Statistical Analysis of Nondisjunction Assays in Drosophila

Many advances in the understanding of meiosis have been made by measuring how often errors in chromosome segregation occur. This process of nondisjunction can be studied by counting experimental progeny, but direct measurement of nondisjunction rates is complicated by not all classes of nondisjunctional progeny being viable. For X chromosome nondisjunction in Drosophila female meiosis, all of the normal progeny survive, while nondisjunctional eggs produce viable progeny only if fertilized by sperm that carry the appropriate sex chromosome. The rate of nondisjunction has traditionally been estimated by assuming a binomial process and doubling the number of observed nondisjunctional progeny, to account for the inviable classes. However, the correct way to derive statistics (such as confidence intervals or hypothesis testing) by this approach is far from clear. Instead, we use the multinomial-Poisson hierarchy model and demonstrate that the old estimator is in fact the maximum-likelihood estimator (MLE). Under more general assumptions, we derive asymptotic normality of this estimator and construct confidence interval and hypothesis testing formulae. Confidence intervals under this framework are always larger than under the binomial framework, and application to published data shows that use of the multinomial approach can avoid an apparent type 1 error made by use of the binomial assumption. The current study provides guidance for researchers designing genetic experiments on nondisjunction and improves several methods for the analysis of genetic data.MEIOSIS is a specialized cell division, where a diploid cell undergoes a single round of replication followed by two rounds of segregation to produce four haploid gametes. During this segregation, chromosomes must correctly separate (or disjoin) from their homologs at meiosis I, followed by sister chromatids disjoining at meiosis II. When chromosomes fail to disjoin from their partners, the resultant nondisjunction produces aneuploid gametes with the wrong number of chromosomes. The study of meiotic nondisjunction in Drosophila has a long and distinguished history of publication in genetics, with the inaugural article published in this journal being Calvin Bridges'' use of nondisjunction to prove the chromosome theory of heredity (Bridges 1916). The first study that screened variants isolated from natural populations used nondisjunction to identify meiotic mutants (Sandler et al. 1968), as did the first EMS-induced mutant screen (Baker and Carpenter 1972). Subsequent screens using new mutagens or techniques have also relied on measuring nondisjunction to identify mutants of interest (Sekelsky et al. 1999). Indeed, much of the progress that has been made in the study of meiosis would not have been possible without the use of nondisjunction to identify new mutations that are defective at some step in chromosome segregation.However, one difficulty in estimating nondisjunction rates is that in most instances the resulting aneuploid progeny cannot survive. Fortunately, in Drosophila it is possible to design crosses to recover them. Sex determination in flies is based on the number of X chromosomes, rather than a masculinizing Y chromosome as in mammals. This means that XO flies are viable (but sterile) males, while XXY flies are viable females. Therefore, it is possible to recover both normal and nondisjunctional progeny, as a nullo-X egg fertilized by an X-bearing sperm will survive as an XO male, while a diplo-X egg fertilized by a sperm lacking an X will be female (XXY). By using visible markers on the sex chromosomes, these exceptional progeny are straightforward to identify. However, if those eggs are fertilized by the other class of sperm, the resulting OY or XXX progeny are inviable. Therefore, the nondisjunction rate that occurs during meiosis is not equal to the proportion of nondisjunctional progeny, as only 50% of nondisjunctional eggs receive sperm compatible with viability, while all normal eggs are viable.Given this experimental limitation, what is the correct method to calculate the error rate during meiosis? For this discussion, let N be the total number of progeny produced in an experiment, let X₁ be the number of inviable nondisjunctional progeny (OY and XXX), let X₂ be the number of viable nondisjunctional progeny (XO and XXY), and let X₃ be the number of normal progeny (XY and XX), such that N = X₁ + X₂ + X₃. If all progeny could be counted, then the nondisjunction rate would simply be (X₁ + X₂)/N.However, only flies that survive to adulthood can be counted, and therefore both X₁ and N are unknown. As X- and Y-bearing sperm are produced in equal numbers, live and dead nondisjunctional progeny are also expected in equal numbers. Therefore, K.W. Cooper (Cooper 1948) proposed the widely used estimator for the X chromosome nondisjunction rate, where X₂ is substituted for X₁ in the above formula, giving the rate as:(1)While this estimator works, the statistical properties of this estimator are not clear. Instead of following the early literature to combine X₁ and X₂ and use a binomial distribution, we go back to the three original categories and model the process as a multinomial distribution with latent number of progeny N, considering all three possible phenotypes for each progeny (nondisjunctional dead, nondisjunctional living, and normal). Whether a nondisjunctional oocyte becomes a nondisjunctional dead or nondisjunctional living progeny depends on the sex chromosome content of the sperm that fertilized it. As X- and Y-bearing sperm are produced in equal numbers during male meiosis, the usual genetic expectation for the rates of nondisjunctional dead and living progeny will be . However, even assuming that the rates of nondisjunctional dead and living progeny are different, with a Poisson assumption of N, we can derive the maximum-likelihood estimators (MLEs) for the nondisjunctional dead and nondisjunctional living rates. Under the usual genetic expectation of equality, the MLE of the nondisjunctional rate coincides with Cooper''s estimator, and we furthermore derive the exact distribution of . Under another set of reasonable assumptions, we show the consistency and asymptotic normality of Cooper''s estimator, and derive asymptotic results when comparing two nondisjunction rates. All these distributional results enable us to develop confidence interval and hypothesis testing related to p, or p_x − p_y in the case of comparing two nondisjunction rates from populations x and y.     

Radiation-Sensitive Mutants of CAENORHABDITIS ELEGANS

Nine rad (for abnormal radiation sensitivity) mutants hypersensitive to ultraviolet light were isolated in the small nematode Caenorhabditis elegans. The mutations are recessive to their wild-type alleles, map to four of the six linkage groups in C. elegans and define nine new games named rad-1 through rad-9. Two of the mutants—rad-1 and rad-2—are very hypersensitive to X rays, and three—rad-2, rad-3 and rad-4—are hypersensitive to methyl methanesulfonate under particular conditions of exposure. The hypersensitivity of these mutants to more than one DNA-damaging agent suggests that they may be abnormal in DNA repair. One mutant—rad-5, a temperature-sensitive sterile mutant—shows an elevated frequency of spontaneous mutation at more than one locus; rad-4, which shows a cold-sensitive embryogenesis, reduces meiotic X-chromosome nondisjunction tenfold and partially suppresses some but not all mutations that increase meiotic X-chromosome nondisjunction; the viability of rad-6 hermaphrodites is half that of rad-6 males at 25°; and newly mature (but not older) rad-8 hermaphrodites produce many inviable embryo progeny. Meiotic recombination frequencies were measured for seven rad mutants and found to be close to normal.     

The Genetic Analysis of a Chromosome-Specific Meiotic Mutant That Permits a Premature Separation of Sister Chromatids in DROSOPHILA MELANOGASTER

mei-G87 is a recessive meiotic mutant that increases second chromosome nondisjunction in both males and females. A significant proportion of the diplo-2 exceptions are equational. In females, diplo-2 reductional exceptions are usually noncrossovers, but, in equational exceptions, crossover frequency and distribution are the same as that found in the haplo-2 controls. The frequencies of nondisjunction are relatively low: 0.6% in females and 1.3% in males. Nondisjunction frequency is affected by environmental conditions (possibly humidity). The defect in mei-G87, as in other "second division" mutants, appears to be a failure to maintain sister-chromatid cohesion. mei-G87 increases nondisjunction of only the second chromosome. This may indicate either a weak mutant with only the second chromosome being sensitive enough to misbehave or it may indicate that chromosome-specific regions responsible for sister-chromatid cohesion exist.     

Dominant X-Chromosome Nondisjunction Mutants of CAENORHABDITIS ELEGANS

Eight dominant X-chromosome nondisjunction mutants have been identified and characterized. Hermaphrodites (XX) heterozygous for any one of the mutations produce 20–35% male (XO) self-progeny compared with the wild-type frequency of 0.2%. Seven of the eight mutants carry X-autosome translocations. Three of these, represented by mnT2, involve linkage group (LG) II and show severe crossover suppression for X-linked markers. The two half-translocations comprising mnT2 are separable and of very unequal size. The smaller one includes the left tip of X and the right end of LGII and can exist as a free duplication, being present in addition to the normal chromosome complement, in either hermaphrodites or males; it has no effect on X nondisjunction. The reciprocal half-translocation of mnT2 includes the bulk of both LGII and X chromosomes; it disjoins regularly from a normal LGII and confers the property of X-chromosome nondisjunction. A fourth translocation, mnT10(V;X), is also reciprocal and consists of half-translocations that recombine with V and X, respectively. Either half-translocation of mnT10 can exist in heterozygous form in the absence of the other to give heterozygous duplication-deficiency animals; the property of X-chromosome nondisjunction is conferred, in homozygotes as well as heterozygotes, solely by one of the half-translocations, which is deficient for the left tip of the X. The final three translocations have X breakpoints near the right end of X and autosomal breakpoints near the right end of LGIV, the left end of LGV and the right end of LGI, respectively. All three are homozygous inviable. Males hemizygous for the X portion of any of the seven translocations are viable and fertile. The final mutant, mn164, maps as a point at or near the left tip of the X and causes X-chromosome nondisjunction in both heterozygotes and homozygotes. In heterozygotes, mn164 promotes equational nondisjunction of itself but not its wild-type allele. The mutants are discussed in light of the holocentric nature of the C. elegans chromosomes. It is proposed that the left end of the X chromosome plays a critical structural role in the segregation of X chromosomes during meiosis in XX animals.     

The synaptonemal complexes of Caenorhabditis elegans

Normal synaptonemal complexes (SCs), consisting of two lateral elements and a central element, are present in wild-type, him-4 and him-8 mutant strains in both hermaphrodites and males of Caenorhabditis elegans. Thus, the increase in rate of nondisjunction in the him mutants is not related to aberrant SC morphology. The wild-type hermaphrodite has six SCs, as determined from 3-D reconstruction analysis of serial sections from electron microscopy. Thus, n = 6 and this confirms early reports based on cytological studies with the light microscope. Only one end of the SC is attached to the nuclear envelope while the other end is free in the nucleoplasm and there is no apparent bouquet formation. Either end of the SC can attach to the nuclear envelope. The pairing behavior of the XX bivalent is normal and occurs synchronously with the autosomes. Electron dense bodies, or knobs, are associated with the SC via the central element and displace the chromatin for a distance of 200 nm. Each pachytene nucleus of the wild-type hermaphrodite has six such structures that are randomly dispersed along the bivalents such that some SCs have one or two knobs while others have none. Their function is unknown.     

Inseparability of X-Heterochromatic Functions Responsible for X:Y Pairing, Meiotic Drive, and Male Fertility in Drosophila melanogaster

Deficiencies encompassing part or all of the X heterochromatin of Drosophila melanogaster have been linked to three abnormalities in male meiosis and spermatogenesis: X-Y nondisjunction, skewed sperm recovery ratios favoring sperm with reduced chromatin content, and sterility in males carrying either a Y-autosome translocation or mal ⁺Y. In this study, 18 X heterochromatic deficiencies of varying sizes were tested in XY males for their spermatogenic phenotypes. All 18 proved to be either mutant for all three phenotypes or wild type for all three. Although variable among mutant deficiencies, expression levels of all three phenotypes were strongly correlated. Deficiencies that cause high levels of nondisjunction also cause severe recovery ratio distortion and are completely sterile in conjunction with mal⁺ Y. Low nondisjunction deficiencies cause comparable mild effects for the other phenotypes. The same deficiencies were also tested in males carrying a large heterochromatic free X duplication Dp(1; f)3. For all deficiencies which induce nondisjunction in XY males, the Y and free duplication pair regularly and the X fails to pair in XYDp males. Drive levels are constant across deficiencies in these males. Thus elimination of variability in the pairing phenotype also eliminates variability in sperm recovery ratios.     

Suppression and Function of X-Linked Lethal and Sterile Mutations in CAENORHABDITIS ELEGANS

We have expanded our collection of recessive lethal and sterile mutants in the region of the X chromosome balanced by mnDp1(X;V), about 15% of the X linkage map, to a total of 54 mutants. The mutations have been mapped with respect to 20 overlapping deficiencies and five X duplications, and they have been assigned to 24 genes by complementation testing. Nine mutants are hermaphrodite-sterile: one of these is a sperm-defect mutant, two have abnormal gonadogeneses and six, in five genes, are maternally influenced mutants, producing inviable zygote progeny. One of the gonadogenesis mutants and two of the maternally influenced mutants are male fertile. All but one of the maternally influenced mutants give cross progeny when mated with wild-type males. Forty-three mutants were tested for suppression by homozygous sup-5(e1464), which is believed to be specific for null alleles. Ten mutants that were judged by independent criteria not to be null mutants are not suppressed. Nine of the other 33 mutants, in nine genes, are suppressed, five in both heterozygous and homozygous suppressor stocks and four only in homozygous suppressor stocks.     

Recombination between Small X Chromosome Duplications and the X Chromosome in Caenorhabditis Elegans

Twelve new X chromosome duplications were identified and characterized. Eight are translocated to autosomal sites near four different telomeres, and four are free. Ten include unc-1(+), which in wild type is near the left end of the X chromosome, and two of these, mnDp72(X;IV) and mnDp73(X;f), extend rightward past dpy-3. Both mnDp72 and mnDp73 recombined with the one X chromosome in males in the unc-1-dpy-3 interval at a frequency 15- to 30-fold higher than was observed for X-X recombination in hermaphrodites in the same interval. Recombinant duplications and recombinant X chromosomes were both recovered. Recombination with the X chromosome in the unc-1-dpy-3 interval was also detected for five other unc-1(+) duplications, even though their right breakpoints lie within the interval. In hermaphrodites, mnDp72 and mnDp73 promoted meiotic X nondisjunction and recombined with an X chromosome in the unc-1-dpy-3 interval at frequencies comparable to that found for X-X recombination; mnDp72(X;IV) also promoted trisomy for chromosome IV. A mutation in him-8 IV was identified that severely reduced recombination between the two X chromosomes in hermaphrodites and between mnDp73 and the X chromosome in males. Recombination between the X chromosome and duplications of either the right end of the X or a region near but not including the left end was rare. We suggest that the X chromosome has one or more elements near its left end that promote meiotic chromosome pairing.     

Sex Attraction and Mating in Bursaphelenchus okinawaensis and B. xylophilus

The fungal feeding, hermaphroditic Bursaphelenchus okinawaensis is a laboratory model to understand the biology of Bursaphelenchus. The extent to which B. okinawaensis can be used to model Bursaphelenchus xylophilus mating was investigated. A chemotaxis assay was conducted to examine whether B. xylophilus and B. okinawaensis produce and respond to volatile sex attractants. Unmated B. xylophilus females were found to attract B. xylophilus males. Similarly, old (sperm depleted) but not young (sperm repleted) B. okinawaensis hermaphrodites attract B. okinawaensis males. Thus, in both species, sperm status corresponds to its ability to attract males. B. xylophilus males also produce a volatile pheromone that attracts both mated and unmated females. A second assay, in which the behavior of males on petri plates in the presence of different females or hermaphrodites of Bursaphelenchus was observed, revealed that B. xylophilus unmated females attract B. okinawaensis males, and B. okinawaensis old hermaphrodites attract B. xylophilus males. These observations suggested that the pheromones of Bursaphelenchus work to some extent across species. Mating behavior through spicule insertion occurs across species, suggesting that postcopulatory mechanisms prevent production of interspecific progeny. The hermaphroditic B. okinawaensis will be a useful model to conduct genetic studies for the understanding of the molecular mechanisms underlying mating behavior in Bursaphelenchus nematodes.     

More Sex-Determination Mutants of CAENORHABDITIS ELEGANS

Sex determination in Caenorhabditis elegans is controlled by the X chromosome: autosome ratio, i.e. 2A;XX animals are hermaphrodite, and 2A;XO animals are male. A procedure for isolating 2A;XO animals that are transformed into hermaphrodites has been developed. Nine mutations causing this transformation have been obtained: eight are recessive, and all of these fall into a new autosomal complementation group, her-1 V. The remaining mutation (her-2) is dominant and has a genetic map location similar to that of tra-1 III. Recessive mutations of tra-1 cause the reverse transformation, transforming 2A;XX animals into males. Therefore, the her-2 mutation may result in constitutive expression of tra-1. Mutations in her-1 are without effect on XX animals, but the her-2 mutation prevents sperm production in both XX and XO animals, in addition to its effect on the sexual phenotype of XO animals. The epistatic relationships between tra and her genes are used to deduce a model for the action of these genes in controlling sex determination.     

CAENORHABDITIS ELEGANS Fertilization-Defective Mutants with Abnormal Sperm

Seven new fertilization-defective mutants of C. elegans have been isolated and characterized; six are temperature sensitive, one is absolute and all are autosomal recessive. One mutation is in a previously described gene, while the other six define six new fer genes that appear to code for sperm-specific functions necessary for normal fertilization. In all fer mutants, both males and hermaphrodites accumulate sperm in near normal numbers. In hermaphrodites, mutant sperm contact the oocytes, but fail to fertilize them. Instead, the sperm are swept into the uterus by the passing oocytes and are expelled when oocytes are laid. Males of two fer mutants do not transfer sperm during copulation, but the other mutant males transfer sperm that fail to move to the spermatheca. Spermatozoa from fer-1 and fer-4 mutants are motility-defective in vitro as well as in vivo, and their pseudopods have an altered morphology. The period of development during which mutant hermaphrodites are temperature sensitive for fertility overlaps the time of sperm development. Some mutants are temperature sensitive throughout the entire period, and others are temperature sensitive during or just prior to spermiogenesis. In fer-4/+ and fer-7/+ males, the fertility of the mutation-bearing sperm is diminished, reducing the transmission ratio. This implies some post-meiotic expression of these genes.—This set of mutants provides a variety of functional and structural alterations in nematode sperm that should help identify and analyze gene products involved in sperm morphogenesis and motility.     

Lethals, Steriles and Deficiencies in a Region of the X Chromosome of CAENORHABDITIS ELEGANS

Twenty-one X-linked recessive lethal and sterile mutations balanced by an unlinked X-chromosome duplication have been identified following EMS treatment of the small nematode, Caenorhabditis elegans. The mutations have been assigned by complementation analysis to 14 genes, four of which have more than one mutant allele. Four mutants, all alleles, are temperature-sensitive embryonic lethals. Twelve mutants, in ten genes, are early larval lethals. Two mutants are late larval lethals, and the expression of one of these is influenced by the number of X chromosomes in the genotype. Two mutants are maternal-effect lethals; for both, oocytes made by mutant hermaphrodites are rescuable by wild-type sperm. One of the maternal-effect lethals and two larval lethals are allelic. One mutant makes defective sperm. The lethals and steriles have been mapped by recombination and by complementation testing against 19 deficiencies identified after X-ray treatment. The deficiencies divide the region, about 15% of the X-chromosome linkage map, into at least nine segments. The deficiencies have also been used to check the phenotypes of hemizygous lethal and sterile hermaphrodites.     

An Autosomal Gene That Affects X Chromosome Expression and Sex Determination in CAENORHABDITIS ELEGANS

Recessive mutant alleles at the autosomal dpy-21 locus of C. elegans cause a dumpy phenotype in XX animals but not in XO animals. This dumpy phenotype is characteristic of X chromosome aneuploids with higher than normal X to autosome ratios and is proposed to result from overexpression of X-linked genes. We have isolated a new dpy-21 allele that also causes partial hermaphroditization of XO males, without causing the dumpy phenotype. All dpy-21 alleles show hermaphroditization effects in XO males that carry a duplication of part of the X chromosome and also partially suppress a transformer (tra-1) mutation that converts XX animals into males. Experiments with a set of X chromosome duplications show that the defects of dpy-21 mutants can result from interaction with several different regions of the X chromosome. We propose that dpy-21 regulates X chromosome expression and may be involved in interpreting X chromosome dose for the developmental decisions of both sex determination and dosage compensation.     

WormGender – Open-Source Software for Automatic Caenorhabditis elegans Sex Ratio Measurement

Fast and quantitative analysis of animal phenotypes is one of the major challenges of current biology. Here we report the WormGender open-source software, which is designed for accurate quantification of sex ratio in Caenorhabditis elegans. The software functions include, i) automatic recognition and counting of adult hermaphrodites and males, ii) a manual inspection feature that enables manual correction of errors, and iii) flexibility to use new training images to optimize the software for different imaging conditions. We evaluated the performance of our software by comparing manual and automated assessment of sex ratio. Our data showed that the WormGender software provided overall accurate sex ratio measurements. We further demonstrated the usage of WormGender by quantifying the high incidence of male (him) phenotype in 27 mutant strains. Mutants of nine genes (brc-1, C30G12.6, cep-1, coh-3, him-3, him-5, him-8, skr-1, unc-86) showed significant him phenotype. The WormGender is written in Java and can be installed and run on both Windows and Mac platforms. The source code is freely available together with a user manual and sample data at http://www.QuantWorm.org/. The source code and sample data are also available at http://dx.doi.org/10.6084/m9.figshare.1541248.     

A Highlights from MBoC Selection: Mutations in Caenorhabditis elegans him-19 Show Meiotic Defects That Worsen with Age

From a screen for meiotic Caenorhabditis elegans mutants based on high incidence of males, we identified a novel gene, him-19, with multiple functions in prophase of meiosis I. Mutant him-19(jf6) animals show a reduction in pairing of homologous chromosomes and subsequent bivalent formation. Consistently, synaptonemal complex formation is spatially restricted and possibly involves nonhomologous chromosomes. Also, foci of the recombination protein RAD-51 occur delayed or cease altogether. Ultimately, mutation of him-19 leads to chromosome missegregation and reduced offspring viability. The observed defects suggest that HIM-19 is important for both homology recognition and formation of meiotic DNA double-strand breaks. It therefore seems to be engaged in an early meiotic event, resembling in this respect the regulator kinase CHK-2. Most astonishingly, him-19(jf6) hermaphrodites display worsening of phenotypes with increasing age, whereas defects are more severe in female than in male meiosis. This finding is consistent with depletion of a him-19-dependent factor during the production of oocytes. Further characterization of him-19 could contribute to our understanding of age-dependent meiotic defects in humans.     

Meiotic Mutants That Cause a Polar Decrease in Recombination on the X Chromosome in Caenorhabditis Elegans

Recessive mutations in three autosomal genes, him-1, him-5 and him-8, cause high levels of X chromosome nondisjunction in hermaphrodites of Caenorhabditis elegans, with no comparable effect on autosomal disjunction. Each of the mutants has reduced levels of X chromosome recombination, correlating with the increase in nondisjunction. However, normal or elevated levels of recombination occur at the end of the X chromosome hypothesized to contain the pairing region (the left end), with recombination levels decreasing in regions approaching the right end. Thus, both the number and the distribution of X chromosome exchange events are altered in these mutants. As a result, the genetic map of the X chromosome in the him mutants exhibits a clustering of genes due to reduced recombination, a feature characteristic of the genetic map of the autosomes in non-mutant animals. We hypothesize that these him genes are needed for some processive event that initiates near the left end of the X chromosome.     

Phenotypic and Genetic Characterization of Two New Mutants of Heterorhabditis bacteriophora

Two new "dumpy" mutants (Hbdpy-2 and Hbdpy-3) of Heterorhabditis bacteriophora were induced and characterized. Mutants (hermaphrodites and males) that hatched from eggs were shorter and wider than the wild-type strain. This phenotype was not discernible in young animals until 24 hours after hatching from eggs or in mutants that developed from infective juveniles. Scanning electron microscopy revealed that the tails of the two mutants are much more slender than in the wild-type. In addition, the vulva of Hbdpy-3 nematodes appeared to be sunken; that of Hbdpy-2 animals was protruding, like in the wild-type. Upon self fertilization, individual Hbdpy-3 hermaphrodites produced fewer progeny than the wild-type. Crosses between virgin Hbdpy-2 and Hbdpy-3 hermaphrodites and wild-type males indicated that the two mutations are recessive. Complementation tests indicated that Hbdpy-1, Hbdpy-2, and Hbdpy-3 affect different genes. The ratio (1.03:1) of wild-type to dumpy phenotype among the F₂ progeny of self-fertilizing heterozygotes suggested linkage among the three genes. The genetic map distance was estimated only between Hbdpy-1 and Hbdpy-2 genes, approximately 29 map units.     

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.     

Modifier Genes of the Sex Ratio Trait in Drosophila pseudoobscura

The msr trait of Drosophila pseudoobscura occurs when "sex-ratio" males produce a very high frequency of null-X sperm which give rise to sterile male (X/O) progeny. The trait involves dramatically lowered fecundity due to spermiogenic failure. The msr trait is multigenic and the genes are located on autosomes II, III and IV of the L116 laboratory stock. This stock also carries genes on the Y chromosome that lower the level of msr. When the genes on the L116 autosomes are present together or with those on the Y chromosome of other stocks, they interact cooperatively to produce very high levels of msr. The msr genes require the presence of a sex-ratio X chromosome to have any effect and thus may be regarded as modifiers of the "sex-ratio" phenotype. Crosses show that the genes causing msr are primarily recessive but have some expression when heterozygous. Sex chromosome nondisjunction is proposed as the mechanism underlying the msr trait.