In the BC-8 mice which comprise the virus resistant strain coisogenic with C3H mice, the single gene differentiating the two strains appears to be the gene conferring virus resistance, and its presence is apparently expressed in macrophages which cannot support multiplication of West Nile and probably all other Arbor B viruses.Summary.-Tissue cultures of peritoneal macrophages prepared from individual mice of the eighth generation of backcrossing between virus-resistant hybrids and virus-susceptible C3H mice were exposed to West Nile virus. Half of the cultures failed to support virus multiplication, while the remaining cultures yielded infectious virus. This distribution of resistance and susceptibility in macrophage cultures reflected on the cellular level genes segregating for virus resistance and susceptibility on the whole animal level.A knowledge of the relation between synapsis, exchange, and disjunction is basic to an understanding of the meiotic process. That exchange in the female of Drosophila melanogaster is not a prerequisite for regular disjunction has been demonstrated by Sturtevant and Beadle' and by Cooper.2 It is equally clear that when more than two chromosomal elements are mutually involved in disjunction, as happens with heterologues in the case of translocation heterozygotes3 or with heteromorphs in the case of secondary nondisjunction,' the frequencies of exchange and regular disjunction are positively correlated. The role of a heterologue or of a heteromorph in these situations has been variously interpreted.Bridges4 postulated that competitive X,X,Y pairing, initiated prior to exchange (since secondary exceptions are almost invariably noncrossovers), is responsible
A temperature-sensitive recombination-defi- The time of meiotic recombination in eukaryotes remains unresolved. Traditional concepts continue to place the event at pachytene during meiotic prophase, but persuasive evidence now exists to fix the time much earlier, during premeiotic interphase. Part of this evidence comes from studies of the temporal relationship between recombination and the premeiotic-S phase, arrived at by means of a temperature probe which locates the sensitive period for increasing recombination. Reliable results with this method require that the temperature treatment be applied to a well-synchronized meiocyte population, that response be measured genetically via crossovers, and that the effective treatment period be sufficiently short and the S phase sufficiently long to permit discrimination between response at replication and response at a postreplication prophase stage. Studies meeting these criteria have shown that the heat-sensitive period for induction and enhancement of recombination coincides with premeiotic-S (1-6) or premeiotic interphase (7). The simplest interpretation of these data is that recombination occurs during premeiotic-S. The less likely possibility, that heat acts indirectly and that recombination occurs at a later time when response is absent, has not been rigorously excluded. An independent approach to the problem appeared possible with a temperature-sensitive recombination-deficient mutant that would permit high frequencies of recombination at the permissive temperature but would decrease or eliminate recombination at the restrictive temperature. The time of recombination would then be revealed by identifying the effective period for reduction of recombination by the restrictive temperature.A search for such a mutant led to the recovery of a new locus designated recombination-1 (rec-1). The locus is located on the right arm of chromosome 3 within the segment defined by the deficiency, Df(3R)sbdt°5. It is represented by three mutant alleles-namely, rec-1 6, rec-1 16, and rec-1 26. Only rec-1 26 is temperature-sensitive, and this property is dominant to the temperature insensitivity of the two other alleles. None of the alleles displays any evidence of mutator activity as is seen with the mutant mutator (8), and all complement fully with mutants at the neighboring c(3)G locus. Electron microscopic examination reveals the presence of synaptonemal complexes. The method used to select the mutants and a description of their properties will be published elsewhere.The present experiments were designed to delineate the period during female germ-cell development when the temperature-sensitive rec-1 26 mutant responds to the restrictive temperature. Identification of the sensitive period as coincident with S now provides an independent criterion for localizing the time of meiotic recombination in a eukaryotic system as occurring during the S phase. EXPERIMENTAL PROCEDUREThe Pupal System. To study the ability of heat to alter recombination frequency when applied to germ...
The extrapolation of the "distributive-pairing hypothesis" of meiosis in Drosophila melanogaster females to human gametogenesis is proposed to account for the co-occurrence of rare karyotype abnormalities in human families. A description of the hypothesis is presented with its application to some established cases in the cytogenetic literature.
all of the multiple enzyme types resulting from the treatment are more negatively charged than the original enzyme. With the exception of an enzyme that migrates slower than SS, the converted enzymes have migration rates identical to the autoand allodimer isozymes occurring in the homozygous and heterozygous combinations of the EF, E1N, and E18 alleles. It is postulated that these three alleles specify polypeptides with identical charges but differing in the number of conjugated positively charged side groups. Loss of varying numbers of these side groups after treatment with NaBH4 is proposed to explain the isozyme interconversions. The assistance of K.
The first investigation into the time of crossing-over in the Drosophila female was made by Plough.' From his results he estimated that crossing-over took place in "the very early o6cytes, just after the last oogonial division." Recent studies of the time of the last DNA replication in animal germ cells place this synthesis almost exclusively at the premeiotic interphase immediately preceding meiotic prophase.2-4 These findings are consistent with the possibility that crossing over and DNA synthesis are coincident.In the present experiment, the time relationship between crossing-over and DNA replication in the Drosophila o6cyte has been examined. The induction of an increase in crossing-over by elevated temperature has been used as a genetic "marker" for the time of crossing-over. Incorporation of tritiated thymidine has been used as a cytological "marker" for the time of DNA replication. The results show that, within the limits of resolution permitted by the method, the two processes are close or coincident in time.General Method.-Heat treatments have been shown to increase the frequency of crossing-over in the proximal regions of chromosome two.' Once the interval between heat treatment and the deposition of affected eggs is determined, this time can be read on a cytological timetable (see below) to provide information about the stage of cytological development at which heat treatment produces its genetic effect. The assumption is made that the genetic effect of heat treatment results from alterations which occur at the time of normal crossing-over. Although no direct evidence exists for this assumption, it seems to be a reasonable one.The cytological timetable is based upon detection of incorporated H3-thymidine which provides a means for identifying those germ cells which were completing their last DNA replication at the time tritiated thymidine was available, and hence must have been in the oocyte stage. Tritiated thymidine incorporated into oogonia is expected to be diluted by subsequent replication, and such oogonia will, in general, follow behind the front of heavily labeled oocytes. The progression of the heavily labeled o6cytes through successive stages until maturity is expected to provide information about: (1) the time interval from the last DNA replication to maturity, and (2) the time interval between the last DNA replication and any maturation stage. In the present experiments labeled odcytes in heat-treated and untreated females progressed through the ovarioles at the same rates so that data from both could be combined. A detailed description of the cytological studies is in preparation by Ann C. Chandley and will be published separately.
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