A marked ring-Y-chromosome in Drosophila hydei with a new type of white variegation

A ring-Y chromosome, R(Y)w ^m, of D. hydei is described which carries a complete set of fertility genes, a NOR region and a small X-chromosomal insertion (w ^m), which may be used as a marker. The ring has been characterized by various staining techniques. It was derived from a w ^mCo Y chromosome by X-ray treatment of spermatocytes. Its mode of origin allows to fix the gene order in the distal region of the long arm of the w ^mCoY chromosome. The white ⁺ gene included in the ring shows a new type of position-effect variegation which is described and discussed in the context of an earlier hypothesis on a dual function of the white locus.     

An analysis of white-mottled mutants inDrosophila hydei,with observations on X-Y exchanges in the male

Chromosomes and phenotypes of four different sex-linkedwhite-mottled mutants of the position-effect variogation type were studied. Three mutants (w ^m1,w ^m2,w ^m3) are X-chromosomal rearrangements which shift the w⁺ locus into a position close to heterochromatin, but which have different ouchromatic and heterochromatic breaks. The fourth, a spontaneous derivative ofw ^m1, is an insertional duplication of part of the X chromosome, including thew ⁺ andN ⁺loci. The duplicated segment is inserted into the distal part of the long arm of the heterochromatic Y chromosome. It is designated,w ^m CoY, orXw ^m Co when transferred to the X chromosome.Three chromosomal types (w ^m1,w ^m CoY) and (Xw ^m Co) having the same cuchromatic break near thew ⁺ locus, cause large-spotted eyes whereas two others (w ^m2,w ^m3) produce a popper-and-salt type of mottling. From the position of the various eu- and heterochromatic breaks, it appears that the distance of thew ⁺ locus to the point of reunion with heterochromatin, rather than the amount or type of adjoining heterochromatin, dietates the phenotypic action of the displacedw ⁺ locus, in the sense of a spreading effect on two proposed functional subunits within thew ⁺ locus.The pigmentation background against which the mottling effect is produced, i.e., a givenw-allele with its characteristic colour, or other eye colour mutations, does not seem to affect the type of mottling. Drosopterins and ommochromes react in the same way to modifing factors like temperature and supernumerary Y chromosomes. Two mutants (w ^m2 andw ^m CoY) while reacting in the same manner to Y chromosomes showed an opposite temperature response.By exchange between the heterochromatin of the Y and X chromosome inw/w ^m CoY males thew ^m Co duplication was transferred between the sex chromosomes with a certain regularity. It is not yet known wether the exchanges are mitotic or meiotic in origin but their heterochromatic nature has been demonstrated cytologically.     

The action of the Notch locus in Drosophila hydei

The phenotypic effects of different doses of the dominant, sex-linked mutant Notch (N) and its wildtype allele (N ⁺) were studied in Drosophila hydei, N being lethal in homozygous or hemizygous condition. Various dosage combinations were made by using N ⁺ N and N ⁺ N ⁺ attached-X chromosomes as well as X and Y N ⁺-duplication chromosomes (w ^mCoY, Xw^mCo,and DpCo ^Nt). The N mutant used, N ⁶⁸, is associated with a small inversion: In (I) N ⁶⁸.The wing phenotype was found to depend solely on the number of functional (N ⁺) alleles present, irrespective of the dose of N. Females with a single dose of N ⁺ are phenotypically Notch, females with three or four doses of N ⁺ show a Confluens wing phenotype. The latter occurs in varying degrees of expression which seem to be correlated with the relative amounts of sex-chromosomal heterochromatin present. In males the N ⁺ locus behaves as a dosage compensated locus either on the X or the Y chromosome.In the w ^mCo (w⁺N⁺) duplication, the w ⁺ locus shows variegation when placed over white, whereas N ⁺ placed over N ⁶⁸ does not. The former being situated closer to the heterochromatin in this aberration, this is consistent with the idea of gene inhibition by heterochromatin but at the same time would imply a very limited spreading effect.     

Y chromosome-specific mutations induced by a giant transposon in Drosophila hydei

Summary The w ^m Co duplication of Drosophila hydei (Dp (1; Y) 16B2-17B1) contains 13–16 bands in salivary gland chromosomes. The duplication resides preferentially in the X heterochromatin or on the Y chromosome. In some stocks frequent (up to 4×10^-3) exchanges of the duplication occur between different Y chromosomes (T(X; Y) and free Y) or between the X and the Y chromosome. About 60% of the T(X; Y)-Y exchanges induce mutations in the Y chromosomal male fertility genes of the recipient Y chromosome. From the mutational spectrum generated by the T(X; Y)-Y transpositions and from the variable efficiency as acceptor of different X-Y translocations it can be concluded that the exchanges show a remarkable site specificity: distal positions in the long arm of the Y chromosome are occupied preferentially. More proximal positions in the long arm of insertions into the short arm of the Y chromosome are found only with a lower frequency. No transpositions to the autosomes have been recovered. Duplications are lost with highly differing frequencies. The losses are not linked with insertions of the w ^m Co element into a new position and are more frequent than transpositions. Therefore, we regard the w ^m Co element as a giant transposon.     

The genetic basis of resistance and sensitivity to the meiotic drive gene D in the mosquito Aedes aegypti L.

A study has been made on the genetic basis of meiotic drive at the Distorter (D) locus which, in coupling with the male-determining gene (or region) M on the Y chromosome, causes production of excess male progeny. Its effect is regulated by the sensitivity/resistance of the X chromosome. This study demonstrates that there are two major loci controlling resistance/sensitivity to MD: (1) the m gene (or region) on the X chromosome (allelic with M) which may be either m ^R or m ^S (resistant or sensitive), (2) the t (tolerance) gene (or genes) which recombines with m and, if present, largely counteracts the effect of m ^S . There is also evidence that MD itself is capable of limited adaptation.The conclusions were derived from using MD males of the T30 or ACCRA strains (from Trinidad and Ghana respectively). The work involved the use of the CHIPEI and RED strains with sensitive X chromosomes, the latter also carrying the t (tolerance) gene which is linked to re (red eye) and m (the sex-determining locus or region) but recombines with both. The implications of these findings for using MD as a method of population control are discussed.     

Genetic instability in Drosophila melanogaster

Summary An unstable long tandem duplication which includes the white locus twice, marked with w ^sp in the left and w ^17G in the right locus, when kept in males has been found to produce red-eyed sons which have lost the long duplication and with it the w ^sp and w ^17G mutants. Such exceptions were produced also when w ^17G had been exchanged for w ^a.Stocks originating from these exceptions are unstable, producing: 1) zeste males, also unstable, 2) w ^- deletions, stable, 3) transpositions of the white locus to sites in other chromosomes.The instability is interpreted as the effect of an IS element, within or adjacent to the white locus, which is supposed to retain a duplication of the proximal zeste interacting part of this locus. According to the orientation of the IS element the duplicated part can be active or inactive, giving a zeste or red eye phenotype.The frequency of exceptional offspring after X-ray treatment of the red and zeste unstable stocks have been compared to stable stocks with corresponding genotypes.     

The incidence of sex-chromosome anomalies following irradiation of mouse spermatogonia with single or fractionated doses of x-rays

Attempts to recover XO offspring resulting from 600 R irradiation of spermatogonia proved negative. X-Rays were administered either in a single dose or in 100+ 500 R fractions separated by 24 h, and controls were strictly comparable in all respects. Altogether 14016 offspring were scored, including a small group derived from irradiated mature spermatozoa. The breeding scheme allowed phenotypic detection of paternal or maternal sex-chromosome loss, paternal nondisjunction, and certain translocations. All phenotypically exceptional mice were examined cytologically and through breeding tests. Similar tests of the mothers of presumed O/X^p exceptionals revealed that in 9 of 14 cases there was a pre-existing XO condition, indicating the importance of performing such a test. Two of the 3a X^m/O-appearing mice were probably X^m/O///X^m/X^p mosaics, with the integument predominantly XO and the germinal tissue predominantly X/X.The frequency of paternal sex-chromosome loss was 2.4 · 10^?3 in the controls and 2.0 · 10^?3 in the two irradiated groups, where X^m/O's must therefore be assumed to be of spontaneous origin. Since translocation experiments provide evidence that chromosome breaks are induced by irradiation of spermatogonia, the failure to recover XO's is explained in one of two possible ways. (1) Breakage in spermatogonia does not lead to recoverable single-chromosome loss, either because no sister-chromatid joining occurs, or, if it does, because this leads to cell-division failure. (2) Alternatively, sex chromosome loss does occur, but resulting X/O and O/Y cell progeny is inviable in the testis—a suggestion supported by evidence from natural and artificial mosaics.The results of the present experiment lend further support to our earlier suggestion that most spontaneously occurring X^m/O mice are the result of events occurring after sperm entry into the egg. The spontaneous frequency of paternal sex-chromosome loss has ranged over two orders of magnitude in various reports. On the other hand, the frequencies of spontaneous X^m loss (O/X^p daughters of X/X females/total daughters) and of paternal nondisjunction (2 · X^m/X^p/Y frequency)     

Neutrons and X-rays, comparative studies with Drosophila melanogaster. 2. Sex-chromosome loss and partial loss, evidence for the induction of chromatid aberrations in spermatozoa

Losses and duplications of BSY y+-chromosome markers were induced by irradiation of spermatozoa with either 0.5-MeV neutrons or 100-kV X-rays. These 2 types of radiation are known to induce significantly different ratios of double:single strand breaks in DNA. Exceptional progeny were grouped into 3 categories; no Y marker, one Y marker, and Y marker duplications + mosaics. The last combination consisted of exceptions derived from only chromatid-type rearrangements. All other classes of exceptions may be derived from either chromatid- or chromosome-type rearrangements. Doses of 15 Gy neutrons and 27 Gy X-rays induced identical frequencies of exceptional progeny, giving an RBE of 1.8. The ratios of the 3 classes of exceptions were similar for both types of radiation. This observation can be interpreted as indicating that, under the conditions used here, chromosome and chromatid rearrangements are not derived directly from double and single DNA strand breaks, respectively.     

Tests for parental imprinting in the nematode Caenorhabditis elegans

Summary The mutation him-6(e1423) leads to generalized chromosomal nondisjunction during meiosis in oogenesis and spermatogenesis of C. elegans. As a result, gametes nullisomic or disomic for each of the six chromosomes occur at appreciable frequency. Crosses utilizing marked him-6 strains were used to generate and identify exceptional euploid progeny which had received both homologues of a marked autosome either from the male parent or from the female parent. Examples of all ten possible exceptions were identified and found to be viable and fertile. These results (together with previous data for the X chromosome) indicate that major chromosomal imprinting effects do not occur during gametogenesis in this organism.     

Maternal influence upon the V-type gene position effect in Drosophila melanogaster.

Summary We have studied the influence of various factors on the V-type position effect of the white gene transposed to heterochromatin as a result of different chromosome rearraugements in D. melanogaster. Variegation due to the white gene position effect is much weaker if the flies have received Dp(1;3)w^vco from parental males, and not females. The origin of the chromosome rearrangement does not have this influence in the case of T(1;4)w^m5 or has it to insignificant extent in the case of In(1)w^m4. The Y-chromosome in maternal genome strongly suppresses Dp(1;3)w^vco-induced variegation even in the progeny which has not received an extra Y-chromosome but only if this progeny gets Dp(1;3)w^vco from the same female. The low temperature (16° C) at which parental females have developed, considerably affects the position effect in the progeny with Dp(1;3)w^vco, whereas the temperature of males' development has no influence at all. The maternal temperature effect occurs also when Dp(1;3)w^vco has come down from the father, though it is stronger if the mother subjected to low temperature treatment bore the rearrangement. The influence of temperature seems to take effect at the final stages of oogenesis.The data obtained lead one to suppose that the influence of genotypic and external factors on variegation is passed to the next generation of flies in different ways. The direction of crosses and additional Y-chromosome heterochromatin in the maternal genome seem to affect variegation in the progeny through changes in the properties (structure) of the chromosome rearrangement expressing the position effect. As to the temperature of the mothers development, only a small part of its influence may be accounted for by the same mechanism, whereas most of the temperature influence seems to be passed on through other components of the nucleus or through the cytoplasm.     

Bestimmung des Heterochromatisierungsstadiums beim white-Positionseffekt mittels r?ntgeninduzierter mitotischer Rekombination in der Augenanlage von Drosophila melanogaster

Summary Position-effect variegation of eye pigmentation in the examined Dp(1;3)N ^264-58 females is due to an insertion of a X-chromosome section including the white-locus into the proximal heterochromatic region of the third chromosome. The light and dark pigmented areas have a cell lineage basis (Fig. 2). Flies bearing the w ⁺-duplication had two X-chromosomes marked with w ^a lz ⁵⁰ e and w ^a rb rux ² respectively (Fig. 3). X-ray induced mitotic recombination in presumptive eye cells of larvae resulted in w ^a lz ^50e /w ^a rb rux ² twin mosaic spots in the adult eyes. After young larvae were treated twin spots appeared, which had one partner light colored and one dark. Such combinations were rarely found after older larvae were treated. Treatment of young larvae in addition produced twin spots with one or both partners variegated (Figs. 5 and 6). Sometime after the stage at which younger larvae were treated and before the stage at which older larvae were treated the translocated w ⁺-gene in each cell was determined for function or no function. As a result the progeny of each of these cells synthesized pigment or not during the pupal stage. At a temperature of 25.5° C the developmental phase during which determination, i.e. heterochromatization of the white gene, takes place, begins not earlier than 39 hours after egg laying and ends about 8 hours later (Fig. 7). In females heterozygous for the short arm of the heterochromatic Y-chromosome linked distally to the X-chromosome (Y ^S X/X) one twin spot partners is homozygous for this arm (Y ^SX/Y^S X), the other lacks it (X/X; Fig.4a). The Y ^SX/Y^S X-partner were more frequently dark pigmented than the X/X-partners (Tables 3 and 4). This shows that heterochromatization of the translocated w ⁺-genes is markedly influenced by the genotype of the single cell. When two genotypes with varied amounts of heterochromatin were compared (Fig. 4) no difference in the phases of heterochromatization could be observed (Table 5). Therefore, when position-effect variegation is modified by varying the amount of heterochromatin in the genome the modification is probably not due to a shift in the phase of heterochromatization.

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Vorgelegt von E. Hadorn     

Univalent sex chromosomes in spermatocytes of Sxr-carrying mice

Pachytene configurations of the sex chromosomes were studied in whole-mount, silver-stained preparations of spermatocytes in mice with XY,Sxr, XX,Sxr, XO,Sxr, XO,Sxr+5¹² and T(X;4)37H,YSxr chromosomes, and non-Sxr-carrying controls. XY,Sxr males showed an increased number of X and Y univalents and of self-synapsed Y chromosomes. In T(X;4)37H,YSxr males an increased proportion of trivalent+Y configurations was also accompanied by higher numbers of self-paired Y univalents; the proportion of trivalent+X⁴ was not increased, but that of self-synapsed X⁴ univalents was. There was more selfsynapsis in cells containing one univalent than in cells containing two univalents. Spermatocytes of XX,Sxr mice contained single univalent X, which was never seen to be self-synapsed, but self-synapsis of the X occurred in a proportion of cells in XO,Sxr males. There were no self-paired X chromosomes in the XO,Sxr+5¹² mouse although lowlevel pairing of the 5¹² chromosome occurred. All four XX,Sxr and XO,Sxr males contained testicular sperm, and testicular sperm were also present in one T(X;4)37H male, while another such male had sperm in the caput. It is concluded that (1) self-synapsis of univalents is affected by variable conditions in the cell as well as by the DNA sequences of the chromosome, and (2) that the level of achievable spermatogenesis is not always rigidly predetermined by a chromosome anomaly but can be modulated by the genetic background.     

The genetics of a new mutable allele at the white locus in Drosophila melanogaster

Summary The genetics of a third case of high mutation frequency at the white locus in Drosophila melanogaster has been analyzed. The new mutable allele, w ^+u, mutates from a wild-type to a white-eyed phenotype in both males and females. The mutational event is 1) premeiotic, 2) not associated with crossingover, 3) sensitive to genetic modification, and 4) restricted to germinal tissue. The only mutants produced by w ^+u are deletions of the white locus. These deficiencies include subsites 4 and 5 of the white locus, but are cytologically unobservable. The mutable allele itself maps to subsite 4.The mutational properties of w ^+u are unlike those of the other highly mutable white alleles which have been interpreted in terms of phage-like controlling elements. Rather, the properties of w ^+u favor a model based on the premature termination of chromosome replication near the terminus of a replicon which leads to a chromosome deficient for the material between the point of premature termination and the end of the replicon.Supported by NIH predoctoral traineeship GM-150 and by NIH research grant GM-07428 to Dr. W. K. Baker.From a dissertation submitted to the Division of Biological Sciences of The University of Chicago in partial fulfillment of the requirements for the degree of Doctor of Philosophy.     

An X‐linked Myh11‐CreERT2 mouse line resulting from Y to X chromosome‐translocation of the Cre allele

The Myh11‐CreER^T2 mouse line (Cre⁺) has gained increasing application because of its high lineage specificity relative to other Cre drivers targeting smooth muscle cells (SMCs). This Cre allele, however, was initially inserted into the Y chromosome (X/Y^Cre+), which excluded its application in female mice. Our group established a Cre⁺ colony from male ancestors. Surprisingly, genotype screening identified female carriers that stably transmitted the Cre allele to the following generations. Crossbreeding experiments revealed a pattern of X‐linked inheritance for the transgene (k > 1000), indicating that these female carries acquired the Cre allele through a mechanism of Y to X chromosome translocation. Further characterization demonstrated that in hemizygous X/X^Cre+ mice Cre activity was restricted to a subset arterial SMCs, with Cre expression in arteries decreased by 50% compared to X/Y^Cre+ mice. This mosaicism, however, diminished in homozygous X^Cre+/X^Cre+ mice. In a model of aortic aneurysm induced by a SMC‐specific Tgfbr1 deletion, the homozygous X^Cre+/X^Cre+ Cre driver unmasked the aortic phenotype that is otherwise subclinical when driven by the hemizygous X/X^Cre+ Cre line. In conclusion, the Cre allele carried by this female mouse line is located on the X chromosome and subjected to X‐inactivation. The homozygous X^Cre+/X^Cre+ mice produce uniform Cre activity in arterial SMCs.     

Intragenic somatic recombination in the lozenge cistron of Drosophila

Summary In Drosophila melanogaster intragenic mitotic recombination between the two lozenge alleles, lz ³⁶ and lz ^y4, separated from each other by 0.14 meiotic recombination units, was observed. Among 48 725 females of the genotype w ⁺lz³⁶/w lz^y4 which had been irradiated by a dose of 1000 r X-rays as larvae 41–47 hours after oviposition, a total of 11 faceted eye spots (not lz) were induced. All 11 spots were w ⁺, none w. Possible reasons for the lack of the expected w, faceted spots were checked. Inversion of the X chromosome which would suppress recombination between the w and lz loci was not involved. Gene order of lz ³⁶-lz^y4-kinetochore was confirmed by meiotic recombination test. Nonautonomy of lz gene action was not a factor, as tested by gynanders which showed that lz ^y4 and lz ^s were autonomous. Possibility of reverse mutation was not likely as shown by the large scale control experiments. Gene conversion is suggested as a likely mechanism for the lack of the expected w, faceted spots although the possibility of unequal crossing over induced by X-rays can not be excluded, nor can the preferential z-segregation hypothesis.     

Third chromosome suppressor of position-effect variegation loci in Drosophila melanogaster

Summary As a result of a genetic analysis of 63 third chromosome suppressor mutations of position-effect variegation 12 different loci showing dominant suppression have been identified and their map positions determined. A compilcation of the genetic data available for each suppressor locus is given. The strong suppressor effects of the mutations have been quantified by measurements of white variegation inw ^m4h /w ^m4h ,w ^m4h /Y andw ^m4h /O flies. Mutant alleles of three loci were found in these studies to dominate over the strong enhancer effect of complete loss of the Y chromosome. Most of the identified loci suppressing position-effect variegation represent essential genetic funtions; only three loci represent nonessential functions. Mutations of two loci display recessive butyrate sensitivity and lethal interaction with the heterochromatic Y chromosome suggesting that these genes affect chromosomal condensation. Studies with deficiencies and triploids revealed that most of the loci represent haplo-abnormal suppressor functions. The use of the isolated mutant material for genetic, developmental and molecular studies of processes connected with gene inactivation in position-effect variegation is discussed.Dedicated to Prof. H.J. Becker on the occasion of his 6th birthday     

Changes in the structure of B A chromosomes in maize

Summary A class of mosaic endospersm involving the marker Su _I was observed among the progeny of individuals hyperploid for the chromosome B ⁴ and genetically analyzed. The exceptional individuals showing mosaic endosperm were found when the hyperploid material was used as pollen source. While in some cases mosaicism was limited to the endosperm tissue, with no apparent consequences in the embryo, in others the mosaicism was transmitted to the progeny, which showed changes in the structure of the B ⁴ chromosome, with the formation of unstable chromosomes whose genetic behaviour was similar to that of ring chromosomes. This interpretation was cytologically confirmed. In other cases the B ⁴ chromosome analyzed in mosaic endosperm individuals underwent altered transmission frequencies or loss, suggesting that its original structure had been modified by breakage-fusion-bridge cycles. The changes in this chromosome revealed by the mosaic phenotype are discussed in relation to the original structure of the B chromosome and the B ⁴ hyperploid condition.The author dedicates the present paper to Prof. Marcus M. Rhoades with esteem and gratitude.     

Dumpy Mutations following X-Irradiation of DROSOPHILA MELANOGASTER Mature Sperm in Oxygen or in Nitrogen

A comparison was made of the oxygen enhancement pattern among the different kinds of dumpy mutations (olv, ov, ol, lv, o and v types), yellow mutations on the scute-8 chromosome, white, miniature and forked mutations, and the marker losses in the doubly marked Y chromosome [B^S Y sc⁸ (y⁺)], all of which were induced by X rays in mature sperm of Drosophila melanogaster. The results indicate that (1) an essential difference does not exist in the oxygen enhancement pattern between the different kinds of dumpy mutations, except for the ov exceptions. For these exceptions, relatively high enhancement by oxygen is elucidated; (2) a similarity exists in the oxygen enhancement pattern among the different kinds of dumpy mutations (except for the ov exceptions), yellow, miniature and forked mutations, and B^S and y⁺ marker losses; and (3) the oxygen enhancement pattern elucidated for the ov exceptions is similar to that for the white mutations. These findings suggest that the nature of the different kinds of dumpy mutations is not different from one another, except for the ov exceptions, and that except for these ov exceptions and the white mutations, there seems to be some kind of similarity in the nature of mutation among the different kinds of mutations studied.     

The relationship between first and second chromosome segregation ratios from Drosophila melanogaster males bearing Segregation Distorter

Summary Drosophila melanogaster males heterozygous for the second chromosome locus Segregation Distorter preferentially transmit this chromosome to their progeny due to a dysfunctioning of SD ⁺-bearing sperm. SD males with a normal sex chromosome constitution produce more females than males among SD ⁺ progeny. This report shows that this unequal recovery of sexes is enhanced from XY/Y; SD/SD ⁺ males and enhanced still further from XY/O; SD/SD ⁺ males. It is argued that the probability that a SD ⁺-bearing sperm will dysfunction is related to its sex chromsome complement, with the relative probabilities of dysfunction ranked O> Y> X> XY. It is shown that a modified probit analysis accounts for the relationship between sex ratio and second chromosome segregation frequency for all paternal genotypes. Finally, SD/SD ⁺ males show no increase in sex chromosome nondisjunction with respect to a control.R. E. Denell was supported by U.S.P.H.S. Training Grant No. GM00337 and by a U.S.P.H.S. Postdoctoral Fellowship; George L. Gabor Miklos was supported by A.E.C. Contract No. AT (04-3)-34 PA150.