Homogeneously Staining Regions (HSR) in Chromosome 1 of the House Mouse: Synapsis and Recombination at Meiosis

Amplified sequences constitute a large part of mammalian genomes. A chromosome 1 containing 2 large (up to 50 Mb) homogeneously staining regions (HSRs) separated by a small inverted euchromatic region is present in many natural populations of the house mouse (Mus musculus musculus). The HSRs are composed of a long-range repeat cluster, Sp100-rs, with a repeat length of 100 kb. In order to understand the organization and function of HSRs in meiotic chromosomes, we examined synapsis and recombination in male mice hetero- and homozygous for the HSR-carrying chromosome using FISH with an HSR-specific DNA probe and immunolocalization of the key meiotic proteins. In all homozygous and heterozygous pachytene nuclei, we observed fully synapsed linear homomorphic bivalents 1 marked by the HSR FISH probe. The synaptic adjustment in the heterozygotes was bilateral: the HSR-carrying homolog was shortened and the wild-type homolog was elongated. The adjustment was reversible: desynapsis at diplotene was accompanied by elongation of the HSRs. Immunolocalization of H3K9me2/3 indicated that the HSRs in the meiotic chromosome retained the epigenetic modification typical for C-heterochromatin in somatic cells. MLH1 foci, marking mature recombination nodules, were detected in the proximal HSR band in heterozygotes and in both HSR bands of homozygotes. Unequal crossing over within the long-range repeat cluster can cause variation in size of the HSRs, which has been detected in the natural populations of the house mouse.

ASY1 acts as a dosage-dependent antagonist of telomere-led recombination and mediates crossover interference inArabidopsis

During meiosis, interhomolog recombination produces crossovers and noncrossovers to create genetic diversity. Meiotic recombination frequency varies at multiple scales, with high subtelomeric recombination and suppressed centromeric recombination typical in many eukaryotes. During recombination, sister chromatids are tethered as loops to a polymerized chromosome axis, which, in plants, includes the ASY1 HORMA domain protein and REC8–cohesin complexes. Using chromatin immunoprecipitation, we show an ascending telomere-to-centromere gradient of ASY1 enrichment, which correlates strongly with REC8–cohesin ChIP-seq data. We mapped crossovers genome-wide in the absence of ASY1 and observe that telomere-led recombination becomes dominant. Surprisingly,asy1/+heterozygotes also remodel crossovers toward subtelomeric regions at the expense of the pericentromeres. Telomeric recombination increases inasy1/+occur in distal regions where ASY1 and REC8 ChIP enrichment are lowest in wild type. In wild type, the majority of crossovers show interference, meaning that they are more widely spaced along the chromosomes than expected by chance. To measure interference, we analyzed double crossover distances, MLH1 foci, and fluorescent pollen tetrads. Interestingly, while crossover interference is normal inasy1/+, it is undetectable inasy1mutants, indicating that ASY1 is required to mediate crossover interference. Together, this is consistent with ASY1 antagonizing telomere-led recombination and promoting spaced crossover formation along the chromosomes via interference. These findings provide insight into the role of the meiotic axis in patterning recombination frequency within plant genomes.

Male Meiotic Recombination in the Steppe Agama, Trapelus sanguinolentus (Agamidae, Iguania, Reptilia)

Meiotic recombination rates and patterns of crossover distributions along the chromosomes vary considerably even between closely related species. The adaptive significance of these differences is still unclear due to the paucity of empirical data. Most data on recombination come from mammalian species, while other vertebrate clades are poorly explored. Using immunolocalization of the protein of the lateral element of the synaptonemal complex (SYCP3) and the mismatch-repair protein MLH1, which marks mature recombination nodules, we analyzed recombination rates and crossover distribution in meiotic prophase chromosomes of the steppe agama (Trapelus sanguinolentus, Agamidae, Acrodonta, Iguania) and compared them with data obtained for the genus Anolis (Dactyloidae, Pleurodonta, Iguania). We found that, despite a smaller genome size, the total SC length and the MLH1 focus number per cell are much higher in the agama than in the anoles. The distributions of the MLH1 foci in the agama are multimodal in larger chromosomes and bimodal in smaller chromosomes without a significant centromere effect, resembling the patterns known for birds. A possible relationship between karyotype remodeling and the evolution of recombination in Iguania is discussed.

Immunocytological Analysis of Meiotic Recombination in the Gray Goose (Anser anser)

Studies on mammals demonstrate wide interspecific variation in the number and distribution of recombination events along chromosomes. Birds represent an interesting model group for comparative analysis of cytological and ecological drivers of recombination rate evolution. Yet, data on variation in recombination rates in birds are limited to a dozen of species. In this study, we used immunolocalization of MLH1, a mismatch repair protein marking mature recombination nodules, to estimate the overall recombination rate and distribution of crossovers along macrochromosomes in female and male meiosis of the gray goose (Anser anser). The average number of MLH1 foci was significantly higher in oocytes than in spermatocytes (73.6 ± 7.8 and 58.9 ± 7.6, respectively). MLH1 foci distribution along individual macrobivalents showed subtelomeric peaks, which were more pronounced in males. Analysis of distances between neighboring MLH1 foci on macrobivalents revealed stronger crossover interference in male meiosis. These data create a framework for future genetic and physical mapping of the gray goose.

Meiotic Recombination in the Giraffe (G. reticulata)

Recently, the reticulated giraffe (G. reticulata) was identified as a distinct species, which emphasized the need for intensive research in this interesting animal. To shed light on the meiotic process as a source of biodiversity, we analysed the frequency and distribution of meiotic recombination in 2 reticulated giraffe males. We used immunofluorescence detection of synaptonemal complex protein (SYCP3), meiotic double strand breaks (DSB, marked as RAD51 foci) in leptonema, and crossovers (COs, as MLH1 foci) in pachynema. The mean number of autosomal MLH1 foci per cell (27), which resulted from a single, distally located MLH1 focus observed on most chromosome arms, is one of the lowest among mammalian species analysed so far. The CO/DSB conversion ratio was 0.32. The pseudoautosomal region was localised in the Xq and Yp termini by FISH and showed an MLH1 focus in 83% of the pachytene cells. Chromatin structures corresponding to the nucleolus organiser regions were observed in the pachytene spermatocytes. The results are discussed in the context of known data on meiosis in Cetartiodactyla, depicting that the variation in CO frequency among species of this taxonomic group is mostly associated with their diploid chromosome number.

Variation of Meiotic Recombination Rates and MLH1 Foci Distribution in Spermatocytes of Cattle, Sheep and Goats

Despite similar genome sizes, a great variability in recombination rates is observed in mammals. We used antibodies against SYCP3, MLH1 and centromeres to compare crossover frequency, position along chromosome arms and the effect of crossover interference in spermatocytes of 4 species from the family Bovidae (Bos taurus, 2n = 60, tribe Bovini; Ovis aries, 2n = 54, Capra hircus, 2n = 60 and Ammotragus lervia, 2n = 58, tribe Caprini). Despite significant individual variability, our results also show significant differences in both recombination rates and the total length of autosomal synaptonemal complexes (SC) between cattle (47.53 MLH1 foci/cell, 244.59 µm) and members of the tribe Caprini (61.83 MLH1 foci, 296.19 µm) which can be explained by the length of time that has passed since their evolutionary divergence. Sheep displayed the highest number of MLH1 foci per cell and recombination density, although they have a lower diploid chromosome number caused by centric fusions corresponding to cattle chromosomes 1;3, 2;8 and 5;11. However, the proportion of MLH1 foci observed on the fused chromosomes in sheep (26.14%) was significantly lower than on the orthologous acrocentrics in cattle (27.6%) and goats (28.2%), and their distribution along the SC arms differed significantly. The reduced recombination rate in metacentrics is probably caused by interference acting across the centromere.

Heterologous Synapsis and Crossover Suppression in Heterozygotes for a Pericentric Inversion in the Zebra Finch

In the zebra finch, 2 alternative morphs regarding centromere position were described for chromosome 6. This polymorphism was interpreted to be the result of a pericentric inversion, but other causes of the centromere repositioning were not ruled out. We used immunofluorescence localization to examine the distribution of MLH1 foci on synaptonemal complexes to test the prediction that pericentric inversions cause synaptic irregularities and/or crossover suppression in heterozygotes. We found complete suppression of crossing over in the region involved in the rearrangement in male and female heterozygotes. In contrast, the same region showed high levels of crossing over in homozygotes for the acrocentric form of this chromosome. No inversion loops or synaptic irregularities were detected along bivalent 6 in heterozygotes suggesting that heterologous pairing is achieved during zygotene or early pachytene. Altogether these findings strongly indicate that the polymorphic chromosome 6 originated by a pericentric inversion. Since inversions are common rearrangements in karyotypic evolution in birds, it seems likely that early heterologous pairing could help to fix these rearrangements, preventing crossing overs in heterozygotes and their deleterious effects on fertility.