Single Crossovers Research Articles

Exchange of terminal portions of X- and Y-chromosomal short arms in human XX males

In most human 'XX males', DNA sequences normally found on Yp, the short arm of the Y chromosome, are present on Xp, the short arm of the X chromosome. To establish whether this transfer involves a terminal portion of Yp, and whether a terminal portion of Xp is lost in the process, we followed the inheritance of pseudoautosomal restriction fragment length polymorphisms in two XX-male families. One XX male apparently inherited the entire pseudoautosomal region of his father's Y chromosome and no part of the pseudoautosomal region of his father's X chromosome. The second XX male also inherited the entire pseudoautosomal region of his father's Y, but in addition inherited a proximal portion of the pseudoautosomal region of his father's X. These findings argue that XX males result from the transfer of a terminal portion of Yp onto Xp in exchange for a terminal portion of Xp (ref. 7). This implies that the testis-determining factor gene (TDF) maps distally in the strictly sex-linked portion of Yp, near the pseudoautosomal domain. The XX males described here appear to result from single (and, at least in the second case, unequal) crossovers proximal to the pseudoautosomal region on Yp and proximal to or within the pseudoautosomal region on Xp.

Fine structure recombinational analysis of cloned genes using yeast transformation.

We describe a general method for analyzing the genetic fine structure of plasmid-borne genes in yeast. Previously we had reported that a linearized plasmid is efficiently rescued by recombination with a homologous restriction fragment when these are co-introduced by DNA-mediated transformation of yeast. Here, we show that a mutation can be localized to a small DNA interval when members of a deletion series of wild-type restriction fragments are used in the rescue of a linearized mutant plasmid. The resolution of this method is to at least 30 base pairs and is limited by the loss of a wild-type marker with proximity to a free DNA end. As a means for establishing the nonidentity of two mutations, we determined the resolution of two-point crosses with a mutant linearized plasmid and a mutant homologous restriction fragment. Recombination between mutations separated by as little as 100 base pairs was detected. Moreover, the results indicate that exchange within a marked interval results primarily from one of two single crossovers that repair the linearized plasmid. These approaches to mapping the genetic fine structure of plasmids should join existing methods in a robust approach to the mutational analysis of gene structure in yeast.

Multiple sites of recombination within the RNA genome of foot-and-mouth disease virus

Recombinant foot-and-mouth disease viruses were isolated from cells infected with a mixture of temperature-sensitive ( ts) mutants belonging to different subtype strains. In order to select for recombination events in many different regions of the genome, crosses were performed between various pairs of mutants, with ts mutations in different regions of the genome. ts + progeny were analysed by electrofocusing virus-induced proteins and RNase T 1 fingerprinting of their RNA. All but 5 out of 43 independent isolates, from nine crosses, proved to have recombinant RNA genomes. Maps of these genomes, based on a knowledge of the locations of the unique oligonucleotides, were constructed. Most could be interpreted as being the products of single genetic cross-overs, although three recombinants were formed by two cross-overs each. Cross-overs in at least twelve distinct regions of the genome were identified. This evidence of a large number of recombination sites suggests that RNA recombination in picornaviruses is a general, as opposed to a site-specific, phenomenon.

Construction and analysis of a two-chromosome double reciprocal translocation that functions as a ‘balancer’ inNeurospora crassa

T(IIL; VL;IIR; VR) BLNC-1 is a compound chromosome rearrangement inNeurospora crassa that combines two reciprocal translocations:T(IIL; VL) AR30 which interchanges the left end of linkage group II with the left end of linkage group V, andT(IIR;VR) ALS154 which interchanges the right end of linkage group II with the right end of linkage group V.BLNC-1 acts as a crossover suppressor for most of both linkage groups II and V since single crossovers between the rearrangement breakpoints result in progeny with lethal unbalanced duplications and deficiencies. The integrity ofBLNC-1 following meiosis was tested in crosses of markedBLNC-1 by marked Normal sequence, with markers located at critical points on linkage groups II and V. Although recombination between distal markers in the four arms was reduced markedly, double crossovers in the long intervening regions occurred with a frequency of 21%. Of these double crossovers, most were coincidental crossovers, one in each of the long intervening regions, resulting in the resolution of the complex into its component rearrangements (16%), while a minority of the double crossovers (5%) were crossovers involving only one of the two component linkage groups, and resulted in the insertion of a segment between the breakpoints. - TheBLNC-1 balancer can be used for: (1) mapping new loci to linkage groups II and V, especially for identifying markers mapping near the tips of the linkage groups; (2) for isolating genetically intact chromosomes from natural populations or for quantitative genetic studies; and (3) for studying recombinational hot-spots which can be detected as escapes from crossover suppression. -Based on experience withBLNC-1, future two-chromosome balancers should be designed with two breakpoints near, but not at, the opposite ends of the chromosome to be balanced, and the other two breakpoints close to, but spanning, the centromere of a second chromosome. Such a construction when combined with appropriately placed selective markers should prevent breakdown of the complex, and should resemble an inversion in eliminating crossover products.

Recombination in RNA

The aphthovirus genome consists of a single molecule of single-stranded RNA that encodes all the virus-induced proteins. We isolated recombinant aphthoviruses from cells simultaneously infected with temperature-sensitive mutants of two different subtype strains. Analysis of the proteins induced by 16 independently generated recombinants revealed two types of protein pattern, which were consistent with single genetic crossovers on the 5′ side and 3′ side, respectively, of the central P34-coding region. Recombinants invariably inherited all four coat proteins from the same parent, and novel recombinant proteins were not observed. RNAase T1 fingerprints of virus RNA, prepared from representatives of each recombinant type, confirmed the approximate crossover sites that had been deduced from the inheritance of proteins. These fingerprints provide molecular evidence of recombination at the level of RNA and demonstrate the potential of RNA recombination for producing genetic diversity among picornaviruses.

CLUSTERING OF SPECIFIC CROSSOVERS IN MAIZE MICROSPOROCYTES

Analysis of systematic scan data from microsporocyte smear preparations suggests that cells with a crossover in a specific region tend to be geographically clustered to some extent within anthers. Crossover classes observed include single crossovers within a heterozygous inversion, three-strand double crossovers within and proximal to the inversion, and four-strand double crossovers within the inversion. Evidence for clustering within scans of cells of the first two classes is reported, but results are inconclusive for the third class. No evidence for within-scan correlation of frequencies of any two of the crossover classes was found. It is inferred that a large part of the variability in crossover frequency observed is probably due to factors which alter crossover interference. Implications related to the frequency and distribution of crossover sites are discussed.

Inversion heterozygosity and the origin of XO daughters of Bpa/+female mice.

THERE has long been an interest in developing mouse stocks carrying chromosome inversions for mutation testing. This interest stems from the fact that heterozygosity for a paracentric inversion, that is, a structural rearrangement in which a chromosome segment that does not include the centromere is rotated through 180°, results in suppression of recombination in the inversion region. This is caused by the failure of recovery of the products of single crossovers in this region, because dicentric chromatids and acentric fragments are produced that either fail to give functional meiotic products or result in the formation of unbalanced zygotes which are generally eliminated. Recombination between the pair of chromosomes concerned is, therefore, confined to any crossovers in the non-inverted regions and to the rare two-strand double crossovers which may occur within the inversion region if it is a long one. This property of inversion heterozygotes enables the recovery, in the progeny, of the structurally normal homologue largely intact and any recessive mutation located on it can be readily detected if suitable genetically marked stocks are used. In the simplest case, when the mutation is a recessive lethal and the inversion is located in the X chromosome, detection is either by the absence of half the males, or if the inversion itself is marked by a recessive lethal (as in the C1B method devised by Muller for Drosophila melanogaster), by the absence of all the males.

Studies on the genetic mechanism of radiation-induced mitotic segregation in yeast.

Sectoring was induced with x-rays or ultraviolet in a diploid yeast strain heterozygous for seven genes located on one chromosome arm. The frequencies of sectoring of different genes were approximately linearly related to their distance from the centromere. If two or more adjacent genes sectored, the event could be explained by mitotic crossing over. Sectoring of single genes, however, was mostly nonreciprocal and resembled a conversion-type event. Approximately 80% of the sectored colonies could be explained single mitotic crossovers in one of the intergenic regions.

Crossing-over in the males of Drosophila ananassae: exchanges in irradiated pupae.

SummaryCrossing-over in the males of D. ananassae has been studied by treating early and mid-pupal stages with 1000 R and 2000 R of x-rays. Spermatocytes produce single cross-overs distributed evenly between males. Spermatogonia produce clusters of cross-overs, the progeny of particular males. A twofold increase in the frequency of cross-overs from independent events by doubling the dose indicated the breakage of two chromatids by a single ionizing particle. The frequency of these exchanges in early pupae is much greater than of those in later stages of the life-cycle. This has been attributed to the predominant proportion of the spermatocytes in the young pupae. Alternatively these early spermatocytes may be intrinsically more sensitive.

Anomalous Segregation in Crosses of Escherichia coli

Segregation of the linked group of loci, S, Gal, Ara, Xyl and Mal in crosses of E. coli is such that these can not be arranged in linear sequence by the usual methods. Assuming that there is no great deviation from familiar chromosome behavior, this anomaly appears to be due to an excess of double crossovers too great to be attributable to irregularities in chromosome pairing, and not such as could result from a selective mechanism acting against single crossovers (e.g., an inversion, a ring chromosome, or a combination of lethal genes). This negative interference in crossing-over seems to occur in all chromosome regions tested.