Frequency Of Chiasmata Research Articles

Cytogenomic characterization of Euschistus (Heteroptera: Pentatomidae) species and strains reveals low chromosomal and repetitive DNAs divergences

Abstract Euschistus stink bugs are important pests on soybean crops, including the Neotropical representatives Euschistus heros, Euschistus crenator, and Euschistus taurulus. Despite their importance, little genomic and chromosomal information is available. Genomic and chromosomal differences can play an important role in establishing reproductive barriers between species and populations. Here we investigate the chromosomes, some repetitive DNAs, and genome sizes of three Euschistus species, including two E. heros strains (North strain-NS and South strain-SS), to address chromosomal evolution and genomic differentiation. Our data reveal conservative karyotypes and only one possible inversion among Euschistus species. Moreover, we observe Y chromosome reorganization through differential microsatellite accumulation. The nuclear genome sizes are slightly variable among species. We noticed a differential accumulation of satellite DNAs, mainly on the Y chromosome, in the two strains of E. heros. Although typical meiotic behaviour, demonstrating full compatibility, was observed in hybrids despite interference on chiasmata frequency. This data shows that chromosomal and repetitive DNAs do not prevent secondary contact between E. heros strains, contributing to genetic variability in hybrids. Therefore, we provide data about Euschistus pests and their chromosomal characteristics, representing the first step in understanding their genomic organization and evolution.

MUS81 is required for atypical recombination intermediate resolution but not crossover designation in rice.

The endonuclease methyl methanesulfonate and UV-sensitive protein 81 (MUS81) has been reported to participate in DNA repair during mitosis and meiosis. However, the exact meiotic function of MUS81 in rice remains unclear. Here, we use a combination of physiological, cytological, and genetic approaches to provide evidence that MUS81 functions in atypical recombination intermediate resolution rather than crossover designation in rice. Cytological and genetic analysis revealed that the total chiasma numbers in mus81 mutants were indistinguishable from wild-type. The numbers of HEI10 foci (the sites of interference-sensitive crossovers) in mus81 were also similar to that of wild-type. Moreover, disruption of MUS81 in msh5 or msh4 msh5 background did not further decrease chiasmata frequency, suggesting that rice MUS81 did not function in crossover designation. Mutation of FANCM and ZEP1 could enhance recombination frequency. Unexpectedly, chromosome fragments and bridges were frequently observed in mus81 zep1 and mus81 fancm, illustrating that MUS81 may resolve atypical recombination intermediates. Taken together, our data suggest that MUS81 contributes little to crossover designation but plays a crucial role in the resolution of atypical meiotic intermediates by working together with other anti-crossover factors.

Cytological Effect of Gamma Radiation on Selected Mutants of Wheat Triticum aestivum L. in M3 Generation.

Wheat (Triticum aestivum L.) offers some unique opportunities for the induction and exploitation of agronomic value. The use of gamma radiation has been proven to be an effective method to induce genetic variation in crops. We aimed to determine genetically stable mutants of wheat which could be utilized for breeding purposes. We did a cytological investigation of induced mutant's behavior and chiasma frequency. Selected mutant types induced in dry and soaked seeds were treated with different doses of gamma rays. Each treated sample and control were subjected to cytological examination of the fixed pollen mother cells in various meiotic stages. The percentage of the total abnormal cells significantly increased in one mutant and significantly decreased in the other mutant. The percentage of total abnormal cells did not diminish from the first to the second meiotic division. The types of meiotic anomalies found included laggards (56.51%), univalent (9.43%), stickiness (45.45%) and bridges (19.32%). There were genotypic differences in the frequency of occurrence of multivalent (trivalent and quadrivalents). A marked reduction in the number of rod and ring bivalent/cell in some genotypes were noticed. The frequency of chiasmata per pollen mother cell was reduced subsequently. Depression index of mutants was negative compared with controls or treatments except for a few genotypes. Selected mutants of wheat tend to be cytologically stable and can therefore, be utilized for breeding purposes.

Variability of Chiasma Frequencies in Different Tomato Species

This article presents the results of comparative studies of the frequency and distribution of chiasmata in pollen mother cells (PMCs) in five diploid tomato species, Solanum lycopersicum, S. pimpinellifolium, S. peruvianum, S. habrochaites, and S. neorickii, and one autotetraploid species, S. pimpinellifolium. It was established that under the same growing conditions, the total chiasma frequency in the cell depended on the species. At the same time, the green-fruited species S. peruvianum, S. neorickii, and S. habrochaites differed in distal chiasma frequency, while the red-fruited species S. lycopersicum and S. pimpinellifolium differed in interstitial chiasma frequency. It was shown that the total chiasma frequency in PMCs of plants of one species is a stable index of recombination potential that does not depend on the growing conditions. The redistribution between distal and interstitial chiasmata was found to be more variable, depending on the species, year, geographic growth conditions. In autotetraploid, the chiasma frequency per bivalent was lower than that in diploid S. pimpinellifolium plants, primarily due to interstitial chiasmata, the frequency of which remained at the level characteristic for diploid plants. It was concluded that the recombination plasticity of the tomato genomes was due to the redistribution of chiasmata along bivalents, and not to the change in their number in the cell.

Interpopulation variability in Artemisia glauca Pall. ex Willd.- studies on three distinct cytotypes

Three wild populations of Artemisia glauca Pall. ex Willd. occurring in the Pir Panjal mountain ranges and in the Kishtwar Himalayas of Jammu and Kashmir State, India, have been analysed for their meiotic systems and reproductive output. Of the three, two namely Poonch and Rajouri populations have a chromosome count of 2n = 18, while the third population named as Kishtwar population has 2n = 36. These are in all probability diploid and tetraploid cytotypes. While the tetraploid Kishtwar population shows stunted growth, diploid ones show little variation at the morphological level but display prominent interpopulation differences in detail of chromosome size and chiasmata frequency. Meiotically Poonch population is more stable than Rajouri. The differences in ploidy level and chromosome constitution affect the reproductive output in these species. Although all of them show high degree of inbreeding depression (revealed by total absence of healthy seeds on forced selfing), seed and fruit set on open pollination also is extremely low. Of the three, Poonch population sets the highest percentage of healthy seeds.

Chromosome fusion polymorphisms in the grasshopper, Dichroplus fuscus (Orthoptera: Acrididae: Melanoplinae): Insights on meiotic effects

Robertsonian fusions account for many of the changes in the evolution of the orthopteran karyotype; in their origin, a centric fusion is involved between two acro-telocentric chromosomes, forming a single bi-armed chromosome. It is usual for these rearrange- ments to be associated with profound changes in meiosis, such as modification in frequency and distribution of chiasmata. Dichroplus fuscus is a South American grasshopper with a wide distribution. In this work we analyzed nine populations from Misiones Province, north-eastern Argentina. This species presents a standard karyotype of 2n = 23/24 (♂/♀) with all chromosomes acro-telocentric and an X0/XX chromosomal sex determining mechanism. This standard karyotype has been modified by the occurrence of two Robertso- nian fusions involving chromosomes 1/3 and 2/4; values of fusions per individual (fpi) show a significant increase in the presence of karyotypic polymorphisms towards southern populations. In individuals showing chromosomal rearrangements, we observed a clear redistribution of chiasmata towards distal positions; significant differences were noted between Robertsonian homozygotes (Ho) and heterozygotes (Ht) for chromosomes arms L 1 and M 3 , although this was not the case between Ho and Ht for chromosome arms L 2 and M 4 . With regard to the orientation of trivalents, values obtained for non-convergent orientation were low.

Male Horse Meiosis: Metaphase I Chromosome Configuration and Chiasmata Distribution

Chromosome configurations and chiasma frequency during the metaphase I stage of spermatogenesis in the male horse are characterized in this work. The genome-wide frequency and distribution of chiasmata was detected as 49.45 ± 2.07 for 14 fertile stallions. All X and Y chromosomes shared a single chiasma at their pseudoautosomal region, while 1-4 chiasmata were observed in autosomal chromosomes. The chiasma frequency and distribution were further studied for 8 different bivalents identified by FISH in 5 fertile stallions. Genetic length was calculated from chiasmata data for the whole genome as well as for these 8 chromosomes. The findings complement the genetic linkage data and provide insight into the genetic basis of spermatogenesis in normal stallions.

Karyotype evolution in progress: A new diploid number in Belostoma candidulum (Heteroptera: Belostomatidae) from Argentina leading to new insights into its ecology and evolution

A novel chromosome complement (2n = 14 = 12 + XY/XX; male/female sex chromosomes), male meiosis behaviour, het­ erochromatin characterization, and frequency and distribution of chiasmata are described for the first time in specimens from a natural population of the giant water bug, Belostoma candidulum Montandon, 1903 (Heteroptera: Belostomatidae) from Argentina. To date, specimens of B. candidulum have been reported by other authors in a sample from a natural population from Brazil. Our results dem­ onstrate that Argentinean and Brazilian populations have different diploid numbers and chromosomal features. During male meiosis, autosomal bivalents generally show a single chiasma, behave as telokinetic chromosomes (i.e. kinetic activity is restricted to terminal regions), and divide reductionally at anaphase I; in contrast, the sex chromosomes are achiasmatic, behave as univalents and segregate equationally at anaphase I. Among autosomal bivalents of B. candidulum, one is remarkably larger and may present one or two termi­ nal chiasmata, showing rod, V-shaped and ring configurations. Here we propose a new mode of segregation for ring bivalents, since it is not essential that one of the chiasmata is released during anaphase I because alternative sites for microtubule attachment become functional for the normal chromosome segregation to the poles. Heterochromatin content is very scarce in specimens from Argentinean B. candidulum populations, revealing C ­ positive interstitial and terminal dots in three pairs of autosomes and C ­ blocks at both ends of X chromosome, whereas the Y chromosome is mainly C ­ positive. One of the C ­ positive bands from X and Y chromosomes is DAPI ­ dull/CMA-bright, which could represent the nucleolus organizing region (NOR) detected by fluorescent in situ hybridization (FISH). The location of the NORs in both sex chromosomes allowed us to use them as a cytological marker to describe their behaviour during meiosis. Despite the fact that specimens from the Argentinean and Brazilian populations have been classified as a single species due to their morphological similarity, our results suggest that both populations are chromosomal races or even morphologically ­ identical cryptic species. The results obtained support the hypothesis that karyotype of B. candidulum originated through autosomal fusions and the fusion of the X and Y chromosomes with the ancestral NOR ­ autosomal pair. Lastly, the genus Belostoma represents an excellent model for assessing the main mechanisms involved in the karyotype evolution in organisms with holokinetic chromosomes, from which inferences may be made concerning its broader ecology and evolution.

Distribution of MLH1 Foci in Horse Male Synaptonemal Complex

Advances in molecular cytogenetics have provided the opportunity to study events during prophase I of meiosis. Immunofluorescent localization of different meiotic protein components were used to characterize the early stages of the first meiotic division in horse spermatocytes. The frequency and distribution of recombination events during prophase I were investigated using the mutL homolog 1 (MLH1) protein that is known to be associated with these events. The frequency and distribution of MLH1 foci were investigated in pachytene nuclei of 6 fertile stallions, and the average relative synaptonemal complex length was found to be highly correlated with the average number of MLH1 foci. The frequency and distribution of MLH1 foci were found to closely correspond to the frequency and distribution of chiasmata on metaphase I chromosomes, and genetic length, calculated from MLH1 foci data, for the whole genome was 2,505.5 cM.

Identifying crossover-rich regions and their effect on meiotic homologous interactions by partitioning chromosome arms of wheat and rye

Chiasmata are usually formed in the distal half of cereal chromosomes. Previous studies showed that the crossover-rich region displays a more active role in homologous recognition at early meiosis than crossover-poor regions in the long arm of rye chromosome 1R, but not in the long arm of chromosome 5R. In order to determine what happens in other chromosomes of rye and wheat, we have partitioned, by wheat-rye translocations of variable-size, the distal fourth part of chromosome arms 1BS and 2BL of wheat and 1RS and 2RL of rye. Synapsis and chiasma formation in chromosome pairs with homologous (wheat-wheat or rye-rye) and homoeologous (wheat-rye) stretches, positioned distally and proximally, respectively, or vice versa, have been studied by rye chromatin labelling using fluorescence in situ hybridisation. Chromosome arm partitioning showed that the distal 12 % of 1BS form one crossover in 50 % of the cells, while the distal 6.7 % of 2RL and the distal 10.5 % of 2BL account for 94 % and 81 % of chiasmata formed in these arms. Distal homoeologous segments reduce the frequency of chiasmata and the possibility of interaction between the intercalary/proximal homologous segments. Such a reduction is related to the size of the homoeologous (translocated) segment. The effect on synapsis and chiasma formation was much lower in chromosome constructions with distal homology and proximal homoeology. All of these data support that among wheat and rye chromosomes, recombining regions are more often involved in homologous recognition and pairing than crossover-poor regions.

Cytotoxic action of lead nitrate on cytomorphology of Trigonella foenum-graecum L.

The present experiment was undertaken to determine the mutagenic potential of lead on the cytomorphology of Trigonella foenum-graecum L. and to determine the maximum concentration of lead nitrate that induces maximum genetic variability. The seeds were treated with 6 different concentrations (10, 25, 50, 100, 200, and 400 ppm) of lead nitrate. The higher concentrations of lead nitrate significantly reduced plant height, number of branches per plant, pollen fertility, and yield; higher concentrations also caused variation in the plants at the seedling and mature stages. In addition, concentrations affected the pairing of homologous chromosomes and spindle formation. This resulted in laggards, bridges, fragments, multipolar orientation of chromosomes, and reduced chiasmata frequency. On the basis of these results, it was concluded that lower concentrations of metal did not significantly affect the cytomorphology of Trigonella, while higher concentrations of lead nitrate were found to be more mutagenic and cytotoxic.

Seasonal variation in chiasma frequency among three morphotypes of Eclipta alba

Natural variation in recombination frequency in Eclipta alba has been assessed by analyzing chiasma frequency variation among three morphologically diverse morphotypes i.e. erect, intermediate and prostrate. The study revealed that the chiasma frequency varied between 18.8-21.4, 18.9-20.9 and 19.3-21.7 in the three morphotypes, respectively. It is found that the chiasmata frequency for prostrate type remain higher throughout the year in comparison to intermediate and erect types. There was no significant variation in mean chiasma frequency between plants within accessions, but there was significant variation between accessions. Presently observed the cyclic variation in the average chiasma frequencies, recombination index and excess chiasma in the three morphotypes of Eclipta alba clearly depict the influence of environmental factors on the process of recombination.

Selection in tetraploid rye

A comparative study of two populations selected for both seed set and meiosis regularity, and an unselected population, showed variable relationships in euploid plants between chiasma frequency and the frequencies of metaphase-I configurations. In the population selected for high seed set and meiosis regularity, quadrivalent frequency had a significant positive regression on the frequency of chiasmata. In the unselected population an increase in chiasma frequency was associated with a significant increase in bivalent frequency. In the population selected for low seed set and irregular meiosis the regression, based on individual plant means, of chiasma frequency on quadrivalents was negative and on bivalents was positive and significant. The same regressions, when based on cell population means showed the opposite relationships. That is the chiasma frequency had a significant positive regression on quadrivalent frequency and a negative regression on bivalent frequency. These results are discussed in relation to chromosomal differentiation within the sets of four homologous chromosomes of an autotetraploid.

Metaphase I orientation of Robertsonian trivalents in the water-hyacinth grasshopper, Cornops aquaticum (Acrididae, Orthoptera)

Trivalents resulting from polymorphic Robertsonian rearrangements must have a regular orientation in metaphase I if the polymorphisms are to be maintained. It has been argued that redistribution of proximal and interstitial chiasmata to more distal positions is necessary for a convergent orientation, the only one that produces viable gametes. Cornops aquaticum is a South-American grasshopper that lives and feeds on water-hyacinths, and has three polymorphic Robertsonian rearrangements in its southernmost distribution area in Central Argentina and Uruguay. The orientation of trivalents in metaphase I, the formation of abnormal spermatids and the frequency and position of chiasmata in the trivalents, was analysed in a polymorphic population of C. aquaticus. In this study we observed a correlation between the number of trivalents with the frequency of abnormal spermatids; additionally, the number of chiasmata, especially proximal and interstitial ones, was strongly correlated with the frequency of the linear orientation. Therefore we confirmed our previous assumption, based on other evidence, that the chiasmata redistribution in fusion carriers is essential to the maintenance of the polymorphisms.

Freqüência, distribuição e terminalização de quiasmas em aveias hexaplóides (Avena sativa L.)

Chiasma counting has been regarded as the most straightforward method of scoring the total number of crossing-over events in the genome. In organisms in which genetic analysis is difficult or impossible to perform, it is a good estimator of the level of genetic recombination. In this paper we evaluated the chiasma frequency and distribution at diakinesis of twelve Brazilian oat varieties ( Avena sativa ), a hexaploid species with cytologically well-defined meiotic chromosomes. There was a high homogeneity of chiasma formation among varieties. The chiasma frequency per microsporocyte ranged from 41.66 to 42.00 among varieties. The frequency of interstitial and terminal chiasmata was more variable. The bivalents may present one, two or three chiasmata, but those with two terminal chiasmata were more frequent. The frequency of univalents was very low at diakinesis but high in metaphase I, suggesting precocious chiasma terminalization. The implications of these findings for oat breeding are discussed

Meiotic behaviour in reciprocal tetraploid hybrids between perennial and Italian ryegrasses

Chromosome associations and chiasma frequencies at first metaphase (MI) were studied in allotetraploid reciprocal hybrids between perennial ryegrass (Lolium perenne, Lp) and Italian ryegrass (L. multiflorum, Lm), and parent autotetraploid perennial ryegrass. Preferential chromosome pairings were assessed in Lp × Lm and Lm × Lp hybrids. The reciprocal hybrids were found to have significantly higher frequencies of bivalent pairing and lower frequencies of quadrivalent pairing and chiasmata. The mean frequencies of total bivalents at MI were 6.820, 9.885 and 10.040 and total quadrivalent frequencies were 3.320, 1.775 and 1.760 in the autotetraploid parent and the Lp × Lm and Lm × Lp hybrids, respectively. The frequencies of chiasmata were 25.075, 23.455 and 23.315 in the parent and the Lp × Lm and Lm × Lp hybrids, respectively. The frequencies of preferential chromosome pairing in the Lp × Lm and Lm × Lp hybrids were 42.69 and 44.85%, respectively and the results indicate a significant degree of preferential bivalent association in the hybrids. An inverse relationship was found between the frequency of total bivalent and quadrivalent associations in the parent and the Lp × Lm and Lm × Lp hybrids. These reciprocal hybrids are potentially important breeding materials for developing new hybrid cultivars.

Effects of polymorphic Robertsonian rearrangements on the frequency and distribution of chiasmata in the water-hyacinth grasshopper, Cornops aquaticum (Orthoptera: Acrididae)

The New World grasshopper Cornops aquaticum (Leptysminae: Acrididae) shows a geographical pattern for three Robert- sonian polymorphisms in its southernmost area of distribution in Argentina and Uruguay. The frequency and distribution of chias- mata were analysed in five Argentinian populations. This study reveals a strong redistribution of chiasmata in fusion carriers, with a reduction in proximal and increase of distal chiasma frequency in fusion bivalents and trivalents, when all three karyotypes were compared. However, when only fusion bivalents and trivalents were compared, chiasma frequency was significantly higher in the former than in the latter. This higher chiasma frequency in fusion bivalents is due to an increase in proximal chiasma frequency. It is argued that the reduction in proximal chiasma frequency (relative to unfused bivalents) in fusion bivalents may be due to interfer- ence across the centromere. Proximal chiasma reduction in trivalents may be attributed either to a physical effect of structural hetero- zygosity or to an adaptation to the polymorphic condition. Therefore the differences in the distribution of chiasmata in trivalents and Robertsonian bivalents have different causes.

GISH analysis of meiotic chromosome pairing inSolanum lycopersicoidesintrogression lines of cultivated tomato

Meiotic chromosome pairing was studied in introgression lines of cultivated tomato, Lycopersicon esculentum (= Solanum lycopersicum), containing 1 or 2 chromosome segments from the wild species Solanum lycopersicoides. Genomic in situ hybridization (GISH) was used to compare the relative lengths at diakinesis of the different introgressed segments and to measure the chiasmate arm frequency for the chromosome pair involved in the introgression(s). Longer segments generally produced stronger GISH signals than shorter segments. GISH signal intensity also depended on whether or not an introgressed segment encompassed the centromeric region. For example, a 29 cM segment that included the centromeric region produced a stronger GISH signal than a 42 cM segment that did not. In each line the chromosome arm containing the homeologous segment showed a reduction in chiasmate arm frequency that was most pronounced in lines with long segments. This reduction was accompanied by an increased chiasmate arm frequency on the other arm. Double introgression lines, heterozygous in repulsion phase for 2 introgressions on opposite chromosome arms, showed a lower frequency of chiasmata than single introgression lines. Pairing failure, indicated by the presence of univalents, was highest in the double introgression and whole chromosome substitution lines. These results are discussed with respect to observations of suppressed recombination in these stocks and potential practical implications for reducing linkage drag in breeding programs.

Genetic system of two rainy season weeds: Commelina benghalensis L. and Commelina caroliniana Walter

Commelina benghalensis L and C. caroliniana Walter, two allied weedy species of the Commelinaceae vary in their breeding and meiotic systems. The former is predominantly auto- and the latter facultatively xenogamous. Meiotic system of C. caroliniana also provides for higher recombination index, characterized as it is by high chromosome number and higher chiasmata frequency: On the contrary C. benghalensis has fewer chromosomes and closed breeding system for which reason it generates little genetic variability. This difference qualifies C. benghalensis as a “specialist” and C. caroliniana as a “generalist”

Chiasmata and chromosome breakages are related to crossing over in Drosophila ananassae males

A cytogenetic analysis of male crossing over in Drosophila ananassae revealed that cytological exchanges resulted in genetic crossing over, and that chiasma frequency and the genetic recombination correlated positively in chromosomes 2 and 3. Furthermore, the frequency of chromosome breakages correlated positively with chiasma frequency. Paracentric inversion heterozygosity had no detectable influence on the chromosome pairing or exchange events within the inversion loop at meiosis. Scoring of the chiasma demonstrated that males homozygous for the previously mapped enhancers of male crossing over had low frequencies of chiasmata, whereas higher frequencies of chiasmata were observed in males heterozygous for enhancers. The results presented here indicate that the genetic factors controlling male crossing over are involved in the origin of chromosome breakages and in exchange events.