Meiotic crossover patterns: Obligatory crossover,interference and homeostasis in a single process

During meiosis, crossover recombination is tightly regulated. A spatial patterning phenomenon known as interference ensures that crossovers are well-spaced along the chromosomes. Additionally, every pair of homologs acquires at least one crossover. A third feature, crossover homeostasis, buffers the system such that the number of crossovers remains steady despite decreases or increases in the number of earlier recombinational interactions. Here we summarize recent work from our laboratory supporting the idea that all 3 of these aspects are intrinsic consequences of a single basic process and suggesting that the underlying logic of this process corresponds to that embodied in a particular (beam-film) model.     

Creutzfeldt-Jakob Disease with a prion protein gene codon 180 mutation presenting asymmetric cortical high-intensity on magnetic resonance imaging

ABSTRACT. Here we report a genetically confirmed case of Creutzfeldt-Jakob disease with a prion protein gene codon 180 mutation presenting atypical magnetic resonance imaging findings. The present case exhibited an acute onset and lateralized neurologic signs, and progressive cognitive impairment. No myoclonus or periodic synchronous discharges on electroencephalography were observed. Diffusion-weighted images revealed areas of high signal intensity in the right frontal and temporal cortices at onset that extended to the whole cortex and basal ganglia of the right cerebral hemisphere at 3 months. Although the cerebrospinal fluid (CSF) was initially negative for neuron specific enolase, tau protein, 14–3–3 protein, and abnormal prion protein, the CSF was positive for these brain-derived proteins at 3 months after onset.     

Surface Spreading and Immunostaining of Yeast Chromosomes

The small size of nuclei of the budding yeast Saccharomyces cerevisiae limits the utility of light microscopy for analysis of the subnuclear distribution of chromatin-bound proteins. Surface spreading of yeast nuclei results in expansion of chromatin without loss of bound proteins. A method for surface spreading balances fixation of DNA bound proteins with detergent treatment. The method demonstrated is slightly modified from that described by Josef Loidl and Franz Klein^1,2. The method has been used to characterize the localization of many chromatin-bound proteins at various stages of the mitotic cell cycle, but is especially useful for the study of meiotic chromosome structures such as meiotic recombinosomes and the synaptonemal complex. We also describe a modification that does not require use of Lipsol, a proprietary detergent, which was called for in the original procedure, but no longer commercially available. An immunostaining protocol that is compatible with the chromosome spreading method is also described.     

DO MALES MATTER? TESTING THE EFFECTS OF MALE GENETIC BACKGROUND ON FEMALE MEIOTIC CROSSOVER RATES IN DROSOPHILA MELANOGASTER

Meiotic recombination is a critical genetic process as well as a pivotal evolutionary force. Rates of crossing over are highly variable within and between species, due to both genetic and environmental factors. Early studies in Drosophila implicated female genetic background as a major determinant of crossover rate and recent work has highlighted male genetic background as a possible mediator as well. Our study employed classical genetics to address how female and male genetic backgrounds individually and jointly affect crossover rates. We measured rates of crossing over in a 33 cM region of the Drosophila melanogaster X chromosome using a two‐step crossing scheme exploiting visible markers. In total, we measured crossover rates of 10 inbred lines in a full diallel cross. Our experimental design facilitates measuring the contributions of female genetic background, male genetic background, and female by male genetic background interaction effects on rates of crossing over in females. Our results indicate that although female genetic background significantly affects female meiotic crossover rates in Drosophila, male genetic background and the interaction of female and male genetic backgrounds have no significant effect. These findings thus suggest that male‐mediated effects are unlikely to contribute greatly to variation in recombination rates in natural populations of Drosophila.     

Increased Meiotic Crossovers and Reduced Genome Stability in Absence of Schizosaccharomyces pombe Rad16 (XPF)

Schizosaccharomyces pombe Rad16 is the ortholog of the XPF structure-specific endonuclease, which is required for nucleotide excision repair and implicated in the single strand annealing mechanism of recombination. We show that Rad16 is important for proper completion of meiosis. In its absence, cells suffer reduced spore viability and abnormal chromosome segregation with evidence for fragmentation. Recombination between homologous chromosomes is increased, while recombination within sister chromatids is reduced, suggesting that Rad16 is not required for typical homolog crossovers but influences the balance of recombination between the homolog and the sister. In vegetative cells, rad16 mutants show evidence for genome instability. Similar phenotypes are associated with mutants affecting Rhp14^XPA but are independent of other nucleotide excision repair proteins such as Rad13^XPG. Thus, the XPF/XPA module of the nucleotide excision repair pathway is incorporated into multiple aspects of genome maintenance even in the absence of external DNA damage.     

A Critical Component of Meiotic Drive in Neurospora Is Located Near a Chromosome Rearrangement

Neurospora fungi harbor a group of meiotic drive elements known as Spore killers (Sk). Spore killer-2 (Sk-2) and Spore killer-3 (Sk-3) are two Sk elements that map to a region of suppressed recombination. Although this recombination block is limited to crosses between Sk and Sk-sensitive (Sk^S) strains, its existence has hindered Sk characterization. Here we report the circumvention of this obstacle by combining a classical genetic screen with next-generation sequencing technology and three-point crossing assays. This approach has allowed us to identify a novel locus called rfk-1, mutation of which disrupts spore killing by Sk-2. We have mapped rfk-1 to a 45-kb region near the right border of the Sk-2 element, a location that also harbors an 11-kb insertion (Sk-2^INS1) and part of a >220-kb inversion (Sk-2^INV1). These are the first two chromosome rearrangements to be formally identified in a Neurospora Sk element, providing evidence that they are at least partially responsible for Sk-based recombination suppression. Additionally, the proximity of these chromosome rearrangements to rfk-1 (a critical component of the spore-killing mechanism) suggests that they have played a key role in the evolution of meiotic drive in Neurospora.     

Conserved Meiotic Genes Point to Sex in the Choanoflagellates

ABSTRACT. The choanoflagellates are a widespread group of heterotrophic aquatic nanoflagellates, which have recently been confirmed as the sister-group to Metazoa. Asexual reproduction is the only mode of cell division that has been observed within the group; at present the range of reproductive modes, as well as the ploidy level, within choanoflagellates are unknown. The recent discovery of long terminal repeat retrotransposons within the genome of Monosiga brevicollis suggests that this species also has sexual stages in its life cycle because asexual organisms cannot tolerate retrotransposons due to the rapid accumulation of deleterious mutations caused by their transposition. We screened the M. brevicollis genome for known eukaryotic meiotic genes, using a recently established "meiosis detection toolkit" of 19 genes. Eighteen of these genes were identified, none of which appears to be a pseudogene. Four of the genes were also identified in expressed sequence tag data from the distantly related Monosiga ovata . The presence of these meiosis-specific genes provides evidence for meiosis, and by implication sex, within this important group of protists.     

Malformations of the genitalia in male Phlebotomus papatasi (Scopoli) (Diptera: Psychodidae)

Phlebotomus papatasi ( Scopoli, 1786 ) (Diptera: Psychodidae) is a major vector of Leishmania major (Kinetoplastida: Trypanosomatidae), a causative agent of zoonotic cutaneous leishmaniasis. Morphological characters of sand fly genitalia are key indicators for species identification. Various anomalies affecting male genitalia have been previously described. We take advantage of a large sand flies survey conducted in 32 stations in Central and Southern Morocco to systematically quantify the prevalence and spatial distribution of malformations affecting the genitalia of P. papatasi. Among 597 examined males, 122 were abnormal (20.4%). Malformations were widespread and largely concerned the number of spines in the lateral lobes and in the styles. Asymmetrical anomalies in lateral lobes were common. Correspondence analysis of our results highlighted the symmetrical anomalies observed in the lateral lobes, and abnormal styles of the male genitalia were found to be associated with environmental disturbances since they were prevalent in sewage dumps.