21 and 22. Pooled as one group, these six fathers of aneuploid offspring of paternal origin had a statistically significant increase in the frequency of nondisjunction for these chromosomes with respect to control individuals. Our findings indicate that there may be an association between fathering aneuploid offspring and increased frequencies of aneuploid spermatozoa. Such increases do not seem to be restricted to the chromosome pair responsible for the aneuploid offspring. Acrocentric chromosomes and other chromosome pairs that usually show only one chiasma during meiosis seem to be more susceptible to malsegregation.     

Rearrangements of highly polymorphic regions near telomeres of Saccharomyces cerevisiae.

We have examined the mitotic and meiotic properties of telomeric regions in various laboratory strains of yeast. Using a sequence (Y probe) derived from a cloned yeast telomere (J. Szostak and E. Blackburn, Cell 29:245-255, 1982), we found that various strains of Saccharomyces cerevisiae show extensive polymorphisms of restriction endonuclease fragment length. Some of the variation in the lengths of telomeric fragments appears to be under the control of a small number of genes. When DNA from various strains was digested with endonuclease KpnI, nearly all of the fragments homologous to the Y probe were found to be of different size. The pattern of fragments in different strains was extremely variable, with a greater degree of polymorphism than that observed for fragments containing the mobile TY1 element. Tetrad analysis of haploid meiotic segregants from diploids heterozygous for many different Y-homologous KpnI fragments revealed that most of them exhibited Mendelian (2:0) segregation. However, only a small proportion of these fragments displayed the obligate 2:2 parental segregation expected of simple allelic variants at the same chromosome end. From the segregations of these fragments, we concluded that some yeast telomeres lack a Y-homologous sequence and that the chromosome arms containing a Y-homologous sequence are different among various yeast strains. Regions near yeast telomeres frequently undergo rearrangement. Among eight tetrads from three different diploids, we have found three novel Y-homologous restriction fragments that appear to have arisen during meiosis. In all three cases, the appearance of a new fragment was accompanied by the loss of another band. In one of these cases, the rearrangement leading to a novel fragment arose in an isogenic diploid, in which both homologous chromosomes should have been identical. Among these same tetrads we also found examples of apparent mitotic gene conversions and mitotic recombination involving telemetric regions.     

Aneuploidy in human sperm: the use of multicolor FISH to test various theories of nondisjunction.

While it is known that all chromosomes are susceptible to meiotic nondisjunction, it is not clear whether all chromosomes display the same frequency of nondisjunction. By use of multicolor FISH and chromosome-specific probes, the frequency of disomy in human sperm was determined for chromosomes 1, 2, 4, 9, 12, 15, 16, 18, 20, and 21, and the sex chromosomes. A minimum of 10,000 sperm nuclei were scored from each of five healthy, chromosomally normal donors for every chromosome studied, giving a total of 418,931 sperm nuclei. The mean frequencies of disomy obtained were 0.09% for chromosome 1; 0.08% for chromosome 2; 0.11% for chromosome 4; 0.14% for chromosome 9; 0.16% for chromosome 12; 0.11% for chromosomes 15, 16, and 18; 0.12% for chromosome 20; 0.29% for chromosome 21; and 0.43% for the sex chromosomes. Data for chromosomes 1, 12, 15, and 18, and the sex chromosomes have been published elsewhere. When the mean frequencies of disomy were compared, the sex chromosomes and chromosome 21 had significantly higher frequencies of disomy than that of any other autosome studied. These results corroborate the pooled data obtained from human sperm karyotypes and suggest that the sex chromosome bivalent and the chromosome 21 bivalent are more susceptible to nondisjunction during spermatogenesis. From these findings, theories proposed to explain the variable incidence of nondisjunction can be supported or discarded as improbable.     

Mitotic Chromosome Loss in a Disomic Haploid of SACCHAROMYCES CEREVISIAE

Experiments designed to characterize the incidence of mitotic chromosome loss in a yeast disomic haploid were performed. The selective methods employed utilize the non-mating property of strains disomic for linkage group III and heterozygous at the mating type locus. The principal findings are: (1) The frequency of spontaneous chromosome loss in the disome is of the order 10^-4 per cell; this value approximates the frequency in the same population of spontaneous mitotic exchange resulting in homozygosity at the mating type locus. (2) The recovered diploids are pure clones, and thus represent unique events in the disomic haploid. (3) Of the euploid chromosomes recovered after events leading to chromosome loss, approximately 90% retain the parental marker configuration expected from segregation alone; however, the remainder are recombinant for marker genes, and are the result of mitotic exchanges in the disome, especially in regions near the centromere. The recombinant proportion significantly exceeds that expected if chromosome loss and mitotic exchange in the disome were independent events. The data are consistent with a model proposing mitotic nondisjunction as the event responsible for chromosome loss in the disomic haploid.     

Chromosomal Translocations Generated by High-Frequency Meiotic Recombination between Repeated Yeast Genes

We have examined meiotic and mitotic recombination between repeated genes on nonhomologous chromosomes in the yeast Saccharomyces cerevisiae. The results of these experiments can be summarized in three statements. First, gene conversion events between repeats on nonhomologous chromosomes occur frequently in meiosis. The frequency of such conversion events is only 17-fold less than the analogous frequency of conversion between genes at allelic positions on homologous chromosomes. Second, meiotic and mitotic conversion events between repeated genes on nonhomologous chromosomes are associated with reciprocal recombination to the same extent as conversion between allelic sequences. The reciprocal exchanges between the repeated genes result in chromosomal translocations. Finally, recombination between repeated genes on nonhomologous chromosomes occurs much more frequently in meiosis than in mitosis.     

Cytological evidence of maternal meiotic errors in a line of chickens with a high incidence of triploidy.

Direct evidence of the nature of maternal meiotic errors in a selected line of chickens with a high incidence of triploidy was obtained by using cytologically marked paternal gametes derived from a closely related avian species. Matings were made by artificial insemination of female chickens of the selection line and a control line with semen from ring-necked male pheasants. A total of five triploid, one pentaploid, and 21 diploid hybrid embryos were karyotyped. Each triploid hybrid embryo contained one set of paternal pheasant chromosomes and two sets of maternal chicken chromosomes, providing irrefutable cytological evidence that the triploids were derived from diploid ova produced by females of the selection line. The pentaploid hybrid contained one set of paternal pheasant chromosomes and four sets of maternal chicken chromosomes, indicating that it had been derived from a tetraploid ovum. Females of the selection line are thought to have a genetically mediated susceptibility to nondisjunction which is responsible for the high incidence of meiotic errors. Evidence is provided that the non-disjunction occurs at both meiosis I and meiosis II.     

A New Mapping Method Employing a Meiotic Rec- Mutant of Yeast

A rapid new mapping method has been developed for localizing a dominant or recessive mutation to a particular chromosome of yeast. The procedure utilizes the ability of strains homozygous for the spo11-1 mutation to undergo chromosome segregation without appreciable recombination during sporulation. The level of sporulation in spo11-1/spo11-1 diploids is reduced and asci are often immature or abnormal in appearance; spore viability is less than 1%. The first step of the mapping procedure is the construction of a haploid spo11-1 strain carrying a recessive drug-resistance marker and the unmapped mutation(s). This strain is crossed to a set of three spo11-1 mapping tester strains containing, among them, a recessive marker on each chromosome. The resulting spo11-1/spo11-1 diploids are sporulated and plated on drug-containing medium. Viable meiotic products that express the drug-resistance marker due to chromosome haploidization are selectively recovered. These meiotic products are haploid for most, but generally not all, chromosomes. The level of disomy for individual chromosomes averages 19%. Each of the recessive chromosomal markers is expressed in approximately a third of the drug-resistant segregants. Ninety-eight percent of these segregants show no evidence of intergenic recombination. Thus, two markers located on the same chromosome, but on different homologs, are virtually never expressed in the same drug-resistant clone. The utility of this mapping procedure is demonstrated by confirming the chromosomal location of seven known markers, as well as by the assignment of a previously unmapped mutation, spo12-1, to chromosome VIII. In addition, the analysis of the products of spo11-1 meiosis indicates that several markers previously assigned to either chromosome XIV or chromosome XVII are actually on the same chromosome.     

A Highlights from MBoC Selection: Ipl1/Aurora-B is necessary for kinetochore restructuring in meiosis I in Saccharomyces cerevisiae

In mitosis, the centromeres of sister chromosomes are pulled toward opposite poles of the spindle. In meiosis I, the opposite is true: the sister centromeres move together to the same pole, and the homologous chromosomes are pulled apart. This change in segregation patterns demands that between the final mitosis preceding meiosis and the first meiotic division, the kinetochores must be restructured. In budding yeast, unlike mammals, kinetochores are largely stable throughout the mitotic cycle. In contrast, previous work with budding and fission yeast showed that some outer kinetochore proteins are lost in early meiosis. We use quantitative mass spectrometry methods and imaging approaches to explore the kinetochore restructuring process that occurs in meiosis I in budding yeast. The Ndc80 outer kinetochore complex, but not other subcomplexes, is shed upon meiotic entry. This shedding is regulated by the conserved protein kinase Ipl1/Aurora-B and promotes the subsequent assembly of a kinetochore that will confer meiosis-specific segregation patterns on the chromosome.     

Non-isotopic detection of chromosome 1 in human meiosis and demonstration of disomic sperm nuclei

Summary Nonradioactive in situ hybridization with the biotin-labeled chromosome 1-specific probe pUC1.77 was performed on human mitotic and meiotic chromosomes, and on sperm nuclei. The streptavidine-horseradish-peroxidase and diaminobenzidine detection system demonstrated heteromorphisms in the Iq12 heterochromatic region, not only in mitotic cells but also in mature sperm heads. The localization of chromosome 1 could be traced through all meiotic stages and in the sperm nuclei. The frequency of chromosome 1 disomy in human sperm, as indicated by two distinct hybridization signals, was calculated to be 0.41%.     

Triploid planarian reproduces truly bisexually with euploid gametes produced through a different meiotic system between sex

Although polyploids are common among plants and some animals, polyploidization often causes reproductive failure. Triploids, in particular, are characterized by the problems of chromosomal pairing and segregation during meiosis, which may cause aneuploid gametes and results in sterility. Thus, they are generally considered to reproduce only asexually. In the case of the Platyhelminthes Dugesia ryukyuensis, populations with triploid karyotypes are normally found in nature as both fissiparous and oviparous triploids. Fissiparous triploids can also be experimentally sexualized if they are fed sexual planarians, developing both gonads and other reproductive organs. Fully sexualized worms begin reproducing by copulation rather than fission. In this study, we examined the genotypes of the offspring obtained by breeding sexualized triploids and found that the offspring inherited genes from both parents, i.e., they reproduced truly bisexually. Furthermore, meiotic chromosome behavior in triploid sexualized planarians differed significantly between male and female germ lines, in that female germ line cells remained triploid until prophase I, whereas male germ line cells appeared to become diploid before entry into meiosis. Oocytes at the late diplotene stage contained not only paired bivalents but also unpaired univalents that were suggested to produce diploid eggs if they remained in subsequent processes. Triploid planarians may therefore form euploid gametes by different meiotic systems in female and male germ lines and thus are be able to reproduce sexually in contrast to many other triploid organisms.     

The Rec102 Mutant of Yeast Is Defective in Meiotic Recombination and Chromosome Synapsis

A mutation at the REC102 locus was identified in a screen for yeast mutants that produce inviable spores. rec102 spore lethality is rescued by a spo13 mutation, which causes cells to bypass the meiosis I division. The rec102 mutation completely eliminates meiotically induced gene conversion and crossing over but has no effect on mitotic recombination frequencies. Cytological studies indicate that the rec102 mutant makes axial elements (precursors to the synaptonemal complex), but homologous chromosomes fail to synapse. In addition, meiotic chromosome segregation is significantly delayed in rec102 strains. Studies of double and triple mutants indicate that the REC102 protein acts before the RAD52 gene product in the meiotic recombination pathway. The REC102 gene was cloned based on complementation of the mutant defect and the gene was mapped to chromosome XII between CDC25 and STE11.     

Separation of meiotic and mitotic effects of claret non-disjunctional on chromosome segregation in Drosophila.

The claret (ca) locus in Drosophila encodes a kinesin-related motor molecule that is required for proper distribution of chromosomes in meiosis in females and in the early mitotic divisions of the embryo. Here we demonstrate that a mutant allele of claret non-disjunctional (ca(nd)), non-claret disjunctional Dominant (ncdD), causes abnormalities in meiotic chromosome segregation, but is near wild-type with respect to early mitotic chromosome segregation. DNA sequence analysis of this mutant allele reveals two missense mutations compared with the predicted wild-type protein. One mutation lies in a proposed microtubule binding region of the motor domain and affects an amino acid residue that is conserved in all kinesin-related proteins reported to date. This region of the motor domain can be used to distinguish meiotic and mitotic motor function, defining an amino acid sequence criterion for classifying motors according to function. ncdD's mutant meiotic effect, but near wild-type mitotic effect, suggests that interactions of the ca motor protein with spindle microtubules differ in meiosis and mitosis.     

Morphology of a human-derived YAC in yeast meiosis

In meiosis of human males DNA is packaged along pachytene chromosomes about 20 time more compactly than in meiosis of yeast. Nevertheless, a human-derived yeast artificial chromosome (YAC) shows the same degree of compaction of DNA as endogenous chromosomes in meiotic prophase nuclei of yeast. This suggests that in yeast meiosis, human and yeast DNA adopt a similar organization of chromatin along the pachytene chromosome cores. Therefore meiotic chromatin organization does not seem to be an inherent chromosomal property but is governed by the host-specific cellular environment. We suggest that there is a correlation between the less dense DNA packaging and the increased rate of recombination that has been reported for human-derived YACs as compared with human DNA in its natural environment.     

Pds5 is required for homologue pairing and inhibits synapsis of sister chromatids during yeast meiosis

During meiosis, homologues become juxtaposed and synapsed along their entire length. Mutations in the cohesin complex disrupt not only sister chromatid cohesion but also homologue pairing and synaptonemal complex formation. In this study, we report that Pds5, a cohesin-associated protein known to regulate sister chromatid cohesion, is required for homologue pairing and synapsis in budding yeast. Pds5 colocalizes with cohesin along the length of meiotic chromosomes. In the absence of Pds5, the meiotic cohesin subunit Rec8 remains bound to chromosomes with only minor defects in sister chromatid cohesion, but sister chromatids synapse instead of homologues. Double-strand breaks (DSBs) are formed but are not repaired efficiently. In addition, meiotic chromosomes undergo hypercondensation. When the mitotic cohesin subunit Mcd1 is substituted for Rec8 in Pds5-depleted cells, chromosomes still hypercondense, but synapsis of sister chromatids is abolished. These data suggest that Pds5 modulates the Rec8 activity to facilitate chromosome morphological changes required for homologue synapsis, DSB repair, and meiotic chromosome segregation.     

Comparison of the gonadal development and plasma levels of sex steroid hormones in diploid and triploid sea bass, Dicentrarchus labrax L

The goal of this study was to compare the reproductive physiology of triploid and diploid European sea bass (Dicentrarchus labrax L.). Gonads of diploid and triploid fish (males and females) were examined both microscopically and macroscopically, together with the plasma levels of the major sex steroids produced (testosterone and estradiol-17beta) when fish were adults. Prior to sexual maturation, the gonadosomatic index (GSI) of triploid males was similar to that of diploids. However, the GSI in 4-year-old adult triploid males was 1.8 times lower than that of diploids (P < 0.05). All diploid males exhibited normal gonadal development. In contrast, in triploid males spermatogenesis was impaired during late meiosis, affecting severely spermiogenesis. This was achieved by an increasing imbalance in the amount of DNA present in daughter cells of the same type as spermatogenesis progressed, as demonstrated by abnormal cell sizes, culminating in inviable spermatids. Thus, no spermiating triploid fish were observed during 4 years, which included three full consecutive maturation cycles. Furthermore, the germ cells from triploids were significantly larger than those from diploids (P < 0.001). Seasonal profiles of plasma levels of testosterone in 4-year-old males were essentially similar in both ploidies. On the other hand, triploid females had rudimentary ovaries containing oogonia and primary oocytes that were arrested during meiotic prophase I, while diploid females exhibited all stages of ovarian development. Diploid females showed levels of testosterone and estradiol-17beta significantly higher than those of triploids (P < 0.05), in which no endocrine signs of maturation were observed at all. Regarding sex ratios, triploids had 10% more females than diploids (P < 0.05) but in both ploidies males predominated, as is usually found in this species under culture conditions. These results show that triploidy blocked the initial phases of meiosis in females and the latter ones in males, resulting in the absence of or reduced gonadal development, respectively. In conclusion, we provide an explanation for the lack of gonadal development in triploid male fish, and, to the best of our knowledge, we report for the first time a case in which induced triploidy completely blocks meiosis in both sexes, thus conferring functional sterility in the sea bass.     

Homologous chromosome interactions in meiosis: diversity amidst conservation

Proper chromosome segregation is crucial for preventing fertility problems, birth defects and cancer. During mitotic cell divisions, sister chromatids separate from each other to opposite poles, resulting in two daughter cells that each have a complete copy of the genome. Meiosis poses a special problem in which homologous chromosomes must first pair and then separate at the first meiotic division before sister chromatids separate at the second meiotic division. So, chromosome interactions between homologues are a unique feature of meiosis and are essential for proper chromosome segregation. Pairing and locking together of homologous chromosomes involves recombination interactions in some cases, but not in others. Although all organisms must match and lock homologous chromosomes to maintain genome integrity throughout meiosis, recent results indicate that the underlying mechanisms vary in different organisms.