The meiotic process and aneuploidy.

Normal females developing at 25 degrees C produce their first population of oocytes at 132 +/- 2 hr post oviposition. Entrance of the oocytes into premeiotic interphase signals initiation of DNA replication which continues for 30 hr. Coincidentally, extensive SCs appear, averaging 50 microns (132 hr), peaking at 75 microns (144 hr), and continuing into early vitellarial stages. Recombinational response to heat, evidenced by enhancement or induction of exchange, is limited to the S-phase with a peak at 144 hr coinciding with maximal extension of the SC. Coincidence of synapsis and recombination response with S at premeiotic interphase is contrary to their conventional localization at meiotic prophase. The interrelationship between exchange and nondisjunction has been clarified by the Distributive Pairing Model of meiosis. Originally revealed through high frequencies of nonrandom assortment of nonhomologous chromosomes, distributive pairing has been shown to follow and to be noncompetitive with exchange, to be based on size-recognition, not homology, and as a raison d'etre, to provide a segregational mechanism for noncrossover homologs. Rearrangements, recombination mutants, and aneuploids may contribute noncrossover chromosomes to the distributive pool and so promote the nonhomologous associations responsible for nondisjunction of homologs and regular segregation of nonhomologs. Further information concerning early meiotic events and their relation to segregation has been revealed by studies of the ts rec-1(26) mutant. Application of the restrictive temperature (31 degrees C) at sequential times during development shows wild-type activity to be drastically reduced beginning at 126/132 hr, terminating at 162 hr, and so coinciding with S and the heat-sensitive period of the normal genome. EMs of serially sectioned oocyte nuclei maintained at the restrictive or control temperature reveal SCs to be indistinguishable, and implicate recombination rather than synapsis as the target of the mutant. Activity of rec-1(26) in the range 17 degrees to 31 degrees C reveals a sharp decline between 28+ degrees and 31 degrees C, typical of a denaturation curve. If denaturation of the rec protein by the restrictive temperature marks its active phase, it follows that recombination terminates at the end of S when the restrictive temperature becomes ineffective. The notion that synapsis ensures regular segregation (35) is invalidated by rec-1(26) whose normal synapsis at the restrictive temperature is followed by a 200-fold increase in nondisjunction at segregation, as compared to the control.(ABSTRACT TRUNCATED AT 400 WORDS)

Time of Recombination in the DROSOPHILA MELANOGASTER Oocyte. III. Selection and Characterization of Temperature-Sensitive and -Insensitive, Recombination-Deficient Alleles in Drosophila.

The procedure for the selection of a temperature-sensitive recombination mutant in Drosophila is described. Use of this procedure has led to the recovery of three alleles at a new recombination locus called rec-1, located within the region of chromosome 3 circumscribed by Deficiency (3R)sbd(105). One allele, rec-1(26), is temperature sensitive, and the other two alleles, rec-1(6) and rec-1(16), are temperature insensitive. Gene dosage studies reveal rec-1(26) to be a leaky mutant with greater recombination activity in two doses than in one. The other two alleles show no dose response, implying that they may be null mutants. The temperature response curves of rec-1(26) as a homozygote and in heteroallelic combination with rec-1(16) suggest that the sharp decrease in recombination between 28 degrees and 31 degrees indicates temperature denaturation of an enzyme or other protein specified by the mutant and associated with the recombination process. The ability of small changes in temperature to reverse or abolish polarity in recombination along the X chromosome arm in rec-1( 26)/rec-1(16) females brings into question the use of the "polarity" criterion to partition mutants into two functional types, i.e., precondition mutants that display polarity and exchange mutants that do not. Evidence that rec-1 may be part of a complex locus residing in a chromosome segment harboring a variety of recombination-related genes is presented.

Synaptonemal complexes at premeiotic interphase in the mouse spermatocyte.

Male mice were injected intraperitoneally with 125 microCi (1 Ci = 3.7 X 10(10) becquerels) of [3H]thymidine at 1-hr intervals and killed 1 hr after the second injection. Testes were prepared for bright-field and electron microscopic autoradiography. Primary spermatocytes, identified by light microscopy to be at the premeiotic interphase stage, were found to be heavily labeled. Electron microscopic examination disclosed the coincidental occurrence of synaptonemal complexes and label within the nuclei of premeiotic interphase spermatocytes, indicating synapses of homologues had begun during the S phase. The significance of this finding for the traditional view of meiosis is discussed.

Time of recombination in the Drosophila melanogaster oocyte. II. Electron microscopic and genetic studies of a temperature-sensitive recombination mutant.

A test has been carried out to determine if the restrictive temperature (31 degrees) acts to reduce recombination in the temperature-sensitive recombination-deficient genotype rec-l26/rec-l16 by reducing or eliminating the synaptonemal complex. Measurements of the length of synaptonemal complexes in heat-treated and untreated stage 1 oocytes, following termination of the temperature-sensitive period, reveal less than a 5% difference, with the greater length present in the treated oocytes. Alterations are not observed in synaptonemal complex distribution within the nucleus or in its fine structure. Parallel genetic studies confirm earlier observations that the restrictive temperature, whose action is confined to a 36-h sensitive period virtually coextensive with premeiotic-S, drastically reduces recombination to approximately 10% of normal. The results are most simply interpreted to mean that the restrictive temperatures acts directly on the recombination process.

Origin of meiotic nondisjunction in Drosophila females.

Meiotic nondisjunction can be induced by external agents, such as heat, radiation, and chemicals, and by internal genotypic alterations, namely, point mutations and chromosomal rearrangements. In many cases, nondisjunction arises from a reduction or elimination of crossing over, leading to the production of homologous univalents which fail to co-orient on the metaphase plate and to disjoin properly. In some organisms, e.g., Drosophila and perhaps man, distributive pairing (i.e., a psot-exchange, size-dependent pairing) ensures the regular segregation of such homologous univalents. When a nonhomologous univalent is present, which falls within a size range permitting nonhomologous recognition and pairing, distributive nondisjunction of the homologues may follow. Examples of nondisjunction induced by inversion heterozygosity, translocation heterozygosity, chromosome fragments, radiation, heat, and recombination-defective mutants are presented.

Time of recombination in the Drosophila melanogaster oocyte: evidence from a temperature-sensitive recombination-deficient mutant.

A temperature-sensitive recombination-deficient mutant, rec-126, has been isolated that permits high frequencies of recombination at the permissive temperature (25 degrees) but greatly decreases recombination at the restrictive temperature (31 degrees). The sensitive period for response of female germ cells carrying this mutant to the restrictive temperature has been defined. Sensitivity begins very close to the time the oocyte enters premeiotic interphase and initiates DNA synthesis; it continues for the duration of premeiotic-S; and it terminates with the completion of S. This time span precisely coincides with the sensitive period for enhancement of recombination by heat in the normal genome and is further characterized by the presence of the synaptonemal complex. These results provide compelling evidence for identifying premeiotic-S as the time of meiotic recombination.

A Comparison of Heat and Interchromosomal Effects on Recombination and Interference in DROSOPHILA MELANOGASTER.

Heat and interchromosomal effects on recombination have been compared for 23 regions comprising the predominantly euchromatic portions of the five arms of the Drosophila genome. Patterns of response are strikingly similar, with both modifiers causing proximal and distal increases and minimal effects in the middle of the arms. Changes in interference for the same regions in the presence of the two modifiers reveal little similarity, except for the X chromosome. The question of independent control of interference and recombination, as well as alternatives for their temporal sequence, is discussed. Recombination response to the two modifiers in the centric heterochromatin of chromosoaime 2 is markedly different from that found in euchromatin. The interchromosomal effect is absent here, whereas heat induces an increase roughly an order of magnitude greater than that found in euchromatin and totally unlike the lack of response in the proximal heterochromatin of the X chromosome. It is proposed that the sequestering of DNA satellite I (thermal dissociation 9-20 degrees lower than that of the other major satellites) in the centromeric heterochromatin of chromosome 2 (but not in X or 3) may account for the increase.

Synaptonemal complexes during premeiotic DNA synthesis in oocytes of Drosophila melanogaster.

Well-synchronized populations of oocytes obtained by means of the "pupal system" (Grell, 1973a) have been examined to determine the time of appearance of the synaptonemal complex. The complex first appears in the most advanced oocytes between 132 and 138 hr of female development. Between 138 and 156 hr the complex apparently undergoes a fourfold increase in length. At 150 and 156 hr the complex system is extensive and present in virtually all oocytes. Previous studies using the pupal system have placed the period of premeiotic DNA synthesis between 132 and 162 hr. Thus, indirect evidence indicates that a significant portion of synaptonemal complex formation is coextensive with the main DNA replication in the oocyte. Direct evidence that DNA synthesis and complex formation occur simultaneously in oocytes has been obtained by electron microscope autoradiography. By definition, then, the stage of synaptonemal complex formation in Drosophila must include premeiotic interphase.

Studies on tolerance. I. The role of the interval between doses on the development of tolerance to morphine.

The effect of varying the interval between doses on the rate of development of tolerance to a series of injections of morphine was studied in two strains of rats. Morphine sulfate injections (15 mg/kg s.c.) were given at intervals of 1, 2, 3, 7, 10, 14 or 21 days to adult male Wistar/Furth or Wistar-Lewis rats and drug effect was measured by means of the hot-plate assay technique. A diminution of morphine effect on the second or subsequent injections of drug was considered to be the result of the development of tolerance. Little or no tolerance was observed with the Wistar/Furth animals when a second injection or morphine was administered 7 days after the first, although there was marked tolerance when there were shorter or longer intervals between the first two injections. Subsequent injections of morphine sulfate, given to the animals at the predetermined intervals, resulted in wiping out out the differences between all interval groups, with the exception of the 1-day group, by the fifth injection. Although the differences between the 7-day and longer or shorter intervals were not as great with the Wistar-Lewis animals, they were significant and may reflect a slightly different rate of tolerance development in the latter strain. These observations are consistent with the hypothesis that there may be two types of tolerance, one appearing very rapidly, the other taking a longer period of time to develop.

Recombination and DNA replication in the Drosophila melanogaster oocyte.

A method is described that permits the recovery of a well-synchronized population of oocytes. Utilizing this pupal system, the heat-responsive period for increasing crossing-over in the Drosophila genome has been defined for the X chromosome and a portion of chromosome 2. The response is initiated close to the time of oocyte formation (premeiotic interphase) and is terminated after approximately 36 hr. During the 36-hr period different regions show characteristic responses, which vary in degree, in duration, and in initiation and termination points, so as to generate the beginning of a thermal recombination map for the Drosophila genome. Centromere regions exhibit the greatest increases in crossing-over for their respective chromosomes but are distinctly asynchronous in time; interstitial regions respond the least. Correlated autoradiographic studies have localized DNA replication in the oocyte to a approximately 24-hr period, which also begins close to oocyte formation (premeiotic interphase); late labeling in restricted regions, undetectable with the present method, could extend the period, as could prolonged synthesis in the oocyte. The results demonstrate that DNA replication and the heat-sensitive period for enhancement of crossing-over are coincident processes over most and possibly all of their length.

Factors affecting recognition and disjunction of chromosomes at distributive pairing in female Drosophila melanogaster. I. Total length vs. arm length.

The behavior of a compound metacentric fourth chromosome (see PDF) has been examined to determine whether arm length or total length is the basis for recognition in distributive pairing. Recognition was judged by the frequency with which the (see PDF) nondisjoined from a series of X duplications (Dp), ranging in size from 4 times the size of a single fourth chromosome. Dp, (see PDF) nondisjunction was measured in the absence and in the presence of a competitor, a compound metacentric X. In both situations, total length and not arm length, was found to confer the characteristic recognition property to the (see PDF). A comparison of Dp, (see PDF) nondisjunction curves for both the noncompetitive and competitive situations with analogous Dp, 4 curves previously obtained, show the Dp, (see PDF) curves to be similar in shape to those obtained earlier but displaced one unit to the right, corresponding precisely to the difference in size between the (see PDF) and the 4. Rules governing chromosome recognition for acrocentrics were found completely applicable to metacentrics; disjunctive behavior of metacentrics differed from that of acrocentrics in that two arms conferred on a chromosome the capacity to act as the intermediate of a trivalent when size no longer warranted this attribute. This capacity, itself, is size-dependent.

Factors affecting recognition and disjunction of chromosomes at distributive pairing in female Drosophila melanogaster. II. The effect of a second arm.

The behavior of heterozygously inverted X chromosomes that were members of the distributive pool at least 70% of the time was studied when the other pool members were either two free 4's or one compound 4. The X's were structurally modified by additions or deletions of heterochromatin, so that the two homologues differed in both size and configuration or in size alone. In the noncompetitive situation, with two free 4's, recognition between the X's remained high despite the modifications, and primary X nondisjunction was low. In the competitive situation, with the compound 4, distributive nondisjunction of the X's increased approximately two orders of magnitude, and trivalent formation was indicated. Disjunction from the trivalent varied with X size and configuration. When both X's were acrocentric, the smaller X directed the larger X and the very small (see PDF) to the same pole; when the larger X carried a second arm, it assumed the directing role; when the size ratio of the smaller, one-armed X to the larger, two-armed X became less than approximately 5/9, the smaller X again directed the other two.