Showing papers in "Chromosoma in 1972"

The use of proteolytic enzymes for the mapping of structural rearrangements in the chromosomes of man.

Abstract: Chromosome preparations treated for short periods with the proteolytic enzyme trypsin show well defined banding patterns, comparable to those obtained by more elaborate techniques.—With such patterns it is possible to map in detail the position of chromosome rearrangements.—A rare balanced A1–E18 translocation in a phenotypically normal female and the unbalanced product in her abnormal child has been used to demonstrate this mapping method.

Differential spiralization along mammalian mitotic chromosomes. I. BUdR-revealed differentiation in Chinese hamster chromosomes.

Abstract: Morphology of chromosomes replicating in the presence of 5-bromodeoxyuridine was studied using long-term cultures of Chinese hamster cells (line Blld-ii-FAF28). The cytological effect of the analog administered in various concentrations, at different stages of the S period, and during one and two successive mitotic cycles was studied. — The main cytological manifestation of the BUdR action consisted in spiralization delay of certain chromosome regions. The degree of the delay was dependent on the time interval between the introduction of the agent and mitosis, as well as on the agent's concentration. With prolongation of the interval, the spiralization delay diminished and disappeared being therefore always observable only in late replicating chromosome regions. Increased concentration of BUdR (in the range of 25 to 400 μg/ml) produced enhancement of the delay of chromosome spiralization. — After two successive reproduction cycles in the presence of BUdR, a great number of metaphases contained chromosomes the sister chromatids of which showed unequal spiralization delay. Autoradiography of 3H-BUdR distribution showed that the sister chromatid with a more pronounced underspiralization corresponds to the chromatid incorporating BUdR into both strands of the DNA molecule. — Mechanisms of the effect observed, as well as chemical influence on chromosome spiralization as a usefull tool of displaying linear chromosome differentiation, are discussed.

Robertsonian chromosomal variation and identification of metacentric chromosomes in feral mice.

Abstract: Cytogenetic studies of feral mice (M. musculus) from various but predominantly Alpine areas of Switzerland, carried out on random samples collected by spot-checks, established the widespread existence of metacentric chromosomes in the somatic karyotype. Despite the finding of the common occurrence of some of the metacentrics in different places, the examination of the possible homology or heterology by breeding procedures revealed the surprising fact that independence, partial or heterobrachial homology of the metacentric chromosomes prevail among mice from different geographical areas. Thus, the general picture is that of an array of different metacentric chromosomes derived from independent events of Robertsonian variation in the process of evolution. — While heterozygosity with independent metacentrics within a Robertsonian system may have a bearing on the fertility rate of a given mouse population, a more severe impairment of the reproductive capacity must be taken into account in mouse populations which possess different metacentrics with mono- or heterobrachial homologies. These conditions favour the assumption of the existence of a selective system of reproductive barriers further subdividing the species in many, more or less stable, micro-populations. — The chromosomal arms (telocentrics) involved in the formation of the metacentric chromosomes could be identified by Q- and G-banding techniques in combination with the results of crossbreeding, and were assigned to the corresponding telocentric autosomes of the mouse (Comm. Standard. Genet. Nomenclat. for Mice, 1972). Most of the telocentric autosomes of the mouse are included in one or more of the metacentrics found in the feral populations. By means of their isolation in separate lines, these metacentrics may be useful in experimental biology as marker chromosomes of defined identity carrying known linkage groups.

Genome size in mammals.

Abstract: Nuclear DNA amounts of fifteen species of placental mammals were determined by Feulgen cytophotometry. Relative values for several widely used species have been ascertained with an error of only a few percent. Absolute values (picograms or numbers of nucleotide pairs) can be determined with an error of about ten percent. The largest known mammalian genome contains about twice the DNA of the smallest one. The modal diploid DNA amount for mammals is slightly above eight picograms.

Quinacrine fluorescence of specific chromosome regions. Late replication and high A: T content in Samoaia leonensis.

Abstract: The pattern of intense fluorescence of interphase nuclei and metaphase chromosomes after staining with quinacrine is described in Samoaia leonensis. Autoradiographic analysis of interphase nuclei after pulse labeling with tritiated thymidine indicates that there is little or no overlap in the time of replication of the intensely fluorescing and weakly fluorescing regions. Autoradiographic analysis of metaphase figures after continuous labeling with tritiated thymidine shows that the intensely fluorescing regions are late replicating and establishes their order of replication. Autoradiographic analysis of interphase nuclei after pulse labeling with tritiated deoxycytidine and of metaphase figures after continuous labeling with this tracer show that there is little, if any, incorporation of deoxycytidine into those chromosome regions which fluoresce intensely after staining with quinacrine and quinacrine mustard. These results indicate that such chromosome regions are characterized chemically by an extremely high, if not exclusive, content of adenine and thymine.

Reconstruction of the Neurospora crassa pachytene karyotype from serial sections of synaptonemal complexes.

Abstract: Serial sections from isolated asci were used to reconstruct the seven pachytene bivalents of Neurospora crassa. The synaptonemal complex could be traced for its whole length in each bivalent, being attached to the nuclear envelope at both ends in six. The satellite end of the nucleolar chromosome did not appear to be attached to the nuclear envelope. The estimated lengths of the bivalents ranged from 10.7 to 5.1 microns in one nucleus, from 11.5 to 4.2 microns in another, and from 8.5 to 4.4 microns in a third, with total haploid complement lengths of 45.5 microns, 47.3 microns, and 43.9 microns respectively. These values are considerably smaller than published light microscopical measurements.—The synaptonemal complex in N. crassa, as in other ascomycetes, has two banded ca. 400 A wide lateral components held about 1200 A apart by a central region containing the ca. 200 A wide central component. With normal glutaraldehyde/OsO4-phosphate buffered fixation the chromatin of the pachytene bivalents is poorly contrasted. Occasional local thickenings of the central component into electron dense nodes ca. 1000 × 500 A in longitudinal section are characteristic of the complex.

The interphase distribution of satellite DNA-containing heterochromatin in mouse nuclei.

Abstract: The distribution of sites capable of binding mouse satellite-complementary RNA in the cytological hybridization reaction has been examined in mouse liver and testis interphase nuclei. The approach taken has been to combine hybridization with semi-thin sectioning and autoradiography in order to obtain a clear picture of the relationship of satellite DNA-containing structures to the rest of the interphase nucleus. In liver nuclei, hybridization occurs primarily with blocks of heterochromatin associated with the nuclear envelope. The most prominent of these, in terms both of size and intensity of hybridization, is the nucleolar stalk and the rest of the nucleolus-associated heterochromatin. The nucleolar body itself is not labeled, nor is much of the peripheral condensed chromatin ; in fact, a polarized distribution of satellite DNA is evident. In Sertoli and spematid nuclei, satellite DNA is found in a small number of large heterochromatin blocks with which the nucleolus is associated; some of this material bears a relationship to the nuclear envelope in these cells also.

Banding patterns of Chinese hamster chromosomes revealed by new techniques.

Abstract: Two kinds of techniques were newly developed to reveal banding patterns of the Chinese hamster chromosomes. Both techniques were essentially the same as those used for the extraction of proteins, and one of them could induce bands in chromosome arms in only a few seconds. Banding patterns produced by these techniques appeared to be identical to those induced by the methods reported by previous workers, requiring post-fixation incubation of slides in a warm saline. —The banding pattern was typical for each chromosome pair, permitting unequivocal identification of several pairs which were hardly distinguished by the conventional staining procedures. It was confirmed that these patterns had been well preserved in the chromosomes of a cultured cell line.

Patterns of puffing activity in the salivary gland chromosomes of Drosophila. VII. Homology of puffing patterns on chromosome arm 3L in D. melanogaster and D. yakuba, with notes on puffing in D. teissieri.

Abstract: Puffing patterns of chromosome arm 3 L of D. yakuba are compared with those of other members of the melanogaster species subgroup D. melanogaster and D. simulans. Several paracentric inversions on 3L have resulted in a considerable rearrangement of gene order in D. yakuba. However the basic sequence of changes in puffing activity which occurs during late larval and prepupal development is very similar to that of D. melanogaster and D. simulans. A fourth member of this species subgroup (D. teissieri) also has similar puffing patterns to those of D. melanogaster despite considerable chromosome evolution.

Holocentric chromosomes in Oncopeltus: kinetochore plates are present in mitosis but absent in meiosis.

Abstract: Fine structure studies of Oncopeltus fasciatus, an hemipteran with diffuse kinetochores, shows the presence of a kinetochore plate extending for up to 75% of the length of the chromosomes during mitosis. During meiosis, microtubules entered all along the body of the chromosomes and the kinetochore plate was completely missing. It is suggested that in organisms with holocentric chromosomes the formation of the meiotic kinetochore apparatus may have to be suppressed to allow terminalization of chiasmata.

Factors involved in the production of banded structures in mammalian chromosomes.

Abstract: Dozens of reagents were tested for their ability to produce bands in Chinese hamster chromosomes by incubating air-dried preparations in aqueous solutions of these reagents for a definite period prior to the Giemsa staining. Acids were found to be without effect on the band production. Many of salts were able to induce bands if their pH was alkaline. Strong bases were also found to be potent band inducing reagents. They produced bands only in a few seconds. Protein denaturants such as urea, guanidine-HCl and several surface active compounds were also effective in the band production. — In the light of these results, it seems very likely that the solubilization or extraction of some chromosomal proteins, probably of acid nature, would be the primary cause of the appearance of the banded structure in chromosome arms.

Genome size and evolution.

Abstract: Examination of data on genome size for prokaryotic cells suggests an evolutionary scheme.

The effect of changes in the respiratory metabolism upon genome activity in Drosophila. I. The induction of gene activity.

Abstract: Inhibition of hydrogen transfer between NADH and Co Q by rotenone or amytal in salivary gland cells of Drosophila hydei maintained in vitro, results in the activation of a particular group of four loci in the polytene chromosomes (puff formation). The response of these loci to the same treatment is enhanced if Na-malonate is present in the incubation medium. — Three of the loci become active if the glands are kept in a medium supplied with antimycin A or 2-heptyl-4-hydroxyquinoline-N-oxide (H QNO), specific inhibitors of the electron transfer between cytochromes b and c. — It was established that a temperature treatment and DNP raise oxygen consumption of the cells to a certain level. Following the same treatments of glands supplied with Na-malate and Na-succinate the raise in oxygen consumption attains a significantly higher level. Under these conditions no response is observed at the genome level. — Whereas DNP, which uncouples oxidative phosphorylation and enhances the respiratory chain reactions, does induce the initiation of puff formation, oligomycin, which inhibits oxidative phosphorylation and suppresses the respiratory chain reactions, is ineffective in initiating puff formation at the specific loci. However, if oligomycin is supplied to the medium in combination with KCN which inhibits the cytochrome oxidase activity, three of the four loci become active. — The presence in the medium of substances which may act as hydrogen acceptors, e.g. menadione or methylene blue, can also result in activation of the chromosome loci. — These results are interpreted as indications for the existence of a regulatory mechanism between mitochondrial respiratory metabolism and the activity of a particular group of genome loci.

Evolution of karyotypes in snakes.

Abstract: Karyotype analysis and morphometric measurement of the chromosomes of 17 species of snakes have been done. Chromosomes of different species so far worked out in each family have been compared using quantitative methods to derive chromosomal affinities between species of different taxonomic categories. The following conclusions have been drawn: (i) It is suggested that the retention of Xenopeltidae as a separate family is unnecessary and the only species Xenopeltis unicolor referred to in that group should be included in the family Boidae. (ii) The subfamilies, Boinae and Pythoninae cannot be distinguished chromosomally. (iii) On the basis of chromosomal similarities, the cytologically known species of Colubridae. have been put into 13 different groupings which do not always correspond to the views of the present day colubrid taxonomists. (iv) In Hydrophiidae, speciation seems to have occurred through changes in the 4th pair of autosomes and sex chromosomes in general and the W chromosome in particular. Evidences are presented to show that fission and inversion have played an important role in bringing about the structural rearrangements in this group. (v) Family Viperidae according to taxonomists is divided into two subfamilies. Both the subfamilies are chromosomally very similar.

Centromere localization at meiosis and the position of chiasmata in the male and female mouse.

Abstract: Techniques for obtaining differential Giemsa staining of the paracentromeric (p.c.) regions of male and female mouse meiotic chromosomes (centromeric heterochromatin) were explored and standard procedures developed for the different meiotic cells in the two sexes. The best result followed the use of heat at controlled pH in Sorensen's phosphate buffer or in Standard Saline Citrate (SSC) solutions. With these techniques, morphological features of the p.c. regions and their variation were studied in normal animals (CFLP strain) and in a strain (AKR) homozygous for a centric fusion [T(11; ?)-1 Ald] between chromosomes No. 6 and No. 15 (Miller et al., 1971). The Y chromosome was often found to show distinct p. c. staining at first and apparently at second meiotic metaphase, and the X and Y chromosomes were found to associate as bivalents by their long arms. Autosomal p.c. regions showed variation in size which might indicate differences between non-homologous chromosomes but a tendency to similarity between homologues. Differences were found between males and females in respect to proportions and variation of bivalents with single and double chiasmata. The relative positions of chiasmata were different in the two sexes. The presence of the centric fusion in the males did not seem to affect the pairing behaviour of the remaining autosomes or of those taking part in the centric fusion. The possibility is discussed that the p.c. regions, to which also other functions would seem to appertain, may be important for chromosome recognition and pairing, possibly on a quantitative basis.

Ectopic pairing of chromosome regions containing chemically similar DNA.

Abstract: Using genetically controlled stocks ofDrosophila melanogaster we have compared the frequency of ectopic pairing in a line showing intense quinacrine fluorescence at two sites (81F and 83E) on chromosome 3 with one showing such fluorescence at only one of these sites (81F). The frequency of ectopic pairing is an order of magnitude greater in cells from the line showing intense fluorescence in both regions than in the line showing it in only one. These data indicate that ectopic pairing is dependent upon properties of discrete chromosome regions as small as individual bands. Since A: T-rich chromatin is known to fluoresce intensely after quinacrine staining, these data further suggest that ectopic pairing is dependent on similarities of the DNA of the discrete chromosome regions involved.

Chromosome studies in four species of Ratitae (Aves).

Abstract: Chromosomes were studied in female specimens of the ostrich, Struthio camelus L., cassowary, Casuarius casuarius (L.), emu, Dromiceius novaehollandiae (Lath.) and rhea, Rhea americana L. by means of blood and feather pulp culture techniques. Male karyotypes were also studied in the emu and rhea. The diploid chromosome number was most likely 80 in the ostrich and rhea and 82 in the emu, while the exact number could not be determined in the cassowary. Karyotypes of the 4 species were strikingly similar and apparently interchangeable with one another with slight modifications of the centromeric position in one or two pairs of macrochromosomes. No heteromorphic macrochromosomal pair was found either in female specimens or in male ones of the ratite species so far examined, except for a female rhea. This specimen was found to possess an acrocentric chromosome which was evidently a member of nos. 4–6, but considerably smaller than any other chromosome of the group. 3H-thymidine autoradiography provided no more information than the straightforward morphological analysis with regard to the differentiation of the sex-chromosomes.

Banding pattern analysis of human chromosomes by use of a urea treatment technique.

Abstract: A new technique to reveal the banding pattern of human chromosomes is described. Slides prepared by the routine air drying technique were treated with urea-Sorensen buffer solution for ten minutes at pH 6.8 at 37° C. Individual pairs of all human chromosomes exhibited a characteristic banding pattern by this technique, and by its use the karyotypes were analysed. With the exception of some minor differences the banding patterns obtained by the present technique appeared to be identical with those obtained by the Giemsa staining and quinacrine fluorescence methods carried out by previous workers.

Genome size in birds

Abstract: Nuclear DNA amounts of twenty-three species of birds from seventeen families of seven orders were determined by Feulgen cytophotometry. Genome size is constant in these birds, the ratio between the largest and smallest genome in the sample is 1.3 to 1. The modal diploid DNA amount for birds is about 3.6 picograms, slightly higher than previously reported. The data point towards an evolutionary control of genome size regardless of chromosome number. Birds represent an example of a group in which reduction of genome size is correlated with active speciation.

Chiasmata and the breeding system in wild populations of diploid wheats

Abstract: Seven populations of the selfer Triticum longissimum (= Aegilops longissima) and five populations of the closely related outbreeder T. speltoides (= Ae. speltoides) were scored for chiasma frequencies in pollen mother cells. The populations of the selfer have significantly higher frequencies of chiasmata than the outbreeding populations. This difference becomes even clearer when interstitial chiasmata alone are compared. It is argued that an optimal degree of effective recombination is achieved by the balance between outbreeding and interstitial chiasmata. — There are wider differences between the selfing populations than between the outbreeding populations, but the differences between families (within populations) are small in both species. Variation between plants within families seems to be lower in the selfer, but nevertheless high enough to be inexplicable on the basis of selfing alone. — Small populations subject to hardship conditions show a higher frequency of chiasmata than others.

Acridine-orange differential fluorescence of fast- and slow-reassociating chromosomal DNA after in situ DNA denaturation and reassociation

Abstract: A cytological technique based on heat denaturation of in situ chromosomal DNA followed by differential reassociation and staining with acridine orange was developed. Mouse nuclei and chromosomes in fixed cytological preparations show a red-orange fluorescence after thermal DNA denaturation (2–4 minutes at 100° C), and fluoresce green if denaturation is followed by a total DNA reassociation (two minutes or more at 65–66°C). — A reassociation time between a few and 60–90 seconds demonstrates the centromeric heterochromatin of chromosomes (which sometimes aggregate in the form of clusters) and the interphase chromocenters in green, the chromosomal arms fluorescing red-orange. Under the same conditions, the Y chromosome presents a pale green or yellow-green fluorescence along its chromatids, but its centromeric region fluoresces weakly. — The interpretation is suggested that the fast-reassociating chromosomal DNA (as detected by AO in centromeric heterochromatin and interphase chromocenters), represents repetitive DNA.

Ultrastructure and composition of the synaptonemal complex in spread and negatively stained spermatocytes of the golden hamster and the albino rat.

Abstract: The ultrastructure of the synaptonemal complex (SC) has been studied in spermatocytes of the golden hamster and the albino rat, spread on liquid surfaces and negatively stained with uranyl acetate. The conditions for a reproducible procedure for spreading the SC have been specified. Spreading on water causes large losses of material from the complex. Spreading on 0.45–0.9% NaCl in water results in good preservation of the SC. Ethanol dehydration introduces irreversible changes in the shape of the chromatin fibers and the components of the complex. Digestions with DNase and proteases, extraction with 2M NaCl and fixation in an aqueous solution of formaldehyde permit analysis of the components of the SC. The lateral elements of the SC are formed by three components: 1) the bulk material which is protease sensitive, DNase resistant, insoluble in 2M NaCl and partially soluble in water; 2) the axial attachment regions of the chromatin fiber; and 3) an axial and linear filament, 65 A wide, which is DNase sensitive. It is suggested that this linear 65 A filament contains a single linear DNA molecule to which the chromatin fibers are attached. The central element of the SC is made of fibrillar material, most of which is DNase resistant and protease sensitive. Fibrils 25 A wide cross the central space and merge with the central element. The cross fibrils and the central element are labile in solutions containing less than 0.45% NaCl. — From the present results and previous data on diplotene axes (Solari, 1970), it is concluded that the lateral elements of the SC of hamster and rat spermatocytes are undivided during pachytene. It is suggested that the singleness of the axes in the lateral elements is based on the presence of a single DNA molecule axially located in the lateral elements, and that the chromatin fibers are symmetrically attached to this DNA molecule.

Die Assembly-Hypothese der Chromosomenbewegung und die Veränderungen der Spindellänge während der Anaphase I in Spermatocyten von Pales ferruginea ( Tipulidae, Diptera )

Abstract: 30 living first spermatocytes of the crane-fly Pales (Nephrotoma) ferruginea were photographed at intervals of 1 or 2 minutes throughout anaphase In 11 cells the spindle length decreased in very early anaphase, in 4 cells it increased, and in the remaining 15 cells no significant changes occurred There is a positive correlation between the decrease of spindle length during very early anaphase and spindle length at the beginning of anaphase (r = 040; P<005) During mid-anaphase the spindle length increased in all spermatocytes The rate of length increase is again positively correlated with spindle length at the beginning of anaphase (r=047; P< 001) Nevertheless, the total length increase of spindles which have long axes at the beginning of anaphase, is not significantly higher than the length increase of those with short axes This is so because the duration of spindle elongation is negatively correlated with spindle length at the beginning of anaphase (r=-039; P<005) In addition the duration of spindle elongation in mid-anaphase seems to be shorter the more the length of the spindle axes decreases during very early anaphase The average velocity of the syntelically oriented chromosomes in early anaphase is positively correlated with the rate of the spindle elongation during mid-anaphase (r=066; P< 0004) Both velocities are correlated with spindle length at the beginning of anaphase An attempt was made to explain these phenomena on the basis of the assembly hypothesis of mitosis

Genetic duplication in the white-split interval of the X chromosome in Drosophila melanogaster

Abstract: A detailed cytogenetic study of male-viable and lethal deficiencies affecting the w-spl interval in Drosophila melanogaster has revealed the existence of genetic duplication such that, for example, the consequences of the loss of salivary chromosome band 3C3 are essentially compensated for by the presence of band 3C5-6, and vice versa. Although each of the duplicate elements possesses rst + and vt + activity, rst and vt phenotypes appear in males when 3C3 and part, but not all, of 3C5-6 are deleted. The degree of rst and vt expression can be correlated with the amount of material lost from 3C5-6. Deletions removing the entire 3C3-6 interval are male lethal. Despite the duplicate elements, at least one EMS-induced, presumptive point mutation expressing only rst is known; two others express both rst and vt. No loci other than rst and vt occur between W and spl. Band 3C2 appears to be associated with the w locus, which probably extends into the interband space between 3C1 and 3C2. The w locus is not involved in the rst-vt duplication in the 3C3-6 region. — The cytogenetic characteristics of the 3C region—a high coefficient of crossing over, frequent induced chromosome breakage, ectopic pairing, constriction, and an extended replication period—can be correlated with the fact that in 3C a relatively long stretch of DNA, nearly 2% of the entire X chromosome, is highly compacted into but few adjacent bands. These characteristics do not necessarily represent special properties of intercalary heterochromatin; they can be interpreted as reflecting the properties of any similarly organized euchromatic region.

The attachment of the synaptonemal complex to the nuclear envelope. An ultrastructural and cytochemical analysis.

Abstract: An ultrastructural and cytochemical analysis of the attachment sites of the synaptonemal complex to the nuclear envelope in rat spermatocytes is presented. The association is formed by the ends of the lateral elements in the form of a plate associated with the inner membrane, as well as by formations of fibrillar character found in the perinuclear space and in the cytoplasm adjacent to the outer membrane. By means of uranyl-EDTA-lead staining, both the area of association between the lateral elements and the fibrillar formations can be strongly stained, while the same areas appear unstained when the tissue has been previously treated with RNase. Staining with alcoholic PTA also produces a strong staining effect at the level of the attachment plates of the lateral elements, and very weak staining of the perinuclear space and of the formations in the adjacent cytoplasm. On the basis of this analysis of both the associations of autosomes and of the XY pair, we suggest that this associating structure consists partly of RNA and partly proteins, presumably histones.

DNS-Replikation und die Natur der spät replizierenden Orte im X-Chromosom von Drosophila melanogaster

Abstract: Der Verlauf der DNS-Replikation im X-Chromosom von Drosophila melanogaster und im besonderen in der white-Region wurde mit Hilfe der 3H-Thymidin-Autoradiographie untersucht. Die Anzahl der markierten Kerne in den Speicheldrusen schwankt zwischen 20 und 50%; dabei uberwiegen die Partialmarkierungsmuster der Chromosomen. Im einzelnen wurden funf verschiedene Markierungstypen unterschieden, zwei mit kontinuierlicher und drei mit Partialmarkierung. Die funf Markierungstypen lassen sich zwanglos in eine Reihe einordnen, die mit der kontinuierlichen Markierung beginnt und durch die immer weiter abnehmende Anzahl der markierten „spots“ definiert ist.

Mutations affecting meiosis in Podospora anserina. I. Cytological studies.

Abstract: Three meiosis-deficient mutants of gene mei2 (mei2-1, mei-2-2 and mei2-3) are blocked during the prophase I of meiosis, before normal pachytene. The mutant mei-2-2 is leaky and there is a partial complementation in crosses mei2-2xmei-2-1 and mei2-2xmei2-3. It has thus been possible to analyse descendants of these crosses. This analysis shows an important alteration in recombination frequencies on at least three different linkage groups. Recombination frequencies appear to be increased near the centromere and decreased in other regions of the chromosomes. This coincides with a decrease in chiasma interference. Intergenic recombination is increased in a locus located very near to the chromosome II centromere. Moreover, the relative proportion of crossovers among the recombination events is stronger than in the control. Though it is impossible at present to formulate a precise hypothesis for the action of the mei2 gene at the molecular level, it is proposed that it might well control a stage of the DNA repair or synthesis.