Allelic Negative Complementation at the Abruptex Locus of DROSOPHILA MELANOGASTER

The mutations of the Abruptex locus in Drosophila melanogaster fall into three categories. There are recessive lethal alleles and viable alleles. The latter can be divided into suppressors and nonsuppressors of Notch mutations. The recessive lethals are lethal in heterozygous combination with Notch. As a rule the recessive lethals are lethal also in heterozygous combination with the viable alleles. Heterozygous combinations of certain viable alleles are also lethal. In such heterozygotes, one heteroallele is a suppressor of Notch and the other is a nonsuppressor. Other heterozygous combinations of viable alleles are viable and have an Abruptex phenotype. The insertion of the wild allele of the Abruptex locus as an extra dose (carried by a duplication) into the chromosomal complement of the fly fully restores the viability of the otherwise lethal heterozygotes if two viable alleles are involved. The extra wild allele also restores the viability of heterozygotes in which a lethal and a suppressor allele are present. If, however, a lethal and a nonsuppressor are involved, the wild allele only partly restores the viability, and the effect of the wild allele is weakest if two lethal alleles are involved. It seems likely that of the viable alleles the suppressors of Notch are hypermorphic and the nonsuppressors are hypomorphic. The lethal alleles share properties of both types, and are possibly antimorphic mutations. It is suggested that the locus is responsible for a single function which, however, consists of two components. The hypermorphic mutations are defects of the one component and the hypomorphic mutations of the other. In heterozygotes their cumulative action leads to decreased viability. The lethal alleles are supposed to be defects of the function as a whole. The function controlled by the locus might be a regulative function.     

Recombination at the Bar Locus in an Inverted Attached-X System in DROSOPHILA MELANOGASTER

Recombination at the Bar locus in Drosophila melanogaster was investigated in an inverted attached-X system which enhanced the frequency of homozygosis for the Bar region. Females among the progeny of homozygous B mothers were searched for changes of B to BB and to B(+). Marker genes were followed and exceptional half-tetrads were analyzed in regard to two hypotheses: that of exchange between obliquely synapsed members of the duplication, which is associated with exchange of outside markers, and that of intrachromosomal exchange, which does not involve recombination of markers.-Recombinant exceptions of B(+) /BB genotype, carrying the outside marker combinations predicted on the hypothesis of exchange between obliquely synapsed duplication members, were encountered repeatedly. It is established that B(+) and BB strands are reciprocal products of the same event.-Twelve nonrecombinant exceptional strands were isolated; ten of these were B(+) and two were BB. Only one of the nonrecombinant half-tetrads offered the opportunity to test the prediction of reciprocity of the intrachromosomal event. Analysis showed the exceptional female to be of the constitution BB/B, a type not expected on the hypothesis. While it could have arisen through some kind of copy error in the repair of a chromatid break, a valid test of the hypothesis of intrachromosomal exchange must rest on the isolation and analysis of more cases of the appropriate exceptional genotype.-In several cases Bar changes were found to be associated with aberrations; all but one of these involved spontaneous, cytologically identifiable deletions.     

White grapes arose through the mutation of two similar and adjacent regulatory genes

Most of the thousands of grapevine cultivars (Vitis vinifera L.) can be divided into two groups, red and white, based on the presence or absence of anthocyanin in the berry skin, which has been found from genetic experiments to be controlled by a single locus. A regulatory gene, VvMYBA1, which could activate anthocyanin biosynthesis in a transient assay, was recently shown not to be transcribed in white berries due to the presence of a retrotransposon in the promoter. We have found that the berry colour locus comprises two very similar genes, VvMYBA1 and VvMYBA2, located on a single bacterial artificial chromosome. Either gene can regulate colour in the grape berry. The white berry allele of VvMYBA2 is inactivated by two non-conservative mutations, one leads to an amino acid substitution and the other to a frame shift resulting in a smaller protein. Transient assays showed that either mutation removed the ability of the regulator to switch on anthocyanin biosynthesis. VvMYBA2 sequence analyses, together with marker information, confirmed that 55 white cultivars all contain the white berry allele, but not red berry alleles. These results suggest that all extant white cultivars of grape vines have a common origin. We conclude that rare mutational events occurring in two adjacent genes were essential for the genesis of the white grapes used to produce the white wines and white table grapes we enjoy today.     

Molecular and physiological characterization of Arabidopsis GAI alleles obtained in targeted Ds-tagging experiments

Bioactive gibberellin (GA) is an essential regulator of vascular plant development. The GAI gene of Arabidopsis thaliana (L.) Heynh. encodes a product (GAI) that is involved in GA signalling. The dominant mutant gai allele encodes an altered product (gai) that confers reduced GA responses, dwarfism, and elevated endogenous GA levels. Recessive, presumed loss-of-function alleles of GAI confer normal height and resistance to the GA biosynthesis inhibitor paclobutrazol. One explanation for these observations is that GAI is a growth repressor whose activity is opposed by GA, whilst gai retains a constitutive repressor activity that is less affected by GA. Previously, we described gai-t6, a mutant allele which contains an insertion of a maize Ds transposable element into gai. Here we describe the molecular and physiological characterization of two further alleles (gai-t5, gai-t7) identified during the Ds mutagenesis experiment. These alleles confer paclobutrazol resistance and normal endogenous GA levels. Thus the phenotype conferred by gai-t5, gai-t6 and gai-t7 is not due to elevated GA levels, but is due to loss of gai, a constitutively active plant growth repressor.     

白色基因座(I)在中国地方猪种毛色遗传中的作用研究

通过利用PCR—RFLP和PCR—SSCP技术对中国地方猪种KIT基因内含子17、18的序列进行多态性分析。结果表明：内含子17上的替换突变(G→A)发生于毛色为白色的个体——白色五指山猪、大白猪、长白猪上,其基因型(AB型)频率分别为1、1和0．8;其他中国地方猪种的此基因型频率均为0。内含子18上的缺失突变(AGTT)也同样发生在上述3个猪种的白色个体中,其基因型(AA型)频率分别为1、1和0．93;而且同样在其他的地方品种中其基因型频率均为0。这充分证明KIT基因对于猪的白毛色有重要的调控作用,而且I基因座对于其他的经典遗传基因座有上位作用。另一方面,中国地方猪种荣昌猪虽然在表型上与引入猪种大白猪、长白猪相似(白毛色),但是在KIT基因上发生的突变完全不同,推测它们分别属于不同的毛色遗传体系。     

JIL-1 kinase, a member of the male-specific lethal (MSL) complex, is necessary for proper dosage compensation of eye pigmentation in Drosophila

The upregulation of the JIL-1 kinase on the male X chromosome and its association with the male-specific lethal (MSL) complex suggest that JIL-1 may play a role in regulating dosage compensation. To directly test this hypothesis we measured eye pigment levels of mutants in the X-linked white gene in an allelic series of JIL-1 hypomorphic mutants. We show that dosage compensation of w(a) alleles that normally do exhibit dosage compensation was severely impaired in the JIL-1 mutant backgrounds. As a control we also examined a hypomorphic white allele w(e) that fails to dosage compensate in males due to a pogo element insertion. In this case the relative pigment level measured in males as compared to females remained approximately the same even in the most severe JIL-1 hypomorphic background. These results indicate that proper dosage compensation of eye pigment levels in males controlled by X-linked white alleles requires normal JIL-1 function.     

Reversion at the HIS1 Locus of Yeast

The his1 gene (chromosome V) of Saccharomyces cerevisiae specifies phosphoribosyl transferase (E.C.2.4.2.17), the first enzyme of histidine biosynthesis. This hexameric enzyme has both catalytic and regulatory functions. The spontaneous reversion rates of seven his1 mutations were studied. The reversion rates of the alleles at the proximal end of the locus (relative to the centromere) were about 50-fold higher than distal alleles. Spontaneous reversion to prototrophy was studied in diploids homoallelic for each of the seven his1 mutations. Based on tetrad analysis, the prototrophy revertants could be assigned to three classes: (1) revertant tetrads that carried a prototrophic allele indistinguishable from wild type; (2) revertant tetrads that carried a prototrophic allele characterized by histidine excretion and feedback resistance; and (3) revertant tetrads that did not contain a prototrophic spore, but rather a newly derived allele that complemented the original allele intragenically. Four of the seven his1 mutations produced the excretor revertant class, and two mutations produced the complementer revertant class. The significance of these findings to our understanding of gene organization and the catalytic and regulatory functions of gene products are discussed.     

Locus specificity of polymorphic alleles and evolution by a birth-and-death process in mammalian MHC genes

We have conducted an extensive phylogenetic analysis of polymorphic alleles from human and mouse major histocompatibility complex (MHC) class I and class II genes. The phylogenetic tree obtained for 212 complete human class I allele sequences (HLA-A, -B, and -C) has shown that all alleles from the same locus form a single cluster, which is highly supported by bootstrap values, except for one HLA-B allele (HLA-B*7301). Mouse MHC class I loci did not show locus-specific clusters of polymorphic alleles. This was considered to be because of either interlocus genetic exchange or the confusing designation of loci in different haplotypes at the present time. The locus specificity of polymorphic alleles was also observed in human and mouse MHC class II loci. It was therefore concluded that interlocus recombination or gene conversion is not very important for generating MHC diversity, with a possible exception of mouse class I loci. According to the phylogenetic trees of complete coding sequences, we classified human MHC class I (HLA-A, -B, and -C) and class II (DRB1) alleles into three to five major allelic lineages (groups), which were monophyletic with high bootstrap values. Most of these allelic groups remained unchanged even in phylogenetic trees based on individual exons, though this does not exclude the possibility of intralocus recombination involving short DNA segments. These results, together with the previous observation that MHC loci are subject to frequent duplication and deletion, as well as to balancing selection, indicate that MHC evolution in mammals is in agreement with the birth-and-death model of evolution, rather than with the model of concerted evolution.     

DNA Repair Dependence of Somatic Mutagenesis of Transposon-Caused WHITE Alleles in DROSOPHILA MELANOGASTER after Treatment with Alkylating Agents

DNA repair-defective alleles of the mei-9, mei-41, mus-104 and mus-101 loci of Drosophila melanogaster were introduced into stocks bearing the UZ and SZ marker sets. Males with the UZ marker set, z1 (zeste allele) and w+(TE) (genetically unstable white allele presumably caused by a transposable element), or the SZ marker set, z1 and w+R (semistable white allele caused by partial duplication of the w+ locus plus transposon insert), were exposed to EMS at the first instar. After emergence, adult males bearing red spots on lemon-yellow eyes were scored as flies with somatic reversions of w+(TE) or w+R. The relative mutabilities (relative values of reversion frequency at an equal EMS dose) of either w+(TE) or w+R in a repair-proficient strain and in mei-9, mei-41, mus-104 and mus-101 strains were 1: approximately 1.2:0.3:0.3:0.7, despite the fact that w+(TE) reverted two to three times as frequently as w+R under both the repair-proficient and repair-deficient genetic conditions. Similarly, after treatment with MMS, MNNG and ENNG, w+(TE) was somatically more mutable in the mei-9 strain and less mutable in the mei-41 and mus-104 strains than in the repair-proficient strain. From these results, we propose that mutagenic lesions produced in DNA by treatment with these chemicals are converted to mutant DNA sequences via the error-prone repair mechanisms dependent on the products of the genes mei-41+ (mei-41 and mus-104 being alleles of the same locus) and mus-101+, whereas they are eliminated by mei-9+-dependent excision repair. In contrast to the approximately linear responses of induced reversions of w+(TE) with ENNG in the repair-proficient, mei-9, and mei-41 strains, seemingly there were dosage insensitive ranges for induced reversion with MNNG in the repair-proficient and mei-41 strains, but not for reversion in the mei-9 strain; w+(TE) in the mus-104 strain was virtually nonmutable with MNNG and ENNG. These results suggest that O6-methylguanine (O6MeG) produced in DNA with MNNG, but not O6-ethylguanine produced with ENNG, is almost completely repaired in a low dose range by constitutive activity of DNA O6MeG transmethylase. From the distribution of clone sizes of spontaneous revertant spots and other data, we propose that both w+(TE) and w+R have a similar tendency to spontaneously revert more frequently at early rather than at later developmental stages probably reflecting a common property of their inserted transposons.     

The Interactions of Three Widely Separated Loci Controlling Conversion Properties of w Locus I in ASCOBOLUS IMMERSUS

Gene conversion properties of white (w) ascospore locus I in the Pasadena strains of Ascobolus immersus are controlled by complex interactions between three separate conversion control factors ( ccfs), which can give conversion frequencies at wI ranging from less than 1% up to 33%. ccf-2, which has three alleles, is very closely linked to wI but does not usually co-convert with it. ccf-2(K) and ccf-2(91) give lower conversion frequencies than ccf-2(P) and are incompletely dominant to ccf-2(P), with cis/trans position effects on conversion of wI. The "super" factor ( Helmi and Lamb 1979) has two interacting but unlinked components, ccf-3E and ccf-4r, which approximately double the conversion frequency at wI. ccf-2 (linkage group VIII), ccf-3 (linkage group I) and ccf-4 are probably all unlinked but interact and specifically control conversion at wI. ccf-3E could code for a diffusible product that affects the action of different ccf-2 alleles, which probably act by controlling the frequency of initiation of hybrid-DNA, which spreads into the adjacent wI locus. ccf-4R could code for a diffusible inhibitor of ccf-3E product, or be an alternative binding site for ccf-3E product. The dominance of ccf-4R depends on which ccf-2 alleles are present in the cross.