Microsatellites reveal male recombination and neo-sex chromosome formation in Scaptodrosophila hibisci (Drosophilidae)

In drosophilid flies, male recombination and neo-sex chromosome formation are rare. Following the genotyping of full-sib families with 20 microsatellite markers and subsequent cytological work, we found evidence of both male recombination and neo-sex chromosome formation in Scaptodrosophila hibisci. As far as we are aware, this is the first report of male recombination and neo-sex chromosome formation co-occurring in a drosophilid fly. Two autosomal loci, Sh29c and Sh90, showed aberrant segregation of male parental alleles. We describe how an autosomal fission followed by fusion of one of the autosomal fragments to the Y chromosome to create a Y1Y2X1X2/X1X1X2X2 sex determination system provides the most parsimonious explanation of the patterns we observe. Male recombination was observed in three families, including autosomal linkage groups and the Y1/X2 linkage group. In addition to the X1 linkage group, two autosomal linkage groups were identified.

Polyploidy in Stokes Aster (Stokesia laevis)

Segregation analysis of two different F2 families of stokes aster created by hybridizing two blue-flowered cultivars [`Peaches Pick' (PE) and `Omega Skyrocket' (OSR)] with the yellow-flowered cultivar `Mary Gregory' (MG) gave disparate results. The F2 progeny of PE × MG segregated in the expected 3:1 (blue:yellow) ratio. In contrast, all 782 progeny from the MG × OSR F2 family were blue-flowered. Flow cytometric analysis of the parents and F1 hybrids was conducted to determine if ploidy differences existed among the parents, as such differences could account for aberrant segregation behavior in the MG × OSR F2 family. Peak ratios suggested that MG and PE were diploid, OSR was tetraploid, and F1 hybrids of MG × OSR were triploid. Chromosome counts from root tip squashes confirmed that MG and PE were diploid (2n= 2x= 14), OSR was tetraploid (2n= 4x= 28), and F1 hybrid progeny of MG × OSR were triploid (2n= 3x= 21). Karyotype analysis also confirmed these results. We propose that the lack of recovery of yellow-flowered progeny in the MG × OSR F2 family is due to differences in parental chromosome number. These results document the first report of polyploidy in stokes aster, and suggest the absence of a triploid block in this species.

Loss of KLP-19 polar ejection force causes misorientation and missegregation of holocentric chromosomes

Holocentric chromosomes assemble kinetochores along their length instead of at a focused spot. The elongated expanse of an individual holocentric kinetochore and its potential flexibility heighten the risk of stable attachment to microtubules from both poles of the mitotic spindle (merotelic attachment), and hence aberrant segregation of chromosomes. Little is known about the mechanisms that holocentric species have evolved to avoid this type of error. Our studies of the influence of KLP-19, an essential microtubule motor, on the behavior of holocentric Caenorhabditis elegans chromosomes suggest that it has a major role in combating merotelic attachments. Depletion of KLP-19, which associates with nonkinetochore chromatin, allows aberrant poleward chromosome motion during prometaphase, misalignment of holocentric kinetochores, and multiple anaphase chromosome bridges in all mitotic divisions. Time-lapse movies of GFP-labeled mono- and bipolar spindles demonstrate that KLP-19 generates a force on relatively stiff holocentric chromosomes that pushes them away from poles. We hypothesize that this polar ejection force minimizes merotelic misattachment by maintaining a constant tension on pole–kinetochore connections throughout prometaphase, tension that compels sister kinetochores to face directly toward opposite poles.

The Conversion Gradient at HIS4 of Saccharomyces cerevisiae. I. Heteroduplex Rejection and Restoration of Mendelian Segregation

In Saccharomyces cerevisiae, some gene loci manifest gradients in the frequency of aberrant segregation in meiosis, with the high end of each gradient corresponding to a hotspot for DNA double-strand breaks (DSBs). The slope of a gradient is reduced when mismatch repair functions fail to act upon heteroduplex DNA—aberrant segregation frequencies at the low end of the gradient are higher in the absence of mismatch repair. Two models for the role of mismatch repair functions in the generation of meiotic “conversion gradients” have been proposed. The heteroduplex rejection model suggests that recognition of mismatches by mismatch repair enzymes limits hybrid DNA flanking the site of a DSB. The restoration-conversion model proposes that mismatch repair does not affect the length of hybrid DNA, but instead increasingly favors restoration of Mendelian segregation over full conversion with increasing distance from the DSB site. In our experiment designed to distinguish between these two models, data for one subset of well repairable mismatches in the HIS4 gene failed to show restoration-type repair but did indicate reduction in the length of hybrid DNA, supporting the heteroduplex rejection model. However, another subset of data manifested restoration-type repair, indicating a relationship between Holliday junction resolution and mismatch repair. We also present evidence for the infrequent formation of symmetric hybrid DNA during meiotic DSB repair.

Inheritance and Linkage Relationships of Isozymes in Easter Cactus

Polyacrylamide gel electrophoresis was used to study inheritance and linkage of isozymes in Easter cactus (Hatiora species and interspecific hybrids). Five isozyme systems were analyzed: aspartate aminotransferase (AAT), glucose-6-phosphate isomerase (GPI), malate dehydrogenase (MDH), phosphoglucomutase (PGM), and triosephosphate isomerase (TPI). F1, F2, BC1, and S1 progeny were used for inheritance studies. Six polymorphic loci (Aat-1, Gpi-1, Mdh-1, Pgm-1, Pgm-2, and Tpi-2) were identified. Aat-1 and Pgm-1 were linked (recombination frequency = 26% ± 7%), but the other isozyme loci assorted independently. Aberrant segregation ratios were observed in at least one segregating family for all six isozyme loci. We hypothesize that segregation distortion was due to linkage between isozyme loci and other genes subject to pre- or postzygotic selection. The existence of five additional isozyme loci (Aat-2, Gpi-2, Mdh-2, Mdh-3, and Tpi-1) was inferred from segregation patterns and by comparison of isozyme profiles from phylloclades and pollen. These isozyme loci may prove useful for confirming hybridity in intra- and interspecific crosses, determining parentage of cultivars, and assessing genetic diversity in germplasm collections.

Development of a core RFLP map in maize using an immortalized F2 population.

A map derived from restriction fragment length polymorphisms (RFLPs) in maize (Zea mays L.) is presented. The map was constructed in an immortalized Tx303 x CO159 F2 mapping population that allowed for an unlimited number of markers to be mapped and pooled F3 seed to be distributed to other laboratories. A total of 215 markers consisting of 159 genomic clones, 16 isozymes and 35 cloned genes of defined function have been placed on 10 chromosomes. An examination of segregation data has revealed several genomic regions with aberrant segregation ratios favoring either parent or the heterozygote. Mapping of cloned genes and isozymes that have been previously mapped by functional criteria has provided 29 points of alignment with the classical maize genetic map. Screening of all mapped RFLP probes against a collection of U.S. Corn Belt germplasm using EcoRI, HindIII and EcoRV has resulted in a set of 97 core markers being defined. The designation of a set of core markers allows the maize genome to be subdivided into a series of bins which serve as the backbone for maize genetic information and database boundaries. The merits and applications of core markers and bins are discussed.

Genetic evidence that the meiotic recombination hotspot at the HIS4 locus of Saccharomyces cerevisiae does not represent a site for a symmetrically processed double-strand break.

In the yeast Saccharomyces cerevisiae, the binding of the Rap1 protein to a site located between the 5' end of the HIS4 gene and the 3' end of BIK1 stimulates meiotic recombination at both flanking loci. By using strains that contain mutations located in HIS4 and BIK1, we found that most recombination events stimulated by the binding of Rap1 involve HIS4 or BIK1, rather than bidirectional events including both loci. The patterns of aberrant segregation indicate that most of the Rap1-stimulated recombination events do not represent the symmetric processing of a double-strand DNA break located at the Rap1-binding site.

Genetic Analysis of Isoenzymes in Raspberry

The inheritance of five isoenzymes was studied in red and purple raspberry F1 progenies (Rubus idaeus L. and Rubus × neglectus Peck). Isocitrate dehydrogenase (IDH; EC 1.1.1.42) was a dimeric enzyme present in the cytosol and coded for by one locus (Idh-1). Three of the four crosses analyzed at this locus had deviations from expected ratios, possibly caused by its linkage to a recessive lethal gene. Malate dehydrogenase (MDH; EC 1.1.1.37), phosphoglucoisomerase (PGI; EC 5.3.1.9), and triose phosphate isomerase (TPI; EC 5.3.1.1) were dimeric enzymes with two loci. Each of these three enzymes was present in an organelle and in the cytosol for locus 1 and 2, respectively. Phosphoglucomutase (PGM; EC 2.7.5.1) was monomeric with two loci, Pgm-1 and Pgm-2, located in an organelle and the cytosol, respectively. Each allele at Pgm-1 resulted in the formation of two bands. Although most progenies analyzed supported Mendelian inheritance of polymorphic loci (except for Idh-1), there was a higher than expected number of aberrant segregation ratios observed (18.4%). Analysis of 85 pairs of jointly segregating loci revealed a possible linkage group consisting of Mdh-2, Tpi-2, and Pgm-1.

FREQUENCY AND DIRECTIONALITY OF GENE CONVERSION EVENTS INVOLVING THE CYC7-H3 MUTATION IN SACCHAROMYCES CEREVISIAE

ABSTRACT The CYC7-H3 mutation is a 5-kb deletion that causes overproduction of iso-2 cytochrome c. Unlike most mutations in yeast, the CYC7-H3 mutation is preferentially lost when it is involved in a gene conversion event. We have shown that cloned copies of CYC7-H3 DNA that are inserted into the yeast genome are associated with a high frequency of recombination and aberrant segregation events. Since parity in conversion frequency was observed when the extensive insertion/deletion heterozygosity at this locus was eliminated, we conclude that the CYC7-H3 sequences are inherently capable of acting as donors or recipients in gene conversion events, although they are unlikely to act as donors when they are located opposite a large heterology. DNA sequence comparisons revealed similarities between the CYC7-H3 junction region and the 2-µm circle DNA region that is involved in site-specific recombination.