两种白斑小鼠突变基因的染色体定位

以本中心ENU诱变获得的两种白斑突变小鼠W-4Bao与Kitl-1Bao为研究对象[均为C57BL/6J(B6)背景],遗传试验表明它们都为单基因显性遗传,W-4Bao及Kitl-1Bao突变基因纯合子小鼠的表型分别为全白色及“黑头白”;将白斑杂合子小鼠与DBA/2(D2)交配获得具有白斑表型的F1小鼠,F1小鼠再回交D2繁殖[(B6×D2)F1×D2]F2小鼠,利用微卫星标记对F2代小鼠进行连锁分析。结果发现W-4Bao与微卫星D5Mit356、D5Mit308之间的LOD值分别为56.82、51.50,从而把该突变基因定位于第5号染色体D5Mit356与D5Mit308之间;Kitl-1Bao与微卫星D10Mit70、D10Mit68之间的LOD值分别为27.37、21.20,从而把该突变基因定位于第10号染色体上D10Mit70与D10Mit68之间。经过检索小鼠基因组数据库确认它们的候选基因分别为kit及kitl。     

Interval Mapping of High Growth (Hg), a Major Locus That Increases Weight Gain in Mice

The high growth locus (hg) causes a major increase in weight gain and body size in mice. As a first step to map-based cloning of hg, we developed a genetic map of the hg-containing region using interval mapping of 403 F(2) from a C57BL/6J-hghg X CAST/EiJ cross. The maximum likelihood position of hg was at the chromosome 10 marker D10Mit41 (LOD = 24.8) in the F(2) females and 1.5 cM distal to D10Mit41 (LOD = 9.56) in the F(2) males with corresponding LOD 2 support intervals of 3.7 and 5.4 cM, respectively. The peak LOD scores were significantly higher than the estimated empirical threshold LOD values. The localization of hg by interval mapping was supported by a test cross of F(2) mice recombinant between the LOD 2 support interval and the flanking marker. The interval mapping and test-cross results indicate that hg is not allelic with candidate genes Igf1 or decorin (Dcn), a gene that was mapped close to hg in this study. The hg inheritance was recessive in females, although we could not reject recessive or additive inheritance in males. Possible causes for sex differences in peak LOD scores and for the distortion of transmission ratios observed in F(2) males are discussed. The genetic map of the hg region will facilitate further fine mapping and cloning of hg, and allow searches for a homologous quantitative trait locus affecting growth in humans and domestic animals.     

Heterozygosity Mapping of Partially Congenic Lines: Mapping of a Semidominant Neurological Mutation, Wheels (Whl), on Mouse Chromosome 4

We identified a semidominant, chemically induced, mouse mutation with a complex array of abnormal behaviors including bidirectional circling and hyperactivity, abnormal circadian rhythmicity and abnormal responses to light. In this report, we genetically and phenotypically characterized the circling/waltzing component of the abnormal behavior. We mapped the locus controlling this trait by heterozygosity mapping of partially congenic lines carrying the mutagenized chromosome outcrossed to different inbred strains for three generations. Analysis of 68 PCR-based markers in 13 affected individuals indicated that the mutant locus, named Wheels (Whl), resides in the subcentromeric portion of mouse chromosome 4. The statistical evaluation of data obtained by heterozygosity mapping validates this efficient mapping approach. Further characterization of the Whl mutation demonstrated that Whl/Whl homozygotes die during embryonic life and that the penetrance of circling behavior depends on genetic background. Morphological analysis of the inner ears of Whl/+ mice revealed a variable number of abnormalities in the sensory and nonsensory portions of their semicircular canals. Abnormalities ranged from slight atrophy of one or more cristae to complete absence of the lateral crista and canal. The molecular characterization of the gene disrupted in the Whl mutation will provide insight into developmental mechanisms involved in inner ear formation.     

Inheritance of a Mutation which Suppresses Flowering in Red Clover

A spontaneous mutation isolated from stocks of red clover (cultivarS123) prevents flower initiation unless plants are suppliedexogenously with gibberellin. Mutant plants are also more uprightand densely tillering in their growth habit. Inheritance ofthe non-flowering character was analysed in a series of crossesbetween wild-type S123 and mutant plants. Hybridity followingintercrossing was confirmed using electrophoretic variants ofcytoplasmic phosphoglucose isomerase coded by a- and b-allelesof the nuclear gene Pgi-2. All F₁ plants flowered normally andwere heterozygous at the Pgi-2 locus. However, F₂ segregationsdid not provide the expected ratios, with flowering plants exceedingpredicted levels. One back-cross involving an F₁ plant and themutant parent gave flowering:non flowering and ab:aa Pgi-2 ratiosof 2:1 rather than the expected 1:1. The results are consistentwith the existence of a zygotic lethal factor, originally presentin heterozygous (non-lethal) form in the mutant (non-flowering)parent and tightly linked to the mutated gene. Segregants whichwere non-flowering always displayed the characteristic mutantgrowth form and seeds borne on these plants were lighter incolour than those borne on normal plants. Thus, there existsin red clover a gene designated ‘dig’ (developmentinfluencing gibberellin) which has several pleiotropic effectsincluding suppressing the initiation of flowering in normallyflorally-inductive environments. There are at least two allelicforms of the gene, F (flowering) and f (non-flowering).     

Characterization of a Mutation in Yeast Causing Nonrandom Chromosome Loss during Mitosis

Diploid strains of the yeast Saccharomyces cerevisiae homozygous for a recessive chromosome loss mutation (chl) exhibit a high degree of mitotic instability. Cells become monosomic for chromosome III at a frequency of approximately one percent of all cell divisions. Chromosome loss at this high frequency is also found for chromosome I, and at lesser frequencies for chromosomes VIII and XVI. In contrast, little or no chromosome loss is found for six other linkage groups tested (II, V, VI, VII, XI and XVII). The chl mutation also induces a ten-fold increase in both intergenic and intragenic mitotic recombination on all ten linkage groups tested. The chl mutation does not cause an increase in spontaneous mutations, nor are mutant strains sensitive to UV or γ irradiation. The effects of chl during meiosis are observed primarily in reduced spore viability. A decrease in chromosome III linkage relationships is also found.     

Isolation and Partial Characterization of Peanut Top Paralysis Virus (PTPV), a Destructive Virus Causing a New Disease in Peanut (Arachis hypogaea L.)

A destructive virus, causing top paralysis to peanut, was discovered in the wild germplasm collection growing in the USDA-ARS greenhouses, Stillwater, Oklahoma, USA. The symptoms observed on the wild plant were restricted to a few leaves as green batches in a light green to yellow background with some leaflets having lost most of the basal part of the laminae leaving the top portion rolling upwards forming a cone. The virus was mechanically transmitted to cultivated peanut ( Arachis hypogaea L,.) where it caused more severe and destructive symptoms including stunting, severe malformation of leaves and partial or complete disappearance of leaflet laminae. This virus differed in symptomology, host range, and/or serological reactivity from allpeanut viruses reported in the literature, particularly those causing leaf malformation and stunting. The virus induced necrotic local lesions on Phaseolus vulgaris L. cv. "Topcrop" and chlorotic local lesions with necrotic centres bordered withvery bright intense red color on Chenopodium amaranticolor. In both passive indirect enzyme-linked immunosorbent assay (PAS-ELISA) and Ouchterlony double immunodiffusion test, the virus did not react with antisera against brome mosaic, bean yellow mosaic, peanut stripe, potato Y, tobacco mosaic, watermelon mosaic 1, watermelon mosaic 2, wheat soilborne mosaic, wheat streak mosaic, and zucchini yellow mosaic viruses.
However, in reciprocal cross reactions the virus seemed to share a common antigenic determinant with a peanut mottle virus isolate from Oklahoma (PMV-OK). The virus had flexuous filamentous particles with a length of 750–850 nm, falling within the range reported for the potyvirus group. The virus was successfully purified and the molecular weight of its protein subunit was found to be 30000 d. A polyclonal antiserum was raised in rabbits against the virus and used for reciprocal serological tests.     

Identification of a Novel Mouse Brachyury (<Emphasis Type="Italic">T</Emphasis>) Allele Causing a Short Tail Mutation in Mice

Mutations in T-box genes are associated with numerous disease states in humans. The objective of this paper was to characterize the T ^shao , a specific T-box mutation, in mice. T ^shao , a short-tailed mutant mouse strain in a B6 background, was obtained by ethylnitrosourea mutagenesis. Microsatellite genomic scans mapped the location of the mutation. RT–PCR was used to amplify the identified region and the product was sequenced. DNA of the region was sequenced and scanned for mutations. Tails of T ^shao mice were mostly curly with tail length ranging from less than 1 cm (tail bud) to half of the normal length. T ^shao presented single dominance gene inheritance, and homozygous mutant mice died approximately at E10. Scans of the F2 generation mapped the mutant gene to chromosome 17, near D17Mit143. The Brachyury (T) gene was identified as a potential candidate gene in this location. To confirm this, RT–PCR was performed on RNA from intercrossed 8.5-day embryos, and products were sequenced. A 67-nucleotide deletion in exon 2 of the mutant T gene was identified. Further sequencing of the genomic DNA from this region identified a T to A transversion at the 67th nucleotide of exon 2. The T ^shao mutation is a result of a deletion in exon 2 causing the early termination and loss of function of protein encoded by the T gene, manifesting as a short tail phenotype.     

Inheritance of Hair Constrictions in Mice

The frequencies of multi-constricted hairs in the pelage is the same (75%) in the caudal mid-dorsum of C57BL/10 and CBA inbred mice, but there are more single-constricted hairs in C57BL mice (9.41%) than in CBA mice (0.13%). This structural feature is used as a basis for genetic analysis of the hair coat.-The frequency of single-constricted hairs (4.41%) in F(1) mice is intermediate between the frequencies observed in the CBA and C57BL parent strains. Data from the F(2) crosses and backcrosses to the respective parent strains indicate that the presence of numerous single-constricted hairs in the pelage is an inherited characteristic. The mode of inheritance is controlled primarily by a semi-dominant autosomal gene (single-constriction; Hct), without the involvement of maternal factors.     

Identification and Mapping of a T-DNA Induced Flower Mutation in Arabidopsis Thaliana

Collection of the T-DNA tagged lines of Arabidopsis thaliana have been created by Agrobacterium-mediated root transformation. Transgenic lines produced by this method have been screened for morphogenic mutations. A flower mutation with increased number of stamens and carpels (scaf1) was identified. This mutation has similar but weaker phenotype than the known mutant superman. Two mapping procedures, with visible and molecular markers, were used to locate scaf1 flower mutation. Genetic analysis showed that this mutation is located on chromosome 3 near gl1 gene. It is probably one of the SUPERMAN epigenetic alleles. This revised version was published online in June 2006 with corrections to the Cover Date.     

Sulfhydryl Compounds in Melanocytes of Yellow (Ay/a ), Nonagouti (a/a), and Agouti (A/A) Mice

CLEFFMANN (1953, 1963a,b) has reported that yellow but not black melanocytes of agouti (A/A) rabbits contained reducing sulfhydryl compounds. We have attempted to repeat CLEFFMANN's observations in mouse melanocytes of the lethal yellow (Ay/a), nonagouti (a/a) and agouti (A/A) genotypes. Our results contradict those of CLEFFMANN and reveal that yellow and black melanocytes, regardless of genotype, possess equivalent amounts of histochemically detectable sulfhydryl compounds. These results do not support the hypothesis that agouti-locus genes act by controlling the sulfhydryl metabolism of pigment cells.     

Interval Mapping of Viability Loci Causing Heterosis in Arabidopsis

The genetic basis of heterosis has implications for many problems in genetics and evolution. Heterosis and inbreeding depression affect human genetic diseases, maintenance of genetic variation, evolution of breeding systems, agricultural productivity, and conservation biology. Despite decades of theoretical and empirical studies, the genetic basis of heterosis has remained unclear. I mapped viability loci contributing to heterosis in Arabidopsis. An overdominant factor with large effects on viability mapped to a short interval on chromosome I. Homozygotes had 50% lower viability than heterozygotes in this chromosomal region. Statistical analysis of viability data in this cross indicates that observed viability heterosis is better explained by functional overdominance than by pseudo-overdominance. Overdominance sometimes may be an important cause of hybrid vigor, especially in habitually inbreeding species. Finally, I developed a maximum likelihood interval mapping procedure that can be used to examine chromosomal regions showing segregation distortion or viability selection.     

Inheritance of Plasma Cholesterol Levels in Mice

Mean plasma cholesterol levels were determined at two ages in mice from eight unrelated inbred strains (BALB/cJ, BDP/J, CBA/J, C57BL/6J, LP/J, RF/J, SJL/J, and 129/J). Significant strain, sex, and age differences were observed. Estimates of the degree of genetic determination of the trait obtained from an analysis of the strain data averaged 58 +/- 4% for the males and 54 +/- 8% for the females.-Selection for high and low plasma cholesterol levels produced two significantly different and distinct lines. Selection was initiated in a genetically heterogeneous population derived from an eight-way cross of the inbred strains listed above. After five generations of selection the divergence of the high and low lines amounted to 4 phenotypic standard deviations of the foundation population. Realized heritability estimated from the regression of divergence on the combined cumulative selection differential was 51 +/- 5% for the males and 50 +/- 3% for the females. The results indicate that genetic factors are important in controlling plasma cholesterol levels in the mouse and that the majority of these factors act additively.     

Mutation Load under Vegetative Reproduction and Cytoplasmic Inheritance

For reasons that remain unclear, even multicellular organisms usually originate from a single cell. Here I consider the balance between deleterious mutations and selection against them in a population with obligate vegetative reproduction, when every offspring is initiated by more than one cell of a parent. The mutation load depends on the genomic deleterious mutation rate U, strictness of selection, number of cells which initiate an offspring n, and the relatedness among the initial cells. The load grows with increasing U, n and strictness of selection, and declines when an offspring is initiated by more closely related cells. If Un >> 1, the load under obligate vegetative reproduction may be substantially higher than under sexual or asexual reproduction, which may account for its rarity. In nature obligate vegetative reproduction seems to be more common and long term in taxa whose cytological features ensure a relatively low load under it. The same model also describes the mutation load under two other modes of inheritance: (1) uniparental transmission of organelles and (2) reproduction by division of multinuclear cells, where each daughter cell receives many nuclei. The load declines substantially when the deleterious mutation rate per organelle genome gets lower or when the number of nuclei in a cell sometimes drops. This may explain the small sizes of organelle genomes in sexual lineages and the presence of karyonic cycles in asexual unicellular multinuclear eukaryotes.