The inter-relationship between heterochromatin distribution and chiasma distribution

Centromere-proximal suppression of meiotic crossovers in Drosophila is robust to changes in centromere number, repetitive DNA content, and centromere-clustering

Accurate segregation of homologous chromosomes during meiosis depends on both the presence and the regulated placement of crossovers (COs). The centromere effect, or CO exclusion in pericentromeric regions of the chromosome, is a meiotic CO patterning phenomenon that helps prevent nondisjunction, thereby protecting against chromosomal disorders and other meiotic defects. Despite being identified nearly a century ago, the mechanisms behind this fundamental cellular process remain unknown, with most studies of the Drosophila centromere effect focusing on local influences of the centromere and pericentric heterochromatin. In this study, we sought to investigate whether dosage changes in centromere number and repetitive DNA content affect the strength of the centromere effect, using phenotypic recombination mapping. Additionally, we studied the effects of repetitive DNA function on centromere effect strength using satellite DNA-binding protein mutants displaying defective centromere-clustering in meiotic nuclei. Despite what previous studies suggest, our results show that the Drosophila centromere effect is robust to changes in centromere number, repetitive DNA content, as well as repetitive DNA function. Our study suggests that the centromere effect is unlikely to be spatially controlled, providing novel insight into the mechanisms behind the Drosophila centromere effect.

Centromere-Proximal Suppression of Meiotic Crossovers in Drosophila is Robust to Changes in Centromere Number and Repetitive DNA Content

Accurate segregation of homologous chromosomes during meiosis depends on both the presence and regulated placement of crossovers (COs). The centromere effect (CE), or CO exclusion in pericentromeric regions of the chromosome, is a meiotic CO patterning phenomenon that helps prevent nondisjunction (NDJ), thereby protecting against chromosomal disorders and other meiotic defects. Despite being identified nearly a century ago, the mechanisms behind this fundamental cellular process remain unknown, with most studies of the Drosophila CE focusing on local influences of the centromere and pericentric heterochromatin. In this study, we sought to investigate whether dosage changes in centromere number and repetitive DNA content affect the strength of the CE, using phenotypic recombination mapping. Additionally, we also studied the effects of repetitive DNA function on CE strength using satellite-DNA binding protein mutants shown to have defective centromere clustering. Despite what previous studies suggest, our results show that the Drosophila CE is robust to dosage changes in centromere number and repetitive DNA content, and potentially also to repetitive DNA function. Our study suggests that the CE is unlikely to be spatially controlled, providing novel insight into the mechanisms behind the Drosophila centromere effect.

Standing chromosomal variation in Lake Whitefish species pairs: the role of historical contingency and relevance for speciation

The role of chromosome changes in speciation remains a debated topic, although demographic conditions associated with divergence should promote their appearance. We tested a potential relationship between chromosome changes and speciation by studying two Lake Whitefish (Coregonus clupeaformis) lineages that recently colonized postglacial lakes following allopatry. A dwarf limnetic species evolved repeatedly from the normal benthic species, becoming reproductively isolated. Lake Whitefish hybrids experience mitotic and meiotic instability, which may result from structurally divergent chromosomes. Motivated by this observation, we test the hypothesis that chromosome organization differs between Lake Whitefish species pairs using cytogenetics. While chromosome and fundamental numbers are conserved between the species (2n = 80, NF = 98), we observe extensive polymorphism of subtle karyotype traits. We describe intrachromosomal differences associated with heterochromatin and repetitive DNA, and test for parallelism among three sympatric species pairs. Multivariate analyses support the hypothesis that differentiation at the level of subchromosomal markers mostly appeared during allopatry. Yet we find no evidence for parallelism between species pairs among lakes, consistent with colonization effect or postcolonization differentiation. The reported intrachromosomal polymorphisms do not appear to play a central role in driving adaptive divergence between normal and dwarf Lake Whitefish. We discuss how chromosomal differentiation in the Lake Whitefish system may contribute to the destabilization of mitotic and meiotic chromosome segregation in hybrids, as documented previously. The chromosome structures detected here are still difficult to sequence and assemble, demonstrating the value of cytogenetics as a complementary approach to understand the genomic bases of speciation.

Contrasting the Chromosomal Organization of Repetitive DNAs in Two Gryllidae Crickets with Highly Divergent Karyotypes

A large percentage of eukaryotic genomes consist of repetitive DNA that plays an important role in the organization, size and evolution. In the case of crickets, chromosomal variability has been found using classical cytogenetics, but almost no information concerning the organization of their repetitive DNAs is available. To better understand the chromosomal organization and diversification of repetitive DNAs in crickets, we studied the chromosomes of two Gryllidae species with highly divergent karyotypes, i.e., 2n(♂) = 29,X0 (Gryllus assimilis) and 2n = 9, neo-X1X2Y (Eneoptera surinamensis). The analyses were performed using classical cytogenetic techniques, repetitive DNA mapping and genome-size estimation. Conserved characteristics were observed, such as the occurrence of a small number of clusters of rDNAs and U snDNAs, in contrast to the multiple clusters/dispersal of the H3 histone genes. The positions of U2 snDNA and 18S rDNA are also conserved, being intermingled within the largest autosome. The distribution and base-pair composition of the heterochromatin and repetitive DNA pools of these organisms differed, suggesting reorganization. Although the microsatellite arrays had a similar distribution pattern, being dispersed along entire chromosomes, as has been observed in some grasshopper species, a band-like pattern was also observed in the E. surinamensis chromosomes, putatively due to their amplification and clustering. In addition to these differences, the genome of E. surinamensis is approximately 2.5 times larger than that of G. assimilis, which we hypothesize is due to the amplification of repetitive DNAs. Finally, we discuss the possible involvement of repetitive DNAs in the differentiation of the neo-sex chromosomes of E. surinamensis, as has been reported in other eukaryotic groups. This study provided an opportunity to explore the evolutionary dynamics of repetitive DNAs in two non-model species and will contribute to the understanding of chromosomal evolution in a group about which little chromosomal and genomic information is known.

Repetitive DNA chromosomal organization in the cricket Cycloptiloides americanus: a case of the unusual X1X 20 sex chromosome system in Orthoptera

A common placement for most sex chromosomes that is involved in their evolutionary histories is the accumulation of distinct classes of repetitive DNAs. Here, with the aim of understanding the poorly studied repetitive DNA organization in crickets and its possible role in sex chromosome differentiation, we characterized the chromosomes of the cricket species Cycloptiloides americanus, a species with the remarkable presence of the unusual sex chromosome system X1X20♂/X1X1X2X2♀. For these proposes, we used C-banding and mapping through the fluorescence in situ hybridization of some repetitive DNAs. The C-banding and distribution of highly and moderately repetitive DNAs (C 0t-1 DNA) varied depending of the chromosome. The greater accumulation of repetitive DNAs in the X2 chromosome was evidenced. The microsatellites were spread along entire chromosomes, but (AG)10 and (TAA)10 were less enriched, mainly in the centromeric areas. Among the multigene families, the 18S rDNA was spread throughout almost all of the chromosomes, except for pair 5 and X2, while the U2 snDNA was placed exclusively in the largest chromosome. Finally, the 5S rDNA was exclusively located in the short arms of the sex chromosomes. The obtained data reinforce the importance of chromosomal dissociation and inversion as a primary evolutionary mechanism to generate neo-sex chromosomes in the species studied, followed by the repetitive DNAs accumulation. Moreover the exclusive placement of 5S rDNA in the sex chromosomes suggests the involvement of this sequence in sex chromosome recognition throughout meiosis and, consequently, their maintenance, in addition to their avoiding degeneration.

Chromatin organization and remodeling of interstitial telomeric sites during meiosis in the Mongolian gerbil (Meriones unguiculatus)

Telomeric DNA repeats are key features of chromosomes that allow the maintenance of integrity and stability in the telomeres. However, interstitial telomere sites (ITSs) can also be found along the chromosomes, especially near the centromere, where they may appear following chromosomal rearrangements like Robertsonian translocations. There is no defined role for ITSs, but they are linked to DNA damage-prone sites. We were interested in studying the structural organization of ITSs during meiosis, a kind of cell division in which programmed DNA damage events and noticeable chromatin reorganizations occur. Here we describe the presence of highly amplified ITSs in the pericentromeric region of Mongolian gerbil (Meriones unguiculatus) chromosomes. During meiosis, ITSs show a different chromatin conformation than DNA repeats at telomeres, appearing more extended and accumulating heterochromatin markers. Interestingly, ITSs also recruit the telomeric proteins RAP1 and TRF1, but in a stage-dependent manner, appearing mainly at late prophase I stages. We did not find a specific accumulation of DNA repair factors to the ITSs, such as γH2AX or RAD51 at these stages, but we could detect the presence of MLH1, a marker for reciprocal recombination. However, contrary to previous reports, we did not find a specific accumulation of crossovers at ITSs. Intriguingly, some centromeric regions of metacentric chromosomes may bind the nuclear envelope through the association to SUN1 protein, a feature usually performed by telomeres. Therefore, ITSs present a particular and dynamic chromatin configuration in meiosis, which could be involved in maintaining their genetic stability, but they additionally retain some features of distal telomeres, provided by their capability to associate to telomere-binding proteins.

Tracking the evolution of sex chromosome systems in Melanoplinae grasshoppers through chromosomal mapping of repetitive DNA sequences

BACKGROUND

The accumulation of repetitive DNA during sex chromosome differentiation is a common feature of many eukaryotes and becomes more evident after recombination has been restricted or abolished. The accumulated repetitive sequences include multigene families, microsatellites, satellite DNAs and mobile elements, all of which are important for the structural remodeling of heterochromatin. In grasshoppers, derived sex chromosome systems, such as neo-XY♂/XX♀ and neo-X1X2Y♂/X1X1X2X2♀, are frequently observed in the Melanoplinae subfamily. However, no studies concerning the evolution of sex chromosomes in Melanoplinae have addressed the role of the repetitive DNA sequences. To further investigate the evolution of sex chromosomes in grasshoppers, we used classical cytogenetic and FISH analyses to examine the repetitive DNA sequences in six phylogenetically related Melanoplinae species with X0♂/XX♀, neo-XY♂/XX♀ and neo-X1X2Y♂/X1X1X2X2♀ sex chromosome systems.

RESULTS

Our data indicate a non-spreading of heterochromatic blocks and pool of repetitive DNAs (C0t-1 DNA) in the sex chromosomes; however, the spreading of multigene families among the neo-sex chromosomes of Eurotettix and Dichromatos was remarkable, particularly for 5S rDNA. In autosomes, FISH mapping of multigene families revealed distinct patterns of chromosomal organization at the intra- and intergenomic levels.

CONCLUSIONS

These results suggest a common origin and subsequent differential accumulation of repetitive DNAs in the sex chromosomes of Dichromatos and an independent origin of the sex chromosomes of the neo-XY and neo-X1X2Y systems. Our data indicate a possible role for repetitive DNAs in the diversification of sex chromosome systems in grasshoppers.

A novel locus for soybean aphid resistance

The soybean aphid (Aphis glycines Matsumura) is an important pest on soybean [Glycine max (L.) Merr.] in North America. Aphid resistance has recently been found on plant introduction (PI) 567543C, but little is known about its genetic control. The objectives of this study were to identify the resistance genes in PI 567543C with molecular markers and validate them in a different genetic background. A mapping population of 249 F(4) derived lines from a cross between PI 567543C and a susceptible parent was investigated for aphid resistance in both the greenhouse and the field. The broad sense heritability of aphid resistance in the field trial was over 0.95. The segregation of aphid resistance in this population suggests a major gene controlling the resistance. Bulked segregant analysis with molecular markers revealed a potential genomic region. After saturating this putative region with more markers, a genetic locus was mapped in an interval between Sat_339 and Satt414 on chromosome 16 (linkage group J) using the composite interval mapping method. This locus explained the majority of the phenotypic variation ranging from 84.7% in the field trial to 90.4% in the greenhouse trial. Therefore, the aphid resistance in PI 567543C could be mainly controlled by this gene. This aphid resistance gene was mapped on a different chromosome than the other resistance genes reported previously from other resistant germplasms. This gene appears to be additive based on the aphid resistance of the heterozygous lines at this locus. Thus, a new symbol Rag3 is used to designate this gene. Moreover, Rag3 was confirmed in a validation population. This new aphid-resistance gene could be valuable in breeding aphid resistant cultivars.

Male mouse recombination maps for each autosome identified by chromosome painting

Linkage maps constructed from genetic analysis of gene order and crossover frequency provide few clues to the basis of genomewide distribution of meiotic recombination, such as chromosome structure, that influences meiotic recombination. To bridge this gap, we have generated the first cytological recombination map that identifies individual autosomes in the male mouse. We prepared meiotic chromosome (synaptonemal complex [SC]) spreads from 110 mouse spermatocytes, identified each autosome by multicolor fluorescence in situ hybridization of chromosome-specific DNA libraries, and mapped >2,000 sites of recombination along individual autosomes, using immunolocalization of MLH1, a mismatch repair protein that marks crossover sites. We show that SC length is strongly correlated with crossover frequency and distribution. Although the length of most SCs corresponds to that predicted from their mitotic chromosome length rank, several SCs are longer or shorter than expected, with corresponding increases and decreases in MLH1 frequency. Although all bivalents share certain general recombination features, such as few crossovers near the centromeres and a high rate of distal recombination, individual bivalents have unique patterns of crossover distribution along their length. In addition to SC length, other, as-yet-unidentified, factors influence crossover distribution leading to hot regions on individual chromosomes, with recombination frequencies as much as six times higher than average, as well as cold spots with no recombination. By reprobing the SC spreads with genetically mapped BACs, we demonstrate a robust strategy for integrating genetic linkage and physical contig maps with mitotic and meiotic chromosome structure.

Chromosomal rearrangements and evolution of recombination: comparison of chiasma distribution patterns in standard and robertsonian populations of the house mouse

The effects of chromosomal rearrangements on recombination rates were tested by the analysis of chiasma distribution patterns in wild house mice. Males and females of two chromosomal races from Tunisia differing by nine pairs of Robertsonian (Rb) fusions (standard all-acrocentric, 2N = 40 and 2N = 22) were studied. A significant decrease in chiasma number (CN) was observed in Rb mice compared to standard ones for both sexes. The difference in CN was due to a reduction in the number of proximal chiasmata and was associated with an overall more distal redistribution. These features were related to distance of chiasmata to the centromere, suggesting that the centromere effect was more pronounced in Rb fusions than in acrocentric chromosomes. These modifications were interpreted in terms of structural meiotic constraints, although genic factors were likely involved in patterning the observed differences between sexes within races. Thus, the change in chromosomal structure in Rb mice was associated with a generalized decrease in recombination due to a reduction in diploid number, a lower CN, and a decrease in the efficiency of recombination. The effects of such modifications on patterns of genic diversity are discussed in the light of models of evolution of recombination.

Chiasma redistribution in the presence of different sized supernumerary segments in a grasshopper: dependence on nonhomologous synapsis

Eight different sized supernumerary segments located at distal ends of the long arms of chromosomes M4, M5, M6, and S8 of the grasshopper Stenobothrus festivus were studied in males with regard to the synaptic process and chiasma distribution in the bivalents that carry them. The M4, M5, and M6 bivalents heterozygous for extra segments were always monochiasmate, in contrast to their bichiasmate condition observed in basic homozygotes. Furthermore, the presence of any of these extra segments led to chiasma redistribution in the carrier bivalents, so that such chiasmata were formed preferentially further away from the extra segment. The intensity of this effect is dependent on the size of the segment. Not all heteromorphic bivalents exhibited synaptonemal complexes with equalized axes at pachytene, but there was always a variable proportion of heterosynapsis around the distal ends of the long arms that was dependent on both the size of the segment and the size of the carrier chromosome. It is proposed that the absence of chiasmata in nonhomologous synapsed regions is responsible for the results obtained. Length measurements of the different extra segments and their carrier chromosomes between pachytene and diplotene indicated that synaptonemal complex is underrepresented in supernumerary heterochromatin.Key words: chiasma distribution, grasshopper, heterosynapsis, supernumerary segment, synaptonemal complex.

Chiasma distribution in the first bivalent of mice carrying a double insertion of homogeneously-staining regions in homo- and heterozygous states

An examination of the meiotic pattern of chromosome 1 isolated from a feral mouse population and containing a double insertion (Is) of homogeneously-staining regions (HSRs) was carried out. In a previous study is was shown that the region delineated by the proximal breakpoint of Is(HSR;1C5) 1Icg and the distal one of Is(HSR;1D)2Icg is unpaired during early pachytene and heterosynapsed at midpachytene. No synaptic disturbances were revealed in homozygotes in this study. Chiasmata number per first bivalent in heterozygous (1.87) and homozygous (1.88) males was shown to be higher than in normal ones (1.61). In normal males a single chiasma is located in the medial part of chromosome 1. In heterozygotes this segment is heterosynapsed and unavailable for recombination. This leads to a significant decrease in the frequency of bivalents bearing a single chiasma and an increase in the frequency of bivalents bearing double chiasmata located mostly at subcentromeric and subtelomeric regions of the chromosome. In homozygous males the frequency of double chiasmata is also increased, and even triple chiasmata become possible because of the increase in the physical length of the bivalents. Thus insertion of heterochromatic regions, which are inert with respect to recombination, leads to an increase in the length of the genetic map of the chromosome because of relaxation of interference restrictions.

Variation within and between nucleolar organizer regions in Australian hylid frogs (Anura) shown by 18S + 28S in-situ hybridization

Five distinct classes of secondary constriction are found in the hylid frogs from the genera Litoria and Cyclorana, each of which is defined by its C-banding pattern and morphology (King, 1980, 1987). In-situ hybridization experiments utilizing 18S + 28S copy RNA probes derived from Xenopus and Drosophila rDNA templates, were made on nine species of frogs possessing the major constriction types. Types 1, 2, 4, and 5 are confirmed as being NORs. These results also indicate that type 1 and 2 constriction types are not differentially despiralized as previously suggested, but show absolute differences in the quantity of ribosomal DNA present. This variation took two forms, deletion polymorphism and amplification polymorphism. These differences were observed between homologues within cells and between cells within individuals. Animals possessing these 'despiralized' constrictions are therefore mosaics for both deletion and amplification polymorphisms. Polymorphism frequencies vary greatly between constriction types. Some specimens have a higher level of presence/absence heterozygosity, (L. moorei, type 2, L. nannotis type 5, L. raniformis (animal A, pair 8 type 2), than do others (L. peronii, L. rothii, L. caerulea). The above species also vary markedly in the degree and frequency of amplification of the NORs. The type 4 constrictions analysed (L. coplandi, L. lesueuri and C. novaehollandiae) have a particularly low frequency of presence/absence heterozygosity, and they have fewer size heteromorphisms between homologues. The type 3 ephemeral constrictions did not hybridize to cRNA probes at any stage. In all but one of the species studied, a single pair of chromosomes possessed an NOR. However, in L. raniformis these occurred on two pairs of chromosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

A model for quantifying genetic recombination in chromosome polymorphisms due to supernumerary heterochromatic segments

Heterochromatic supernumerary segments can modify chiasma distribution in the affected bivalents. A model is developed for quantifying the genetic recombination in populations polymorphic for such segments. This paper describes this model for a monochiasmate bivalent, and its application in different populations of the grasshopper Chorthippus parallelus. The observations suggest that these polymorphic systems have evolutionary implications.

Intraspecific and interspecific variation in the sequence and abundance of highly repeated DNA among mosquitoes of the Aedes albopictus subgroup

Interspecific variation in abundance of highly repeated DNA sequences has been examined in three species of the Aedes scutellaris and three species of the A. albopictus subgroup of the A. scutellaris group. Sequences from a population of A. albopictus were hybridised with whole genomic content from other species and strains. Copy number estimates were determined by dot-blot hybridisation. Variation in sequence abundance between strains of A. albopictus was as great as between it and the other six species. Two clusters were formed by principal components analysis, each of which contains populations of A. albopictus. Copy numbers of highly repeated sequences do not correlate with genome size. The results indicate extensive sequence divergence and rapid evolution. These findings are discussed in relation to the importance of concerted evolution and natural selection in the evolution of the species group.