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 ≤ 0.3 to > 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.     

The relation of exchange to nondisjunction in heterologous chromosome pairs in the Drosophila female

An analysis of the relationships in Drosophila melanogaster between one set of homologues (third chromosome) undergoing crossing over and a second, independent set (X chromosome) undergoing nondisjunction shows that the nondisjunctional set almost invariably segregates from one of the members of the crossover set and not the other. The results seem contradictory to the expectations based on the "distributive pairing hypothesis" according to which nondisjunctional (i.e., noncrossover) elements form a "distributive pool" whose members behave independently of those which have been involved in exchange.     

Meiotic Behavior of Compound Autosomes in Females of DROSOPHILA MELANOGASTER: Interchromosomal Effects and the Source of Spontaneous Nonsegregation

In females of Drosophila melanogaster, compound autosomes enter the repulsion phase of meiosis uncommitted to a particular segregation pattern because their centromeres are not restricted to a bivalent pairing complex as a consequence of crossing over. Their distribution at anaphase, therefore, is determined by some meiotic property other than exchange pairing, a property that for many years has been associated with the concept of nonhomologous pairing. In the absence of heterologous rearrangements or a free Y chromosome, C(3L) and C(3R) are usually recovered in separate gametes, that is as products of meiotic segregation. Nevertheless, there is a regular, albeit infrequent, recovery of reciprocal meiotic products (the nonsegregational products) that are disomic and nullosomic for compound thirds. The frequency of these exceptions, which is normally between 0.5 and 5.0%, differs for the various strains examined, but remains constant for any given strain. Since previous studies have not uncovered a cause for this base level of nonsegregation, it has been referred to as the spontaneous frequency. In this study, crosses between males and females whose X chromosomes, as well as compound autosomes, are differentially marked reveal a highly significant positive correlation between the frequency of compound-autosome nonsegregation and the frequency of X-chromosome nondisjunction. However, an inverse correlation is found when the frequency of nondisjunction is related to the frequency of crossing over in the proximal region of the X chromosome. These findings have been examined with reference to the distributive pairing and the chromocentral models and interpreted as demonstrating (1) that nonsegregational meiotic events arise primarily as a result of nonhomologous interactions, (2) that forces responsible for the segregation of nonhomologous chromosomes are properties of the chromocentral region, and (3) that these forces come into expression after the exchange processes are complete.     

Heterologous interchange and non-disjunction of distributively paired chromosomes in Drosophila melanogaster: immature oocytes

The relationships between interchange-mediated disjunction and segregation of distributively paired chromosomes have been analyzed. Even when an interchange generates a quasi-bivalent, one component of which is either the compound-X or the Y chromosome, the uninvolved sex chromosome disjoins from its regular disjunctive partner more often than not. Interchange between distributively paired heterologs does not remove these chromosomes from the distributive pool, a consequence of which would be regular disjunction of those elements remaining in the distributive pool.     

Meiotic Chromosome Behavior Influenced by Mutation-Altered Disjunction in DROSOPHILA MELANOGASTER Females

The effects of a female-specific meiotic mutation, altered disjunction (ald: 361), are described. Although ald females show normal levels of meiotic exchange, sex- and 4th-chromosome nondisjunction occurs at an elevated level. A large proportion of the nondisjunction events is the result of nonhomologous disjunction of the sex and 4th chromosomes. These nonhomologous disjunction events, and probably all nondisjunction events occurring in ald females, are the result of two anomalies in chromosome behavior: (1) X chromosomes derived from exchange tetrads undergo nonhomologous disjunction and (2) the 4th chromosomes nonhomologously disjoin from larger chromosomes. There is at best a marginal effect of ald on the meiotic behavior of chromosomes 2 or 3. The results suggest that the ald⁺ gene product acts to prevent the participation of exchange X chromosomes and all 4th chromosomes in nonhomologous disjunction events. The possible role of ald⁺ in current models of the disjunction process is considered.     

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.     

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.     

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.     

Evidence for the Single Phase Pairing Theory of Meiosis

The segregation pattern of an attached X chromosome with several Y-autosome translocations conflicts with the expectations based on the distributive pairing hypothesis because the chromosomes segregating from the translocation configuration include both exchange and non-exchange chromosomes. The results of the second experiment involving three compound chromosomes go even further; they suggest that the essential association which determines the segregation of nonhomologous elements is in fact set up prior to the time of crossing over.     

Cytogenetic Analysis of a Segment of the Y Chromosome of DROSOPHILA MELANOGASTER

Males carrying a large deficiency in the long arm of the Y chromosome known to delete the fertility gene kl-2 are sterile and exhibit a complex phenotype: (1) First metaphase chromosomes are irregular in outline and appear sticky; (2) spermatids contain micronuclei; (3) the nebenkerns of the spermatids are nonuniform in size; (4) a high molecular weight protein ordinarily present in sperm is absent; and (5) crystals appear in the nucleus and cytoplasm of spermatocytes and spermatids. In such males that carry Ste⁺ on their X chromosome the crystals appear long and needle shaped; in Ste males the needles are much shorter and assemble into star-shaped aggregates. The large deficiency may be subdivided into two shorter component deficiencies. The more distal is male sterile and lacks the high molecular weight polypeptide; the more proximal is responsible for the remainder of the phenotype. Ste males carrying the more proximal component deficiency are sterile, but Ste ⁺ males are fertile. Genetic studies of chromosome segregation in such males reveal that (1) both the sex chromosomes and the large autosomes undergo nondisjunction, (2) the fourth chromosomes disjoin regularly, (3) sex chromosome nondisjunction is more frequent in cells in which the second or third chromosomes nondisjoin than in cells in which autosomal disjunction is regular, (4) in doubly exceptional cells, the sex chromosomes tend to segregate to the opposite pole from the autosomes and (5) there is meiotic drive; i.e., reciprocal meiotic products are not recovered with equal frequencies, complements with fewer chromosomes being recovered more frequently than those with more chromosomes. The proximal component deficiency can itself be further subdivided into two smaller component deficiencies, both of which have nearly normal spermatogenic phenotypes as observed in the light microscope. Meiosis in Ste ⁺ males carrying either of these small Y deficiencies is normal; Ste males, however, exhibit low levels of sex chromosome nondisjunction with either deficient Y. The meiotic phenotype is apparently sensitive to the amount of Y chromosome missing and to the Ste constitution of the X chromosome.     

The Genetic Analysis of Distributive Segregation in Drosophila Melanogaster. I. Isolation and Characterization of Aberrant X Segregation (Axs), a Mutation Defective in Chromosome Partner Choice

We describe the isolation and characterization of Aberrant X segregation (Axs), a dominant female-specific meiotic mutation. Although Axs has little or no effect on the frequency or distribution of exchange, or on the disjunction of exchange bivalents, nonexchange X chromosomes undergo nondisjunction at high frequencies in Axs/+ and Axs/Axs females. This increased X chromosome nondisjunction is shown to be a consequence of an Axs-induced defect in distributive segregation. In Axs-bearing females, fourth chromosome nondisjunction is observed only in the presence of nonexchange X chromosomes and is argued to be the result of improper X and fourth chromosome associations within the distributive system. In XX females bearing a compound fourth chromosome, the frequency of nonhomologous disjunction of the X chromosomes from the compound fourth chromosome is shown to account for at least 80% of the total X nondisjunction observed. In addition, Axs diminishes or ablates the capacity of nonexchange X chromosomes to form trivalents in females bearing either a Y chromosome or a small free duplication for the X. Axs also impairs compound X from Y segregation. The effect of Axs on these segregations parallels the defects observed for homologous nonexchange X chromosome disjunction in Axs females. In addition to its dramatic effects on the X chromosome, Axs exerts a similar effect on the segregation of a major autosome. We conclude that Axs defines a locus required for proper homolog disjunction within the distributive system.     

The Meiotic Effects of a Deficiency in DROSOPHILA MELANOGASTER: Identification of Two Dosage-Sensitive Sites

Heterozygosity for a deficiency for the entire zeste-white region of the X chromosome of Drosophila melanogaster females causes both reduced recombination and increased nondisjunction. The location of the dosage-sensitive sites responsible for these two meiotic defects has been studied by use of a set of deficiencies that subdivide the region. Recombination is reduced when the zw7-zw11 region is present in one dose, while nondisjunction is increased only if the doses of both the zw8-zw10 and zw6-zw11 segments are reduced. Examination of trans heterozygotes of two deficiencies explicitly demonstrates the compound nature of the meiotic dose effect and further delimits the location of the proximal disjunctional site to the zw12-zw11 interval. In inversion/deficiency heterozygotes, reduced dose of the zw8-zw10 region alone, without reduced dose of the proximal site, yields increased nondisjunction, suggesting that the proximal element that affects disjunction is the same as that which affects recombination. Thus, the zeste-white region contains at least two dosagesensitive loci that affect meiosis: reduced dosage of one locus, in the zw7-zw11 interval, causes reduced recombination; reduced dose of another, in the zw8-zw10 region, increases the probability that nonexchange chromosomes will nondisjoin. A slight effect on the regional distribution of exchange may also be a property of the zw8-zw10 region locus, but could be an effect of yet another locus or of structural heterozygosity. The implications of these results for understanding meiotic control and the prospects for further analysis of the structure of the zeste-white interval are considered.     

Compound Autosomes in DROSOPHILA MELANOGASTER: The Meiotic Behavior of Compound Thirds

Studies of the meiotic distribution of compound-3 chromosomes in males and females of Drosophila melanogaster provided the following results. (1) From females homozygous for the standard arrangement of all chromosomes other than C(3L) and C(3R), less than 5% of the gametes recovered were nullosomic or disomic for compound-3 chromosomes. The frequency of nonsegregation differed between strains, but within a given strain it remained relatively constant. (2) According to egg-hatch frequencies, C(3L) and C(3R) segregate independently during spermatogenesis. (3) In females, structurally heterozygous second chromosomes occasion a marked increase in the recovery of nonsegregational progeny; in males, rearranged seconds have no apparent influence on the distribution of compound thirds. (4) The highest frequencies of nonsegregational progeny were recovered from C(3L);C(3R) females carrying compound-X (plus free Y) chromosomes. (5) In comparing the recovery of nonsegregating compound thirds to the recovery of rearranged heterologs, a definite nonrandom distribution was realized in several crosses. These results are examined in reference to the concepts of distributive pairing (Grell 1962). Moreover, considering the structural nature of compound autosomes, we propose that nonhomologous (distributive) pairing is a property of the centromeric region and suggest that rearrangements involving breaks in this region possibly alter the effectiveness of distributive pairing forces.     

Orientation Disruptor (ord): A Recombination-Defective and Disjunction-Defective Meiotic Mutant in DROSOPHILA MELANOGASTER

The effects of a semidominant autosomal meiotic mutant, orientation disruptor (symbol: ord), located at 2–103.5 on the genetic map and in region 59B-D of the salivary map, have been examined genetically and cytologically. The results are as follows. (1) Crossing over in homozygous females is reduced to about seven percent of controls on all chromosomes, with the reduction greatest in distal regions. (2) Crossing over on different chromosomes is independent. (3) Reductional nondisjunction of any given chromosome is increased to about thirty percent of gametes from homozygous females. The probability of such nondisjunction is the same among exchange and nonexchange tetrads with the exception that a very proximal exchange tends to regularize segregation. (4) Equational nondisjunction of each chromosome is increased to about ten percent of gametes in homozygous females; this nondisjunction is independent of exchange. (5) The distributive pairing system is operative in homozygous females. (6) In homozygous males, reductional nondisjunction of each chromosome is increased to about ten percent, and equational nondisjunction to about twenty percent, of all gametes. (7) Cytologically, two distinct meiotic divisions occur in spermatocytes of homozygous males. The first division looks normal although occasional univalents are present at prophase I and a few lagging chromosomes are seen at anaphase I. However, sister chromatids of most chromosomes have precociously separated by metaphase II. Possible functions of the ord⁺ gene are considered.     

Perceptual learning in an appetitive Pavlovian procedure: Analysis of the effectiveness of the common element

Non-reinforced preexposure to two stimuli often enhances discrimination between them. Analyses of this perceptual learning phenomenon have mainly focused on the role played by the distinctive stimulus features; this study examined the contribution of the non-distinctive common elements. A standard appetitive Pavlovian procedure was used. Rats received two different schedules of exposure - alternated or blocked - to two compound auditory stimuli, AX and BX. In Experiment 1 a generalization test to BX that followed conditioning to AX showed that animals responded less, and hence discriminated better, following alternated exposure, thus extending the generality of this perceptual learning effect to standard appetitive Pavlovian procedures. The degree to which the common element X was mediating this effect was explored in the next three experiments. Experiment 2 assessed the effectiveness of X following conditioning to AX. Experiment 3 explored X's effectiveness throughout extensive conditioning to X. Experiment 4 tested the ability of X to overshadow a novel stimulus Y. The results were consistent with the suggestion that alternated preexposure can reduce the relative effectiveness of the common element.     

Nondisjunction Mutants of the Nematode CAENORHABDITIS ELEGANS

The frequency of males (5AA; XO) among the self progeny of wild-type Caenorhabditis elegans hermaphrodites (5AA; XX) is about one in 500. Fifteen him (for "high incidence of males") mutations have been identified that increase this frequency by a factor of ten to 150, as a result of increased X-chromosome nondisjunction. The mutations define ten complementation groups, which have been mapped: nine are autosomal, and one sex linked. Most of the mutants are superficially wild type in anatomy and behavior; however, him-4 mutants display gonadal abnormalities, and unc-86 mutants, which have a Him phenotype, exhibit a variety of anatomical and behavioral abnormalities. All the mutants segregate fertile 3X hermaphrodite progeny as well as XO male progeny. Some produce large numbers of inviable zygotes. Mutants in all ten genes produce diplo-X and nullo-X exceptional ova, and in the four strains tested, diplo-X and nullo-X exceptional sperm are produced by 2X "transformed" males. It appears likely that most of the mutants have defects in both gamete lines of the hermaphrodite. XO males of him strains other than him-4 and unc-86 are similar to wild-type males in anatomy and behavior, and all produce equal or almost equal numbers of haplo-X and nullo-X sperm, and no diplo-X sperm. Male fertility is reduced to varying extents in all him mutants. In four of the strains, nondisjunction during oogenesis has been shown to occur at a reductional division, and in three of these strains, abnormalities in recombination have been demonstrated. One mutant also exhibits autosomal nondisjunction, but many of the others probably do not. Therefore, the X chromosome of C. elegans may differ from the autosomes in the mechanisms controlling its meiotic behavior.——3X hermaphrodites are shorter and less fertile than 2X hermaphrodites, and they produce many inviable zygotes among their self progeny: these are probably 4X zygotes. Haplo-X and diplo-X ova are produced in 2:1 ratio by 3X hermaphrodites. him mutations are expressed in these animals, increasing the frequency of self-progeny males and 2X hermaphrodites.     

Vitellogenin in locusts (Locusta migratoria): Translation of vitellogenin mRNA in Xenopus oocytes and analysis of the polypeptide products

Cytoplasmic poly(A)⁺-RNA, prepared from fat bodies of reproductively active locust females, directed the synthesis of two large polypeptides in Xenopus oocytes. Occasionally two smaller polypeptides (X₁ and X₂) were also detectable. The two larger polypeptides were immunologically and electrophoretically indistinguishable from the unprocessed Vitellogenin polypeptides (Vg₁ and Vg₂) of locust fat bodies. Peptide patterns generated from these two translation products by Staphylococcus aureus V8 protease digestion were identical to those of Vg₁ and Vg₂. RNA isolated from allatectomized female locusts treated with the juvenile hormone analog (ZR-515) was also able to direct the synthesis of vitellogenin in Xenopus oocytes, whereas RNA isolated from mature males or allatectomized females did not. The molecular weights of fat-body Vg₁ and Vg₂ were 235,000 and 225,000, respectively and the processed vitellogenin polypeptides were found to range from 126,000 to 52,000. Electrophoretic and Chromatographic analysis of [³⁵S]methionine-containing tryptic peptides of Vg₁ and Vg₂ showed two different tryptic peptide fingerprints. Distinctly different peptide patterns were also observed when Vg₁ and Vg₂ were partially digested with V8 protease or papain. However, tryptic peptide mapping and V8 protease limited digestion mapping of fat-body X₁ and X₂ revealed that these two polypeptides were derived from Vg₁ and Vg₂. This suggests that Vg₁ and Vg₂ are products of two different vitellogenin structural genes.     

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.     

Mutation tests in Neurospora crassa: A report of the U.S. environmental protection agency gene-tox program

Many mutation tests have been developed in Neurospora crassa during the almost 40 years of its use in mutation research. These tests detect two major classes of mutation: gene mutation and meiotic nondisjunction. Within the first class, forward- and reverse-mutation tests have been used. The forward-mutation tests include those that detect mutations at many loci and at specific loci. Both kinds of forward-mutation tests have been done in homokaryons (n) and heterokaryons (n + n′). From the publications that were not rejected by our pre-established criteria, data were extracted for 166 chemicals that had been tested for mutagenicity. Only 6 of the 166 chemicals have been tested in one or more gene mutation test and the meiotic nondisjunction test; these 6 chemicals were positive in the first and negative in the second. Of the 102 chemicals tested in one or more gene mutation tests, 94 were positive and 8 were negative. Of the 70 chemicals tested in the meiotic nondisjunction test, 7 were positive and 63 were negative.Two tests, the ad-3) forward-mutation test and the meiotic nondisjunction test, have been used most frequently. These two tests are especially important for hazard evaluation, because each detects a class of mutations that is likely to be deleterious or lethal in the F₁ - disomics by the meiotic nondisjunction test and multilocus deletions by the ad-3 forward-mutation test in heterokaryons. Generally, direct-acting chemicals are mutagenic in the gene mutation tests, but few chemicals that required metabolic activation have been tested. Only 31 of the 166 chemicals tested in N. crassa have been tested for carcinogenicity. Among these chemicals, there is a good association between mutagenicity in gene mutation tests and carcinogenicity but a poorer association between meiotic nondisjunction and carcinogenicity; however, only a small number of chemicals has been tested in the meiotic nondisjunction test. Further use and development of certain mutation tests in N. crassa are desirable.     

Cytogenetics of ticks (Acari: Ixodoidea)

Summary Prior to this paper there have been no reports of a multiple sex chromosome mechanism operative in any tick. The present paper deals with two species of Ixodidae, Amblyomma moreliae and Amblyomma limbatum that exhibit an X₁X₁X₂X₂:X₁X₂Y type of sex chromosome mechanism. Cells from males of both species show nine bivalents plus one sex trivalent. Eleven bivalents were observed in one female A. moreliae. The sex trivalent probably evolved through reciprocal translocation from a system that included ten autosomal bivalents and one sex univalent (the system found in most ixodid species). As a result of the translocation, there are now two X chromosomes (X₁ and X₂) segregating from an unaltered autosome, the neo-Y. A large X chromosome is characteristic of many ticks; in this instance the reciprocal translocation did not change appreciably its relative size.The opinions and assertions contained herein are the private ones of the author and are not to be construed as official or reflecting the views of the Navy Department or the Naval service at large.This study was begun during the tenure of a North Alantic Treaty Organization (National Science Foundation) Postdoctoral Fellowship.