Variation of Spontaneous Occurrence Rates of Chromosomal Aberrations in the Second Chromosomes of DROSOPHILA MELANOGASTER

After accumulating mutations by the aid of marked inversions, spontaneous occurrence rates of chromosome aberrations were estimated for 1148 chromosome lines that originated from five stem line second chromosomes of Drosophila melanogaster. In chromosome lines originating from three stem chromosomes (CH, PQ, and RT), mutations were accumulated for 7550, 7252, and 7256 chromosome generations, respectively, but no structural change was detected. For the chromosome lines that originated from the other two stem chromosomes, the situation was different: Twenty aberrations (19 paracentric inversions and 1 translocation between the second and the third chromosomes) during 45990 chromosome generations took place in the 500 chromosome lines derived from stem line chromosome (AW), and 92 aberrations (83 paracentric inversions, 6 pericentric inversions, 2 translocations between the second and the third chromosomes and 1 transposition) arose during 45006 chromosome generations in the 500 chromosome lines derived from stem line chromosome (JH). For the AW group the occurrence rate becomes 0.00043 per chromosome per generation for all aberrations and 0.00041 for inversions. For the JH group the corresponding rates are 0.00204 and 0.00198, respectively.-A non-random distribution of the breakpoint on the salivary gland chromosome was observed and the breakpoints were concentrated in the regions 26, 29, 33, and 34.-The cytoplasms and the chromosomes (other than the second chromosomes) were made approximately uniform throughout the experiments. Thus, this remarkable variability in the occurrence rate is most probably due to the differences in one or more chromosomal elements on the original five stem chromosomes. The mutable chromosomes (AW and JH) appear to carry a kind of mutator factor such as hi (Ives 1950).     

A P-element insertion screen identified mutations in 455 novel essential genes in Drosophila

With the completion of the nucleotide sequences of several complex eukaryotic genomes, tens of thousands of genes have been predicted. However, this information has to be correlated with the functions of those genes to enhance our understanding of biology and to improve human health care. The Drosophila transposon P-element-induced mutations are very useful for directly connecting gene products to their biological function. We designed an efficient transposon P-element-mediated gene disruption procedure and performed genetic screening for single P-element insertion mutations, enabling us to recover 2500 lethal mutations. Among these, 2355 are second chromosome mutations. Sequences flanking >2300 insertions that identify 850 different genes or ESTs (783 genes on the second chromosome and 67 genes on the third chromosome) have been determined. Among these, 455 correspond to genes for which no lethal mutation has yet been reported. The Drosophila genome is thought to contain approximately 3600 vital genes; 1400 are localized on the second chromosome. Our mutation collection represents approximately 56% of the second chromosome vital genes and approximately 24% of the total vital Drosophila genes.     

Mutational accessibility of essential genes on chromosome I(left) in Caenorhabditis elegans

We have analyzed a region of approximately 5.4 million base pairs for mutations, which under standard laboratory conditions result in developmental arrest, sterility, or maternal-effect lethality in Caenorhabditis elegans. Lethal mutations were isolated, maintained, and genetically manipulated as homozygotes using sDp2– a duplication of the left half of chromosome I. All of the lethals and rearrangements used in this analysis were balanced by sDp2. Relatively low doses of mutagen, (approximately 15 mM ethylmethane sulfate; EMS), were used so as to limit the occurrence of second-site mutations, thus increasing the probability of recovering single nucleotide substitutions. Treatment of over 32,400 marked chromosomes resulted in 486 analyzed mutations. In this paper, we add 133 previously unidentified let genes, isolated in the EMS screens, and one let gene identified by a γ-ray induced mutation, to our collection of 103 essential genes. We also recovered lethal alleles of genes for which visible mutants already existed. In total, eight deficiencies and alleles of 237 essential genes were identified. Eighty-nine of the previously unidentified let genes are represented by more than one lethal allele. Statistical analysis indicates a minimum estimate of 400 essential genes in the region of chromosome I balanced by sDp2. This region occupies approximately half of chromosome I, and contains over 1135 protein-coding genes predicted from the genomic sequence data. Thus, approximately one-third of the predicted genes are estimated to be essential. Of these approximately 60% are represented by lethal alleles. Less than 2% of the lethal-bearing strains recovered in our analysis, including the eight genetically definable deficiencies, carried more than one lethal mutation. Several screens were used to recover mutations for this analysis. Because all the mutations were isolated using the same balancer, under similar screening conditions, it was possible to compare intervals within the sDp2 region with each other. The fraction of essential genes that present relatively large targets for EMS was highest within the central cluster (dpy-5 to unc-13). Received: 12 July 1999 / Accepted: 6 December 1999     

Spontaneous and Ethyl Methanesulfonate-Induced Mutations Controlling Viability in DROSOPHILA MELANOGASTER. I. Recessive Lethal Mutations

The efficiency of the adult feeding method for EMS treatment in Drosophila melanogaster was studied by measuring the frequency of induced recessive lethals on the second chromosome. The treatment was most effective when mature spermatozoa or spermatids were treated and was much less effective on earlier stages. The number of mutations induced was proportional to the concentration except at the highest doses. The recessive lethal rate was estimated to be about 0.012 per second chromosome per 10(-4) M. In addition, about 0.004-0.005 recessive lethals per 10(-4) M were found in a later generation in chromosomes that had not shown the lethal effect in the previous generation. When the experiments are done in a consistent manner and gametes treated as mature sperm or spermatids are sampled, the results are highly reproducible. However, modifications of the procedure, such as starvation before EMS treatment, can considerably alter the effectiveness of the mutagen.     

Isolation and Characterization of Dominant Female Sterile Mutations of Drosophila Melanogaster. II. Mutations on the Second Chromosome

Twenty-four, second chromosome, dominant female sterile (Fs) mutations in Drosophila are described. Fs(2) were isolated at a frequency of approximately 1 per 1000 EMS-treated chromosomes screened. In comparison the isolation of frequency for second chromosome zygotic recessive lethal mutations was approximately 550 per 1000. Complementation analysis of the Fs(2) revertants showed that the 24 Fs(2) mutations identify 13-15 loci, calculated to be about 65-75% of the second chromosome genes EMS mutable to dominant female sterility. Two of the Fs(2) mutations are useful tools for the dominant female sterile technique: Fs(2)1 for induction and detection of germ-line clones and Fs(2)Ugra for follicle cell clones. Several of the Fs(2) mutations bring about novel mutant phenotypes. Seven of them alter egg shape, whereas the others arrest development primarily at two stages: around fertilization by five Fs(2) and during cleavage divisions [by Fs(2) in three loci]. The remaining that allow development to the larval stage of differentiation include four new dorsal alleles and one dominant torso allele. Analysis of germ-line chimeras revealed that with two exceptions all the Fs(2) mutations are germ-line dependent. The Fs(2) mutations were mapped mainly on the basis of mitotic recombination induced in the female germ-line cells of adult females. That most of the Fs(2) may be gain-of-function mutations is indicated by the unusual behavior of the Fs+ germ-line clones and also by the fact that 90% of the could be induced to revert.     

High Rates of Occurrence of Spontaneous Chromosome Aberrations in DROSOPHILA MELANOGASTER

Spontaneous mutations were accumulated for 40 generations in 140 unrelated second chromosomes with the standard gene arrangement. These were extracted from the same population by using the marked inversion technique, and the following findings were obtained: (1) In 42 out of the 140 chromosome lines, chromosome aberrations were detected by examining the salivary gland chromosomes: 40 paracentric and 15 pericentric inversions, 2 reciprocal translocations between the second and the third chromosomes, and 6 transpositions. (2) In 63 out of the 90 originally lethal-free lines, recessive lethal mutations occurred. (3) There were only 3 lines that acquired chromosome aberrations (inversions) with no lethal effects in the homozygous condition. (4) In a comparison of these results with those of the (CH), (PQ), and (RT) chromosomes in which no chromosome aberrations occurred after accumulating mutations for 22058 chromosome.generations (Yamaguchi and Mukai 1974), it was concluded that some of these 140 chromosomes carried a kind of mutator. (5) The frequency of mutator-carrying chromosome lines was estimated to be 0.66 on the basis of the distribution of the break-points on the chromosome lines and the frequency of lines that acquired neither recessive lethal mutations nor chromosome aberrations. Thus, the average number of breaks per mutator-carrying chromosome was estimated to be about 0.19/generation.On the basis of these estimates, the nature of the mutator factor was discussed.     

Heterozygous Effects on Fitness of Ems-Treated Chromosomes in DROSOPHILA MELANOGASTER

The heterozygous effects on fitness of second chromosomes carrying mutants induced with different doses of EMS were ascertained by monitoring changes in chromosome frequencies over time. These changes were observed in populations in which the treated chromosomes, as well as untreated competitors, remained heterozygous in males generation after generation. This situation was achieved by using a translocation which links the second chromosome to the X chromosome; however, only untranslocated second chromosomes were mutagenized. Chromosomes were classified according to their effects on viability in homozygous condition. A preliminary homozygosis identified completely lethal chromosomes; secondary tests distinguished between drastic (viability index < 0.1) and nondrastic chromosomes. Chromosomes that were nondrastic after treatment were found to reduce the fitness of their heterozygous carriers by 3-5%. The data show that flies homozygous for these chromosomes were about 2.7% less viable per treatment with 1 mm EMS than flies homozygous for untreated chromosomes. By comparing the fitness-depressing effects of nondrastic EMS-induced mutants in heterozygous condition with the corresponding viability-depressing effects measured by Temin, it is apparent that the total fitness effects are several times larger than the viability effects alone. Completely lethal chromosomes derived from the most heavily treated material reduced fitness by 11% in heterozygous condition; approximately half of this reduction was due to the lethal mutations themselves.     

Rapid identification of Arabidopsis insertion mutants by non-radioactive detection of T-DNA tagged genes

To assist in the analysis of plant gene functions we have generated a new Arabidopsis insertion mutant collection of 90 000 lines that carry the T-DNA of Agrobacterium gene fusion vector pPCV6NFHyg. Segregation analysis indicates that the average frequency of insertion sites is 1.29 per line, predicting about 116 100 independent tagged loci in the collection. The average T-DNA copy number estimated by Southern DNA hybridization is 2.4, as over 50% of the insertion loci contain tandem T-DNA copies. The collection is pooled in two arrays providing 40 PCR templates, each containing DNA from either 4000 or 5000 individual plants. A rapid and sensitive PCR technique using high-quality template DNA accelerates the identification of T-DNA tagged genes without DNA hybridization. The PCR screening is performed by agarose gel electrophoresis followed by isolation and direct sequencing of DNA fragments of amplified T-DNA insert junctions. To estimate the mutation recovery rate, 39 700 lines have been screened for T-DNA tags in 154 genes yielding 87 confirmed mutations in 73 target genes. Screening the whole collection with both T-DNA border primers requires 170 PCR reactions that are expected to detect a mutation in a gene with at least twofold redundancy and an estimated probability of 77%. Using this technique, an M2 family segregating a characterized gene mutation can be identified within 4 weeks.     

Spontaneous and Ethyl Methanesulfonate-Induced Mutations Controlling Viability in DROSOPHILA MELANOGASTER. II. Homozygous Effect of Polygenic Mutations

Polygenic mutations affecting viability were accumulated on the second chromosome of Drosophila melanogaster by treating flies with EMS in successive generations. The treated chromosomes were later made homozygous and tested for their effects on viability by comparison of the frequency of such homozygotes with that of other genotypes in the same culture. The treated wild-type chromosomes were kept heterozygous in Pm/+ males by mating individual males in successive generations to Cy/Pm females. The number of generations of accumulation was 1 to 30 generations, depending on the concentration of EMS. A similar experiment for spontaneous polygenic mutations was also conducted by accumulating mutations for 40 generations. The lower limit of the spontaneous mutation rate of viability polygenes is estimated to be 0.06 per second chromosome per generation, which is about 12 times as high as the spontaneous recessive lethal mutation rate, 0.005. EMS-induced polygenic mutations increase linearly with the number of treated generations and with the concentration of EMS. The minimum mutation rate of viability polygenes is about 0.017 per 10(-4)m, which is only slightly larger than the lethal rate of 0.013 per 10(-4) m. The maximum estimate of the viability reduction of a single mutant is about 6 to 10 percent of the normal viability. The data are consistent with a constant average effect per mutant at all concentrations, but this is about three times as high as that for spontaneous mutants. It is obvious that one can obtain only a lower limit for the mutation rate, since some mutants may have effects so near to zero that they cannot be detected. The possibility of measuring something other than the lower limit is discussed. The ratio of the load due to detrimental mutants to that caused by lethals, the D/L ratio, is about 0.2 to 0.3 for EMS-induced mutants, as compared to about 0.5 for spontaneous mutants. This is to be expected if EMS treatment produces a large fraction of small deletions and other chromosome rearrangements which are more likely to be lethal.     

Toward a comprehensive genetic analysis of male fertility in Drosophila melanogaster

Drosophila melanogaster is a widely used model organism for genetic dissection of developmental processes. To exploit its full potential for studying the genetic basis of male fertility, we performed a large-scale screen for male-sterile (ms) mutations. From a collection of 12,326 strains carrying ethyl-methanesulfonate-treated, homozygous viable second or third chromosomes, 2216 ms lines were identified, constituting the largest collection of ms mutations described to date for any organism. Over 2000 lines were cytologically characterized and, of these, 81% failed during spermatogenesis while 19% manifested postspermatogenic processes. Of the phenotypic categories used to classify the mutants, the largest groups were those that showed visible defects in meiotic chromosome segregation or cytokinesis and those that failed in sperm individualization. We also identified 62 fertile or subfertile lines that showed high levels of chromosome loss due to abnormal mitotic or meiotic chromosome transmission in the male germ line or due to paternal chromosome loss in the early embryo. We argue that the majority of autosomal genes that function in male fertility in Drosophila are represented by one or more alleles in the ms collection. Given the conservation of molecular mechanisms underlying important cellular processes, analysis of these mutations should provide insight into the genetic networks that control male fertility in Drosophila and other organisms, including humans.     

大豆疫霉菌的EMS化学诱变

以甲基磺酸乙酯(ethylmethane sulfonate,EMS)为诱变剂,通过其对大豆疫霉菌Phytophthora sojae休止孢萌发的影响,确定化学诱变条件。通过收集单卵孢子,建立了包含640个单卵孢子系的突变体库,其中约有50%的诱变菌系在培养性状和菌落形态方面发生了明显变化,菌落形态多样,表现出较紧密或松散,近圆形或不规则;气生菌丝减少,生长速度较慢或快;在卵孢子产量方面,8.13%的菌系有增加,20.41%的菌系减少,27.82%的菌系极少或者没有卵孢子产生,43.64%的菌系卵孢子产量类似野生型。以质膜氢离子泵蛋白基因PsPMA1(plasma membrane H+-ATPase1)为对象,通过TILLING技术,从320个大豆疫霉菌突变体中获得9个突变体,进一步确认了EMS对大豆疫霉菌的诱变效果,并且估算EMS对大豆疫霉菌的诱变频率至多每115kb发生一个核苷酸变异。新构建的突变体库为开展大豆疫霉病菌的功能基因组研究奠定了遗传材料基础。     

Mutations affecting the pattern of the larval cuticle inDrosophila melanogaster

Summary The present report describes the recovery and genetic characterization of mutant alleles at zygotic loci on the third chromosome ofDrosophila melanogaster which alter the morphology of the larval cuticle. We derived 12600 single lines from ethyl methane sulfonate (EMS)-treatedst e orrucuca chromosomes and assayed them for embryonic lethal mutations by estimating hatch rates of egg collections. About 7100 of these lines yielded at least a quarter of unhatched eggs and were then scored for embryonic phenotypes. Through microscopic examination of unhatched eggs 1772 lines corresponding to 24% of all lethal hits were classified as embryonic lethal. In 198 lines (2.7% of all lethal hits), mutant embryos showed distinct abnormalities of the larval cuticle. These embryonic visible mutants define 45 loci by complementation analysis. For 32 loci, more than one mutant allele was recovered, with an average of 5.8 alleles per locus. Complementation of all other mutants was shown by 13 mutants. The genes were localized on the genetic map by recombination analysis, as well as cytologically by complementation analysis with deficiencies. They appear to be randomly distributed along the chromosome. Allele frequencies and comparisons with deficiency phenotypes indicate that the 45 loci represent most, if not all, zygotic loci on the third chromosome, where lack of function recognizably affects the morphology of the larval cuticle.     

The cre1 and cre3 nematode resistance genes are located at homeologous loci in the wheat genome

Differential responses in host-nematode pathotype interactions occur in wheat lines carrying different cereal cyst nematode resistance (Cre) genes. Cre1, located on chromosome 2B, confers resistance to most European nematodes and the sole Australian pathotype, while Cre3, present on chromosome 2D, is highly resistant to the Australian pathotype and susceptible to a number of European pathotypes. Genes encoding nucleotide binding site-leucine rich repeat (NBS-LRR) proteins that cosegregate with the Cre3 locus cross hybridize to homologues whose restriction fragment length polymorphism (RFLP) patterns distinguish near-isogenic Cre1 nematode-resistant wheat lines. Genetic mapping showed that the NBS-LRR gene members that distinguished the Cre1 near-isogenic lines were located on chromosome 2BL at a locus, designated Xcsl107, that cosegregates with the Cre1 locus. A haplotype of NBS-LRR genes from the Xcsl107 locus provides a diagnostic marker for the presence of Cre1 nematode resistance in a wide collection of wheat lines and segregating families. Genetic analysis of NBS-LRR haplotypes that cosegregate with Cre1 and Cre3 resistance, together with flanking cDNA markers and other markers from homoeologous group 2 chromosomes, revealed a conserved gene order that suggests Cre1 and Cre3 are homeoloci.     

The Doublesex Locus of Drosophila Melanogaster and Its Flanking Regions: A Cytogenetic Analysis

The region of the third chromosome (84D-F) of Drosophila melanogaster that contains the doublesex (dsx) locus has been cytogenetically analyzed. Twenty nine newly induced, and 42 preexisting rearrangements broken in dsx and the regions flanking dsx have been cytologically and genetically characterized. These studies established that the dsx locus is in salivary chromosome band 84E1-2. In addition, these observations provide strong evidence that the dsx locus functions only to regulate sexual differentiation and does not encode a vital function. To obtain new alleles at the dsx locus and to begin to analyze the genes flanking dsx, 59 lethal and visible mutations in a region encompassing dsx were induced. These mutations together with preexisting mutations in the region were deficiency mapped and placed into complementation groups. Among the mutations we isolated, four new mutations affecting sexual differentiation were identified. All proved to be alleles of dsx, suggesting that dsx is the only gene in this region involved in regulating sexual differentiation. All but one of the new dsx alleles have equivalent effects in males and females. The exception, dsxEFH55, strongly affects female sexual differentiation, but only weakly affects male sexual differentiation. The interactions of dsxEFH55 with mutations in other genes affecting sexual differentiation are described. These results are discussed in terms of the recent molecular findings that the dsx locus encodes sex-specific proteins that share in common their amino termini but have different carboxyl termini. The 72 mutations in this region that do not affect sexual differentiation identify 25 complementation groups. A translocation, T(2;3)Es that is associated with a lethal allele in one of these complementation groups is also broken at the engrailed (en) locus on the second chromosome and has a dominant phenotype that may be due to the expression of en in the anterior portion of the abdominal tergites where en is not normally expressed. The essential genes found in the 84D-F region are not evenly distributed throughout this region; most strikingly the 84D1-11 region appears to be devoid of essential genes. It is suggested that the lack of essential genes in this region is due to the region (1) containing genes with nonessential functions and (2) being duplicated, possibly both internally and elsewhere in the genome.     

An Analysis of Male-Recombination Elements in a Natural Population of DROSOPHILA MELANOGASTER in South Texas

Genomes from a group of Drosophila melanogaster collected from a natural population at San Benito, South Texas, in March of 1975 were analyzed for the presence of male-recombination elements. All three autosomes and both sex chromosomes were examined, with emphasis placed on the two major autosomes, the second and third chromosomes. In samples of 16 second and 16 third chromosomes, at least half, but not all, of each were found to carry male-recombination elements. It is suggested, although the data are not conclusive, that some of the fourth, X, and Y chromosomes might also be associated with male-recombination elements.—When a male-recombination element, or elements, was located in the second chromosome, relatively more male recombination was induced in the second than in the third chromosome. This situation was reversed when the element(s) was located in the third chromosome.—Distortion of transmission frequency, one of the characteristics of previously studied second chromosome lines associated with male recombination, was confirmed for these second chromosomes that carried male-recombination elements. Similar, but less pronounced, distortion was observed for the third chromosome lines that carried male-recombination elements.     

Genetical variation for enzyme activity in a population of Drosophila melanogaster. IV. Analysis of alcohol dehydrogenase activity in chromosome substitution lines.

Chromosome substitution lines derived from two inbred strains of Drosophila melanogaster homozygous for the AdhS allele of alcohol dehydrogenase but differing significantly in ADH activity have been analysed. Variation in activity can be attributed to all three major chromosomes. The effect of the second chromosome, where the ADH structural gene is located, can be modified significantly by the genotype of both the first and the third chromosomes. The most substantial single effect results from homozygous differences between the third chromosomes. In contrast, differences between the X chromosomes are revealed only when the second or second and third chromosomes are heterozygous.     

Persistence time of loss-of-function mutations at nonessential loci affecting eye color in Drosophila melanogaster

Persistence time of a mutant allele, the expected number of generations before its elimination from the population, can be estimated as the ratio of the number of segregating mutations per individual over the mutation rate per generation. We screened two natural populations of Drosophila melanogaster for mutations causing clear-cut eye phenotypes and detected 25 mutant alleles, falling into 19 complementation groups, in 1164 haploid genomes, which implies 0.021 eye mutations/genome. The de novo haploid mutation rate for the same set of loci was estimated as 2 x 10(-4) in a 10-generation mutation-accumulation experiment. Thus, the average persistence time of all mutations causing clear-cut eye phenotypes is approximately 100 generations (95% confidence interval: 61-219). This estimate shows that the strength of selection against phenotypically drastic alleles of nonessential loci is close to that against recessive lethals. In both cases, deleterious alleles are apparently eliminated by selection against heterozygous individuals, which show no visible phenotypic differences from wild type.     

Extrachromosomal Element Delta in DROSOPHILA MELANOGASTER. IX. Induction of Delta-Retaining Chromosome Lines by Mutation and Gene Mapping

[Delta b], symbolized as [delta(b)], is retained by S(b) chromosome lines and transmitted through the females to their progeny. Transmission through the males is not directly demonstrable (Minamori 1969a). [delta r], symbolized as [delta(r)], is retained by S(r) chromosome lines and transmitted biparentally (Minamori 1971). The multiplication of delta is suppressed at low temperature. All descendant lines derived from S(b)-carrying or S(r)-carrying flies in which the presence of delta cannot be demonstrated gradually accumulate their specific delta factors over many generations (Minamori 1969b, 1972). The delta factors and the sensitive chromosomes are inseparably associated. This observation led to the assumption that delta may be a copy of a chromosomal gene or a certain agent integrated into the chromosome (Minamori 1972). This assumption was examined in the present study by experiments designed to induce delta-retaining sensitive chromosomes, and to map the gene(s) responsible for delta-retention and/or for sensitivity to the killing action of delta factor. One sensitive chromosome which retained [delta(b)] (S(b) chromosome) was obtained in the presence of [delta(b)] out of 2492 insensitive chromosomes which retained no delta; in addition one S(b) chromosome was obtained in the presence of [delta(r)] out of 2131 insensitives. The latter finding suggests that S(b) might be induced by a mutation caused by [delta(b)] or [delta(r)], but not by integration of either delta into the chromosome. Four S(b) chromosomes and one sensitive chromosome which retained [delta(r)] (S(r) chromosome) were obtained out of 1970 insensitives when males carrying the chromosome were fed an alkylating mutagen, ethyl methane sulfonate (EMS). The location of delta-retaining genes was examined by crossing-over experiments employing eight S(b) and five S(r) chromosomes. The genes on these chromosomes were found to be located in the same region or near one another. The gene for [delta(b)], symbolized as Da(b), and the gene for [delta(r)], symbolized as Da(r), are assumed to be multiple alleles of a locus at 2-24.9. The sensitivity of the chromosomes was modified appreciably by recombination; hence, the genes controlling this trait are assumed to be a polygenic system. The findings obtained in this study lead to the hypothesis that delta may be produced by a chromosomal gene (Da) and transmitted extrachromosomally.     

Identification of chromosome inheritance modifiers in Drosophila melanogaster

Faithful chromosome inheritance is a fundamental biological activity and errors contribute to birth defects and cancer progression. We have performed a P-element screen in Drosophila melanogaster with the aim of identifying novel candidate genes involved in inheritance. We used a "sensitized" minichromosome substrate (J21A) to screen approximately 3,000 new P-element lines for dominant effects on chromosome inheritance and recovered 78 Sensitized chromosome inheritance modifiers (Scim). Of these, 69 decreased minichromosome inheritance while 9 increased minichromosome inheritance. Fourteen mutations are lethal or semilethal when homozygous and all exhibit dramatic mitotic defects. Inverse PCR combined with genomic analyses identified P insertions within or close to genes with previously described inheritance functions, including wings apart-like (wapl), centrosomin (cnn), and pavarotti (pav). Further, lethal insertions in replication factor complex 4 (rfc4) and GTPase-activating protein 1 (Gap1) exhibit specific mitotic chromosome defects, discovering previously unknown roles for these proteins in chromosome inheritance. The majority of the lines represent mutations in previously uncharacterized loci, many of which have human homologs, and we anticipate that this collection will provide a rich source of mutations in new genes required for chromosome inheritance in metazoans.