The Waxy Locus in Maize III. Effect of Structural Heterozygosity on Intragenic Recombination and Flanking Marker Assortment

The effect of heterozygosity for structural rearrangements on recombination between two wx heteroalleles (C and 90) and the pattern of flanking markers in the resultant Wx gametes has been examined. The rearrangements are Tp9, an insertional translocation in which a segment of chromosome 3 has been inserted into the short arm of chromosome 9 close to the wx locus; In9a, a long pericentric inversion with wx in the inverted segment; and Rearr 9, a complex rearrangement of chromosome 9. Heterozygosity for rearrangements decreases the frequency of Wx gametes to varying degrees.—Heterozygosity for Tp9 enhances the proportion of Wx gametes that are apparent convertants and allows the conclusion that such gametes do not normally arise from an exchange in the wx locus plus a second exchange distal to wx. Heterozygosity for In9a markedly decreases the frequency of Wx gametes that are recombinant for outside markers but does not decrease the frequency of convertants.—Heterozygosity for Rearr 9 permits a low frequency of Wx gametes, all of which are apparent convertants.—A high proportion of the convertants have the flanking markers that entered the cross with C so recombination is polarized in normal homologs and in heterozygotes for all rearrangements.     

Genetic regulation of somatic mutability of two Mu-induced a1 mutants of maize

Summary Previous studies of stocks of two Mutator-induced mutable a1 alleles (a1-Mum2 and al-Mum3) gave results consistent with the presence of one or more autonomous elements regulating the expression of mutability. This article reports on the results of studies designed to map these autonomous elements by using a series of waxy marked translocations. Linkage of waxy with autonomous elements was found for a1-Mum2 by using the translocations wx T2-9d, wx T4-9e and wx T4-9b. Several different linkage values were found in crosses involving wx T2-9d, suggesting that autonomous elements have transposed to different locations on chromosome 2. Linkage of autonomous elements with waxy was found for a1-Mum3 using translocation wx T2-9d. Again, several different linkage values were found. Some of these values were the same as those observed for a1-Mum2, but some were unique. In some crosses, the number of autonomous elements increased by one or two unlinked elements in addition to the linked element in one generation (i. e. the generation of the cross to the translocation series). Such an increase in number is probably the result of transposition of the original autonomous element to an independent locus while retaining the autonomous element at the original locus. Reduction in the number of autonomous elements is probably the result of the independent assortment in crosses of plants with two or more autonomous elements.Journal Paper No. J-14569 of The Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa Project No 2870     

Chromosome engineering in wheat to restore male fertility in the msH1 CMS system

Pollen fertility restoration of the CMS phenotype caused by H. chilense cytoplasm in wheat was associated with the addition of chromosome 6H^chS from H. chilense accession H1. In order to develop an euploid restored line, different genomic combinations substituting the 6H^chS arm for another homoeologous chromosome in wheat were evaluated, with the conclusion that the optimal combination was the translocation T6H^chS·6DL. The double translocation T6H^chS·6DL in H. chilense cytoplasm was obtained. This line is fertile and stable under different environmental conditions. However, a single dose of the T6H^chS·6DL translocation is insufficient for fertility restoration when chromosome 6D is also present. Restoration in the msH1 system is promoted by interaction between two or more genes, and in addition to the restorer of fertility (Rf) located on chromosome 6H^chS, one or more inhibitor of fertility (Fi) genes may be present in chromosome 6DL.     

MUTATIONS AFFECTING HETEROKARYOSIS IN SCHIZOPHYLLUM COMMUNE

Mutations that affect the basic characteristics of heterokaryons of S. commune occur spontaneously and are preferentially selected in the common-A heterokaryon and in its homokaryotic mimics, strains carrying a mutated B factor or strains disomic for heteroallelic B factors. Nine independent mutations were compared: all segregate independently of A and B incompatibility factors, and at least 3 distinct loci, of which 2 are linked, are involved. None of the mutations is phenotypically expressed in the homokaryon or in the common-AB heterokaryon. All 9 mutations increase vegetative vigor of the common-A the effects of all the mutations are additive in both heteroallelic and homoallelic combinations. At least 1 type-II mutation also affects nuclear distribution in common-B heterokaryons. Type-II mutations appear to reduce common-A, common-B, and compatible heterokaryons to a single type unlike any of the normal heterokaryons. Pseudoclamping often persists for extended periods in modified homokaryons isolated from modified heterokaryons. Several cases of somatic recombination have been observed among components of modified heterokaryons.     

Suppressed recombination rate in 6VS/6AL translocation region carrying the Pm21 locus introgressed from Haynaldia villosa into hexaploid wheat

Pm21 is an effective gene for powdery mildew resistance transferred from Haynaldia villosa into common wheat cultivars. No virulence against this gene has been detected so far. A set of 42 powdery mildew isolates collected in Israel and tested in the current study also revealed no virulence against this gene. Pm21 was previously reported to be located on the short arm of 6VS/6AL translocation chromosome. We constructed a high-density genetic map of chromosome 6A, consisting of 28 PCR markers and the Pm21 gene. A comparison with previously published genetic maps of wheat chromosome 6A revealed that the recombination rate in the 6VS/6AL translocation region was poor. We assume that suppressed recombination caused by the alien H. villosa genetic material is the most reasonable explanation for the tight genetic linkage and the inadequacy between the Pm21 genetic map and the Pm21 physical map of 6A. A large number of sequence-tag sites (STS) and simple sequence repeat markers, which co-segregate with or are closely linked to the Pm21 gene, and the conversion of three resistance gene analog markers into new STS markers, provide a reliable and easy-to-use molecular tool for marker-assisted selection of Pm21 in wheat breeding programs. An additional gene, Pm31, previously reported to be derived from Triticum dicoccoides, was mapped into a similar genomic location to Pm21. Screening of the parental lines and the mapping population with Pm21 diagnostic markers clearly confirmed that the donor line of Pm31 is H. villosa and not T. dicoccoides. Therefore, we conclude that Pm21 and Pm31 refer to the same gene, derived from H. villosa, and that the designation of Pm31 as a new Pm gene was erroneous.     

Selectively recombining B incompatibility factors of Schizophyllum commune

Summary Analysis of genetic crosses among strains of Schizophyllum commune carrying recombining B factors has revealed that not all heteroallelic pairs of B factors are able to recombine with each other. This suppression of recombination is highly specific and appears to be determined by the B factors themselves.     

Biochemical control of recombination in Saccharomyces cerevisiae. I. L-histidine inhibition of intragenic mitotic recombination at the ad 3 locus

Summary A yeast strain heteroallelic at the ad ₃ locus is used to study mitotic intragenic recombination. L-histidine inhibits the recombination at this locus in strains heteroallelic for all possible combinations between the four alleles studied. No other amino acid has this effect. The kinetic of recombination was studied by addition of L-histidine at different times or by compeating the L-histidine present with D-histidine added at different times. The two techniques gave similar results showing that the recombination takes place between the 8th and 24th hour after plating although it is expressed a few days later.Taking advantage of the early interval in which the recombination takes place and of the fact than the petite mutation is induced by acriflavine only in new formed buds, we developed a technique to study the recombination in liquid medium, thanks to which, we were able to show that L-histidine inhibits the genetic event itself.     

Differential activity of the maize mutator Mu at different loci and in different cell lineages

Summary Mutator activity of the maize mutator (Mu) system varies for different loci. Mutation frequencies as high as 7.54x10^–5 and as low as 4.0x10^–6 are observed for 5 loci (i.e., y ₁,yg2, bz1, sh2, and wx). For the waxy locus, a higher mutation frequency is observed in Mu plants crossed as males than when Mu plants function as females. The frequency of unselected mutations also is found to be higher in Mu plants crossed as males than in the first-generation Mu plants crossed as females. The mutation frequency of the y1 locus, however, does not differ in the male or female crosses. Mu-induced mutation frequencies vary with respect to loci and, for some loci, may depend on other factors such as the sex of the Mu parent or the previous crossing history of the Mu parent. More limited data have been obtained for 4 additional loci(su1, c1, c2 and o2).Journal Paper No. J-11487 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 2623     

Meiotic mapping of the nuclear determinant of cell lysis of the osmotic dependent Saccharomyces cerevisiae mutant VY1160

Summary A recessive nuclear mutation, sorb ^-, which determines the ability for lysis of the osmotic dependent Saccharomyces cerevisiae mutant VY1160 has been mapped on the right arm of chromosome I. sorb ^- is not centromere linked and is approximately 31 recombination units from ade1.     

Transmission and phenotypic effect of a duplicate chromosome segment in maize

Summary The chromosome B⁴, extracted from the translocation TB-4a (involving chromosome 4 and a B chromosome) was transferred into stocks with normal complement. This chromosome carried 75% of the short arm of chromosome 4 and was provided with a B centromere. Loss in somatic tissues, in meiotic divisions and through gametophyte competition in the pollen was investigated by cytological and genetical means. Nondisjunction in the second microspore division of the B⁴, in the presence of a normal chromosome 4, was not frequently observed. Sectoring in endosperm tissues, after appropriate crosses, presumably indicated either late replication of this chromosome and loss during endosperm development, and/or inactivation of the Su locus which is near the breakage point of the translocation with the B chromosome. Reduced vigor of the plants carrying one or two B⁴ chromosomes was interpreted as an effect of the duplication. There are indications that hyperploidy for this specific region may affect the kernel size and weight.This work was supported by C.N.R., N.A.T.O. and Indiana University funds.     

Effect of trisomy 6 on recombination in chromosome 9 of maize

The effect of an additional chromosome 6 upon recombination in chromosome 9 was investigated in maize. Trisomic 6 plants and their disomic sibs, heterozygous for three loci of chromosome 9 (yg, sh and wx), were testcrossed, and recombination in the regions yg–sh and sh–wx was analyzed. Single exchanges in the sh–wx region and double exchanges were more frequent in trisomics, particularly in female flowers.——In reciprocal testcrosses, higher male crossover rates were found for the sh–wx region, and the difference was enhanced in trisomic 6 plants.     

Mapping of susceptibility to Marek's disease within the major histocompatibility (B) complex by refined typing of White Leghorn chickens

The major histocompatibility (B) complex of a distinct commercial pure White Leghorn chicken line was characterized using serological, biochemical and restriction fragment length polymorphism (RFLP) typing. Line B chickens displayed a high recombination frequency within the B complex. Three recombinant haplo-types were identified. The influence of these haplotypes was determined in relation to the haplotypes B^l9 and B²¹ on their resistance to Marek's disease (MD) in an experimental infection with the virus. Offspring of sires with a recombinant haplotype in combination with B¹⁹ or B²¹, and dams, which were homozygous B¹⁹/B¹⁹ or B²¹/B²¹ were infected. The B type of the offspring had a significant effect upon survival. Animals with B complex types B²¹/B²¹, B¹³⁴/B²¹ and B²³⁴/B²¹ were relatively resistant to MD (24–32% mortality), whereas B¹⁹/B¹⁹ birds were highly susceptible (68% mortality). Animals with a recombinant halpotype B^19r21 (B-G²¹, B-F¹⁹) were equally susceptible to MD as birds with the complete B¹⁹ haplotype. In contrast to earlier publications, resistance was not inherited as a dominant trait. Apparently, B¹⁹ was associated with a dominant susceptibility. The gene(s) associated with the B complex and involved in resistance to MD were localized within the B-F/B-L region. However, the association with a presumably non-coding subregion of B-G could not be excluded.     

An approach to mapping haplotype-specific recombination sites in human MHC class III

Studies of the major histocompatibility complex (MHC) in mouse indicate that the recombination sites are not randomly distributed and their occurrence is haplotype-dependent. No data concerning haplotype-specific recombination sites in human are available due to the low number of informative families. To investigate haplotype-specific recombination sites in human MHC, we here describe an approach based on identification of recombinant haplotypes derived from one conserved haplotype at the population level. The recombination sites were mapped by comparing polymorphic markers between the recombinant and assumed original haplotypes. We tested this approach on the extended haplotype HLA A3; B47; Bf ^* F; C4A ^* 1; C4B ^* Q0; DR7, which is most suitable for this analysis. First, it carries a number of rare markers, and second, the haplotype, albeit rare in the general population, is frequent in patients with 21-hydroxylase (21OH) defect. We observed recombinants derived from this haplotype in patients with 21OH defect. All these haplotypes had the centromeric part (from Bf to DR) identical to the original haplotype, but they differed in HLA A and B. We therefore assumed that they underwent recombinations in the segment that separates the Bf and HLA B genes. Polymorphic markers indicated that all break points mapped to two segments near the TNF locus. This approach makes possible the mapping of preferential recombination sites in different haplotypes.     

Induction and repair of gene conversion in UV-sensitive mutants of yeast

Summary Photoreactivation effect on UV-induced allelic recombination has been examined using various combinations of leu 1 alleles in UV-sensitive and wild type diploid yeast, Saccharomyces cerevisiae. The frequencies of UV-induced heteroallelic reversion in UV-sensitive strains, presumably lacking dark-repair, are strikingly enhanced compared to those in wild type at the same doses under dark condition. However, these enhanced frequencies of reversion are diminished by photoreactivation almost to the level of those in wild type. The induced frequencies of homoallelic reversion (mutation) of relevant alleles are apparently lower than those of heteroallelic reversion. Phenotypic analysis for linked gene leu 1 on UV-induced heteroallelic revertants has shown that most of the revertants are of the nonreciprocal type recombination (mitotic gene conversion). These results would indicate that most of the dark-repairable damage leading to mitotic gene conversion after UV-light is due to pyrimidine dimers.On leave of absence from Radiation Center of Osaka Prefecture, Shinke-cho Sakai, Osaka, Japan.     

Genetic determination of the donor properties in Escherichia coli K-12. Phenomena of chromosome mobilization and integrative suppression.

Summary The chromosome mobilization is the ability of F⁺ donors to introduce part of the chromosome besides F-plasmids into the recipient cells during conjugation. We studied the genetic determination of this phenomenon. Most efficient almost like true Hfr's are F⁺-cells of the genotype recBC ^- sbcB ^- belonging to the RecF-recombination type. Their ability to chromosome mobilization is 50 fold higher comparing with wild type F⁺ (of the RecBC-recombination type). This property is fully dependent on the recF gene but does not depend on recL. The donors recBC ^-F⁺ but without the mutation sbcB ^- act in mobilization about 4 times weaker than wild type. Hence we see two main levels of mobilization, quantitatively very different: a recF-dependent and recBC-dependent. Both reveal an absolute requirment of the product of recA gene.The efficiency of mobilization of different markers along the chromosome was studied and mapped. The maps were identical, in spite of great difference in absolute frequencies for the RecF- and Rec BC-pathways. They are not at all random. The sites of mobilization are coinsident with the points of interaction of the F-factor leading to stable Hfr's. Therefore it is suggested that these sites of predominant mobilization are IS-sequences and that during chromosome mobilization single-strand integration of the F-factor via a semichiasmus is effected. It gives a pulse to initiate DNA transfer into the recipient but is unstable and transient and does not yield true Hfr's.The suppression of the Dna^ts phenotype in F⁺ cells due to the integration of an F-plasmid into the chromosome (integrative suppression) is increased manyfold on the RecF-pathway of recombination. Probably it is a manifestation of mentioned hot spots of recombination.The regions fre described earlier (Bresler et al., 1978) and confirmed in this paper are regarded as substrates of some recF-dependent endonuclease of recombination. Probably they coinside with clusters of IS-sequences.     

recE-Dependent Gene Amplification Induced by a Tunicamycin-resistant Mutation (tmr A7) in Bacillus subtilis

A recombinant Bacillus subtilis phage, ρ11-AA248, contains the tmr A7-amy R2-amy E⁺-tmr B⁺-aroI⁺ region of the B. subtilis N7 chromosome on a 22.4 kb DNA fragment. The amy E⁺-tmr B⁺ gene region in the phage genome of the B. subtilis 207-21 transductants by ρ11-AA248 was amplified to approximately 10 copies after cultivation in the presence of tunicamycin (10 μg/ml) and to two copies without tunicamycin. The amplification of the gene region caused hyper-production of extracellular α-amylase. In contrast, no amplification of the gene region was detected in the transductants of B. subtilis 207-25, a recE-deficient derivative of 207-21 strain.     

Conformational dependence of vicinal 13C′NCαH coupling constants in peptides: A dirac vector model investigation

Theoretical calculations of the heteronuclear vicinal coupling constant ³J(¹³C′NC^αH) in peptides have been carried out using the Dirac vector model. The results showed an angular dependence for this coupling constant, which can be expressed in the form ³J(¹³C′NC^αH) = A cos² θ + B cos θ + C, where A, B, and C are constants and θ is related to the torsional angle ? of the peptide backbone. The results of the present calculations are in very good agreement with those obtained using finite perturbation theory at the INDO level of approximation.