Genetic Analysis of B2t, the Structural Gene for a Testis-Specific β-Tubulin Subunit in DROSOPHILA MELANOGASTER

Genetic analysis of the B2t locus has resulted in the recovery of four recessive mutations in the B2t structural gene and a deficiency that deletes the locus. Two of the mutations were recovered as suppressors of B2t^D, a dominant male sterile mutation at the locus, and two were induced on wild-type chromosomes. All four mutant genes encode β₂-tubulin subunits that are synthesized at normal rates but do not accumulate. All mutants are completely male sterile as homozygotes.     

Characterisation of a new tumorous-head mutant of Drosophila melanogaster

Summary A new homoeotic mutant, I127, showing abnormal growths in the head region including homoeotic transformation of eye to genitalia and antenna to leg, was isolated in a screen designed to find new alleles of the tumorous head (tuh-3), mutation. Similarities in the phenotype and genetics of the mutant, and complementation studies with tuh-1; tuh-3, suggest that I127 is indeed an allele of tuh-3. In combination with the first chromosome modifier tuh-1, the mutant is temperature-sensitive during the third larval instar, giving an increased penetrance of the tumorous head phenotype when reared at 25° C as opposed to 18° C. The isolation of further alleles at the tumorous-head locus are essential. The types of morphological defects which can result from mutations at this locus would enable us to establish if this is a complex locus, and if null mutations are lethal during development. The interactions of the tumorous-head gene with first chromosome modifiers and other homoeotic mutations will only be understood if we able to induce a number of mutations at this locus, and as a consequence begin to elucidate the role of the wild-type gene product in normal development.     

Characterization of a Recombinant Mouse t Haplotype That Expresses a Dominant Lethal Maternal Effect

The t^wLub2 chromosome was generated by rare recombination between a complete t haplotype and a wild-type form of mouse chromosome 17. This recombinant chromosome expresses a dominant lethal effect in all embryos that inherit the mutant chromosome from their mothers. The phenotype of this maternal effect is indistinguishable from that expressed by the previously described T^hp deletion chromosome. It appears likely that the crossing over event that gave rise to t^wLub2 was unequal and resulted in the alteration or deletion of a gene (which is named the T-associated maternal effect locus, Tme) that must be inherited from the mother in order for normal development to proceed through late stages of gestation. The results presented here allow a mapping of the Tme locus between the quaking and tufted loci which are 3 cM apart within the proximal region of chromosome 17.     

Molecular Characterization of the pun Allele of the Mouse Pink-Eyed Dilution Locus

The mouse pink eyed dilution locus, p, located on chromosome 7, mediates coat and eye color. The human correlate of this gene may underlie some forms of tyrosinase-positive oculocutaneous albinism. Mutations at the p locus result in a reduction in pigmentation of the eyes and coat. Although most mutant p alleles (including all spontaneous mutations) affect only pigmentation, several mutant alleles (all radiation induced) are also associated with a variety of other phenotypes. We have focused our attention on the p^un mutant allele, a spontaneous mutation, exhibiting one of the highest reversion frequencies reported for a mammalian mutation. Using a new technique, genome scanning, we have cloned fragments of genomic DNA from the p locus that are associated with a DNA duplication in p^un DNA. These fragments can now be used to locate the p gene-encoding sequences and aid in the molecular characterization of complex mutant p alleles.     

The Utilization during Mitotic Cell Division of Loci Controlling Meiotic Recombination and Disjunction in DROSOPHILA MELANOGASTER

To inquire whether the loci identified by recombination-defective and disjunction-defective meiotic mutants in Drosophila are also utilized during mitotic cell division, the effects of 18 meiotic mutants (representing 13 loci) on mitotic chromosome stability have been examined genetically. To do this, meiotic-mutant-bearing flies heterozygous for recessive somatic cell markers were examined for the frequencies and types of spontaneous clones expressing the cell markers. In such flies, marked clones can arise via mitotic recombination, mutation, chromosome breakage, nondisjunction or chromosome loss, and clones from these different origins can be distinguished. In addition, meiotic mutants at nine loci have been examined for their effects on sensitivity to killing by UV and X rays.—Mutants at six of the seven recombination-defective loci examined (mei-9, mei-41, c(3)G, mei-W68, mei-S282, mei-352, mei-218) cause mitotic chromosome instability in both sexes, whereas mutants at one locus (mei-218) do not affect mitotic chromosome stability. Thus many of the loci utilized during meiotic recombination also function in the chromosomal economy of mitotic cells.—The chromosome instability produced by mei-41 alleles is the consequence of chromosome breakage, that of mei-9 alleles is primarily due to chromosome breakage and, to a lesser extent, to an elevated frequency of mitotic recombination, whereas no predominant mechanism responsible for the instability caused by c(3)G alleles is discernible. Since these three loci are defective in their responses to mutagen damage, their effects on chromosome stability in nonmutagenized cells are interpreted as resulting from an inability to repair spontaneous lesions. Both mei-W68 and mei-S282 increase mitotic recombination (and in mei-W68, to a lesser extent, chromosome loss) in the abdomen but not the wing. In the abdomen, the primary effect on chromosome stability occurs during the larval period when the abdominal histoblasts are in a nondividing (G2) state.—Mitotic recombination is at or above control levels in the presence of each of the recombination-defective meiotic mutants examined, suggesting that meiotic and mitotic recombination are under separate genetic control in Drosophila.—Of the six mutants examined that are defective in processes required for regular meiotic chromosome segregation, four (l(1)TW-6^cs, ca^nd, mei-S332, ord) affect mitotic chromosome behavior. At semi-restrictive temperatures, the cold sensitive lethal l(1)TW-6^cs causes very frequent somatic spots, a substantial proportion of which are attributable to nondisjunction or loss. Thus, this locus specifies a function essential for chromosome segregation at mitosis as well as at the first meiotic division in females. The patterns of mitotic effects caused by ca^nd, mei-S332, and ord suggest that they may be leaky alleles at essential loci that specify functions common to meiosis and mitosis. Mutants at the two remaining loci (nod, pal) do not affect mitotic chromosome stability.     

Multiple Allele Approach to the Study of Genes in DROSOPHILA MELANOGASTER That Are Involved in Imaginal Disc Development

The phenotypes of five different lethal mutants of Drosophila melanogaster that have small imaginal discs were analyzed in detail. From these results, we inferred whether or not the observed imaginal disc phenotype resulted exclusively from a primary imaginal disc defect in each mutant. To examine the validity of these inferences, we employed a multiple-allele method. Lethal alleles of the five third-chromosome mutations were identified by screening EMS-treated chromosomes for those which fail to complement with a chromosome containing all five reference mutations. Twenty-four mutants were isolated from 13,197 treated chromosomes. Each of the 24 was then tested for complementation with each of the five reference mutants. There was no significant difference in the mutation frequencies at these five loci. The stage of lethality and the imaginal disc morphology of each mutant allele were compared to those of its reference allele in order to examine the range of defects to be found among lethal alleles of each locus. In addition, hybrids of the alleles were examined for intracistronic complementation. For two of the five loci, we detected no significant phenotypic variation among lethal alleles. We infer that each of the mutant alleles at these two loci cause expression of the null activity phenotype. However, for the three other loci, we did detect significant phenotypic variation among lethal alleles. In fact, one of the mutant alleles at each of these three loci causes no detectable imaginal disc defect. This demonstrates that attempting to assess the developmental role of a gene by studying a single mutant allele may lead to erroneous conclusions. As a byproduct of the mutagenesis procedure, we have isolated two dominant, cold-sensitive mutants.     

Transmission ratio distortion in mouse t-haplotypes is due to multiple distorter genes acting on a responder locus

Transmission ratios of male mice heterozygous for various combinations of partial t-haplotypes provide evidence in support of a model for the genetic basis of ratio distortion, involving two or more distorter genes acting on a responder locus. The t form of the responder locus, Tcr, in the medial part of the haplotype, must be present and heterozygous for distortion to occur. When the responder alone is present, as in t^low haplotypes, the chromosome carrying it is transmitted in a low ratio (<50%). The t forms of the distorter loci act additively, in cis or trans, to raise the transmission of whichever chromosome carries Tcr. Identified distorter loci are Tcd-1, in the proximal part of the haplotype, Tcd-2, distal to Tcr, and probably Tcd-3, lying between Tcr and Tcd-2. In the absence of Tcr the distorters are transmitted normally. The system is compared with the SD system of Drosophila.     

Esterase and acid phosphatase polymorphism in the fig tree (Ficus carica L.)

The genetics of two enzymatic loci, esterase (Est-D) and acid phosphatase (AcP-A), were studied by means of polyacrylamide gel electrophoresis in the fig tree (Ficus carica L.). Two codominant alleles are described at the Est-D locus and four codominant alleles at the AcP-A locus. Heterozygotes at the AcP-A locus have a hybrid band, thus showing that the AcP-A allozymes, are at least dimer molecules. Both loci are independent of the male sterility factor in F. carica. The polymorphism in four natural populations was investigated for both loci. A significant deficiency of heterozygotes was observed.     

Male sterility associated with APRT deficiency in Arabidopsis thaliana results from a mutation in the gene APT1

Four mutants of Arabidopsis thaliana that are deficient in adenine phosphoribosyl transferase (APRT) activity have been isolated by selecting for germination of seeds and growth of the plantlets on a medium containing 2,6-diaminopurine (DAP), a toxic analog of adenine. In all mutants, DAP resistance is due to a recessive nuclear mutation at a locus designated apt. The mutants are male sterile due to pollen abortion after meiosis. Furthermore, it has been shown that metabolism of cytokinins is impaired in the mutant BM3, which has the lowest level of APRT activity among the mutants tested. However, three different cDNAs encoding APRT have been isolated in A. thaliana and this raised the question of the nature of the mutation which results in low APRT activity. The mutation was genetically mapped to chromosome I and lies within 6 cM of the phenotypic marker dis2, indicating that the mutation affects the APT1 gene, a result confirmed by sequencing of mutant alleles. The mutation in the allele apt1-3 is located at the 5′ splicing site of the third intron, and eliminates a BstNI restriction site, as verified by Southern blotting and PCR fragment length analysis.     

Genetic Change in Mutations at the T/t-Locus in the Mouse

Recessive lethal or semilethal alleles at the T/t locus in the mouse generate new t-variants, with characteristics different from the parent allele at a rate of about 10^-3. Almost invariably the variant chromosome carries marker genes derived from the opposite parental chromosome. New t-mutations obtained in this way are sometimes recessive lethals that are indistinguishable from those in already known complementation groups. Most derived t-mutations are viable, however. This paper summarizes data on the rate and types of variants produced by members of each of the six lethal complementation groups, and by semilethal alleles. It appears that particular complementation groups preferentially generate certain types of variants, and that in general, the pattern of variant production runs "uphill," that is, to less abnormal states. The data are compatible with the hypothesis that t-mutations represent some extent of altered chromosome and that variants are produced by loss of abnormal material.     

Genetic basis of natural variation in body pigmentation in Drosophila melanogaster

Body pigmentation in insects and other organisms is typically variable within and between species and is often associated with fitness. Regulatory variants with large effects at bab1, t and e affect variation in abdominal pigmentation in several populations of Drosophila melanogaster. Recently, we performed a genome wide association (GWA) analysis of variation in abdominal pigmentation using the inbred, sequenced lines of the Drosophila Genetic Reference Panel (DGRP). We confirmed the large effects of regulatory variants in bab1, t and e; identified 81 additional candidate genes; and validated 17 candidate genes (out of 28 tested) using RNAi knockdown of gene expression and mutant alleles. However, these analyses are imperfect proxies for the effects of segregating variants. Here, we describe the results of an extreme quantitative trait locus (xQTL) GWA analysis of female body pigmentation in an outbred population derived from light and dark DGRP lines. We replicated the effects on pigmentation of 28 genes implicated by the DGRP GWA study, including bab1, t and e and 7 genes previously validated by RNAi and/or mutant analyses. We also identified many additional loci. The genetic architecture of Drosophila pigmentation is complex, with a few major genes and many other loci with smaller effects.     

Mutators in SACCHAROMYCES CEREVISIAE: MUT1–1, MUT1–2 and MUT2–1

The purpose of this study was to characterize two mutator stocks of yeast which were induced and selected on the basis of high spontaneous reversion rates of the suppressible "ochre" nonsense allele lys1-1. In the mutator stock VA-3, a single mutation, designated mut1-1, is responsible for the increase in the reversion rate of the ochre alleles lys1-1 and arg4-17. In stock VA-105, there are two separate mutator mutations. Tetrad analysis data showed these two loci are loosely linked. Based on complementation data, one of these mutations is at the same locus as mut1-1 and designated mut1-2. The second mutator of stock VA-105 was designated mut2-1. All three mutators are recessive. Both mut1-1 and mut1-2 give a high mutation rate for ochre nonsense suppressor (SUP) loci, but not for the ochre nonsense alleles. On the contrary, the mutation rates of the ochre alleles are greatly reduced. With the mutant mut2-1 there were mutations at both the lys1-1 site and its suppressors; mut2-1 is as effective as mut1-2 but not as effective as mut1-1 in inducing reversions of a missense mutant, his1-7. Neither mut1-1, mut1-2 nor mut2-1 were effective in inducing reversions of a putative frameshift mutation, hom3-10, or in inducing forward mutations to canavanine resistance.     

TheD17Tu5 locus in thet complex: implications for the origin oft haplotypes and inbred strains

The mouse × Chinese hamster cell line R4 4-1 contains only one mouse chromosome, the bulk of which corresponds toMus musculus chromosomes 17 and 18 (MMU17 and MMU18, respectively). A genomic library was prepared from the R4 4-1 DNA, and a mouse clone was isolated from the library, which—with the help of somatic cell hybrids-could be mapped to the MMU17. A locus defined by a 2.7-kb longBam HI probe from this clone was designatedD17Tu5 (Tu for Tübingen). The locus proved to be polymorphic among inbred strains and wild mice. By testing of recombinant inbred strains and partialt haplotypes, theD17Tu5 locus could be mapped to a position between theD17Leh66E andD17Rp17 loci within thet complex. Two alleles were found at this locus,D17Tu5 ^a andD17Tu5 ^b , defined byTaq I restriction fragment length polymorphism. Both alleles are present among inbred strains and wild mice of the speciesM. domesticus. All completet haplotypes tested carry theD17Tu5 ^a allele and all tested wild mice of the speciesM. musculus, with the exception of those bearingt haplotypes, carry theD17Tu5 ^b allele. Additional alleles are found in some populations of wild mice and in other species of the genusMus. The distribution of the two alleles among the inbred strains correlates well with their known or postulated genealogy. Their distribution between the two species ofMus and among the mice withT haplotypes suggests a relatively recent origin of thet haplotypes.     

Molecular mapping of the intermedium spike-c ( int-c) and non-brittle rachis 1 ( btr1) loci in barley ( Hordeum vulgare L.)

Lateral spikelet fertility and a non-brittle rachis are key characters in studying the evolution of barley. The fertility of lateral spikelets is controlled predominantly by the alleles at the vrs1 locus on chromosome 2HL and is modified by the alleles at the int-c locus on chromosome 4HS. The non-brittle rachis is controlled by alleles at two tightly linked loci, btr1 and btr2, on chromosome 3HS. This paper presents the integration of the int-c and btr1 loci in molecular linkage maps of barley. The int-c locus was mapped to the end of chromosome 4HS, 8.2 cM distal from the MWG2033 locus. The analysis was followed by a composite interval mapping of quantitative trait loci, which verified the position of the int-c locus. Linkage analysis using recombinant inbred lines showed that the btr1 locus is flanked between two AFLP loci, e14m27.4.1 and e15m19.7, with map distances of 3.1 cM and 4.2 cM, respectively. The molecular markers will expedite further high-density mapping of the int-c and btr1 loci.     

Gene mapping within the T/t complex of the mouse. II. Anomalous position of the H-2 complex in t haplotypes

Naturally occurring t haplotypes are chromosome 17 polymorphisms that suppress genetic recombination in t/+ heterozygotes over a long distance that includes the H-2 complex. There is strong linkage disequilibrium between t haplotypes and H-2 haplotypes; over 20 independently isolated t chromosomes representing eight different complementation groups share only four H-2 haplotypes. Thus t haplotypes and their associated H-2 loci are inherited en bloc as a “supergene” complex, whose frequency is driven in wild mouse populations by their high transmission from male t heterozygotes. This phenomenon must therefore serve as an important regulator of H-2 polymorphisms. Genes within the region of recombination suppression in t haplotypes have been mapped by crossing-over that occurs readily between two different t haplo-types situated in trans, and by this means we show here that the H-2 complex occupies an anomalous position in t haplotypes, mapping proximal to the locus of tf closely flanked by t-lethal mutations.     

Time of Recombination in the DROSOPHILA MELANOGASTER Oocyte. III. Selection and Characterization of Temperature-Sensitive and -Insensitive, Recombination-Deficient Alleles in Drosophila

The procedure for the selection of a temperature-sensitive recombination mutant in Drosophila is described. Use of this procedure has led to the recovery of three alleles at a new recombination locus called rec-1, located within the region of chromosome 3 circumscribed by Deficiency (3R)sbd¹⁰⁵. One allele, rec-1²⁶, is temperature sensitive, and the other two alleles, rec-1⁶ and rec-1¹⁶, are temperature insensitive. Gene dosage studies reveal rec-1²⁶ to be a leaky mutant with greater recombination activity in two doses than in one. The other two alleles show no dose response, implying that they may be null mutants. The temperature response curves of rec-1²⁶ as a homozygote and in heteroallelic combination with rec-1¹⁶ suggest that the sharp decrease in recombination between 28° and 31° indicates temperature denaturation of an enzyme or other protein specified by the mutant and associated with the recombination process. The ability of small changes in temperature to reverse or abolish polarity in recombination along the X chromosome arm in rec-1 ²⁶/rec-1¹⁶ females brings into question the use of the "polarity" criterion to partition mutants into two functional types, i.e., precondition mutants that display polarity and exchange mutants that do not. Evidence that rec-1 may be part of a complex locus residing in a chromosome segment harboring a variety of recombination-related genes is presented.     

Gene Polymorphism in Natural Populations of DROSOPHILA PERSIMILIS

Genetic variation at 43 loci has been studied in six different populations of Drosophila persimilis by electrophoresis of enzymes and proteins. In D. persimilis the mean proportion of polymorphic loci is 0.362, the mean proportion of heterozygous loci per individual is 0.100 and the average number of alleles per locus is 1.651. In all populations, the loci coding for the hydrolytic and other nonspecific enzymes are much more variable than the loci coding for the enzymes of the glycolytic pathway, Kreb's cycle, other specific enzymes and larval proteins. Most loci have similar allele frequency in all populations except the two loci, Amylase and Pt-12, which show a pattern of associations of different alleles with different third chromosome inversions.     

Diversity analysis at MHC class II DQA locus in buffalo (Bubalus bubalis) indicates extensive duplication and trans-species evolution

Variation at MHC Class II-DQA locus in riverine and swamp buffaloes (Bubu) has been explored in this study. Through sequencing of buffalo DQA, 48 nucleotide variants identified from 17 individuals, reporting 42 novel alleles, including one pseudogene. Individual animal displayed two to seven variants, suggesting the presence of more than two Bubu-DQA loci, as an evidence of extensive duplication. dN values were found to be higher than dS values at peptide binding sites, separately for riverine and swamp buffaloes, indicating locus being under positive selection. Evolutionary analysis revealed numerous trans-species polymorphism with alleles from water buffalo assigned to at least three different loci (Bubu-DQA1, DQA2, DQA3). Alleles of both the sub-species intermixed within the cluster, showing convergent evolution of MHC alleles in bovines. The results thus suggest that both riverine and swamp buffaloes share con-current arrangement of DQA region, comparable to cattle in terms of copy number and population polymorphism.     

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.     

Genome-wide association study identified the human leukocyte antigen region as a novel locus for plasma beta-2 microglobulin

Beta-2 microglobulin (B2M) is a component of the major histocompatibility complex (MHC) class I molecule and has been studied as a biomarker of kidney function, cardiovascular diseases and mortality. Little is known about the genes influencing its levels directly or through glomerular filtration rate (GFR). We conducted a genome-wide association study of plasma B2M levels in 6738 European Americans from the Atherosclerosis Risk in Communities study to identify novel loci for B2M and assessed its association with known estimated GFR (eGFR) loci. We identified 2 genome-wide significant loci. One was in the human leukocyte antigen (HLA) region on chromosome 6 (lowest p value = 1.8 × 10^?23 for rs9264638). At this locus, 6 index SNPs accounted for 3.2 % of log(B2M) variance, and their association with B2M could largely be explained by imputed classical alleles of the MHC class I genes: HLA-A, HLA-B, or HLA-C. The index SNPs at this locus were not associated with eGFR based on serum creatinine (eGFRcr). The other locus of B2M was on chromosome 12 (rs3184504 at SH2B3, beta = 0.02, p value = 3.1 × 10^?8), which was previously implicated as an eGFR locus. In conclusion, although B2M is known to be a component of MHC class I molecule, the association between HLA class I alleles and plasma B2M levels in a community-based population is novel. The identification of the two novel loci for B2M extends our understanding of its metabolism and informs its use as a kidney filtration biomarker.