Chiasma frequency effects of structural chromosome change

Three structural chromosome changes in the plant Hypochoeris radicata 2n = 8 have been tested for their effects on chiasma formation: (1) centric fission of chromosome 1, (2) a whole arm exchange between chromosomes 1 and 3, and (3) an interchange between the long arm of chromosome 1 and the short arm of 2 which gives an effectively three-armed pachytene multiple. Mean chiasma frequencies were compared between full-sibs in families segregating for the rearrangements. In each family the chiasma frequency was higher in heterozygotes than basic homozygotes. The size of the chiasma increase is dependant on the number of additional potentially-paired segments in the complement at pachytene. Fission heterozygotes and 1/2 interchange heterozygotes, with one extra pairing region, both form about 0.45 more chiasmata per PMC than full-sib basic homozygotes. The 1/3 exchange, with two additional pairing regions, increases chiasma frequency by twice this, about 0.85 per PMC. Individuals homozygous for the centric fission maintain the raised chiasma level. The chiasma increase appears limited to the chromosome(s) affected by structural change with no detectable interchromosomal effect.     

Cytological, genetic and evolutionary functions of chiasmata based on chiasma graph analysis.

The nature of the chiasma as a cytological parameter for analysing cross-over was reexamined quantitatively by an improved chiasma graph method. It was reconfirmed in Mus platythrix (n =13) that interstitial chiasmata at diakinesis are distributed randomly and almost uniformly along bivalents except for the centromere and telomere regions. The size of these chiasma blank regions was consistently 0.8% of the total length of haploid autosomes in all chromosomes. There was a minimum value of chiasma interference distance between two adjacent chiasmata, which was constantly 1.8% in all chromosomes. The chiasma frequency at diakinesis was 20.1+/-2. 0 by the conventional method including terminal chiasmata. However, the primed in situ labeling technique revealed that terminal chiasmata were mostly telomere-telomere associations. From these data and also from recent molecular data we concluded that the terminal chiasma is cytologically functional for ensuring the normal disjunction of bivalents at anaphase I, but genetically non-functional for shuffling genes. The chiasma frequency excluding terminal chiasmata was 14.6+/-1.8. Reexamination of the chiasma frequency of 106 animal species revealed that the chiasma frequency increased linearly in proportion to the haploid chromosome number in spite of remarkable difference in their genome size. The increase in chiasma frequency would be evolution-adaptive, because gene shuffling is expected to be accelerated in species with high chromosome numbers.     

Chiasmata distribution in the normal karyotype of mice

The number of chiasmata per cell and variance of chiasmata numbers were studied, as well as the recombinational interaction between different bivalents in CBA/Lac mice male line. No competition of bivalents for chiasmata was discovered in mice; at the same time, the chiasmata within one arm of the chromosome interfere with each other. The number of chiasmata per bivalent is estimated for each chromosome independently. The number of chiasmata per chromosome is limited both from below (minimum one chiasma independently of its size) and from above (positive interference of chiasmata).     

The influence of B-chromosomes on chiasma frequencies in Black Mexican Sweet Corn

The mean chiasma frequency at first metaphase of meiosis in pollen mother cells of a maize variety, Black Mexican Sweet Corn, increases with increasing number ofB-chromosomes. The increased chiasma frequency is also reflected by a decrease in the number of rod bivalents at first metaphase. There is some indication that theB-chromosomes also influence the distribution of chiasmata between pollen mother cells. The increase in chiasma frequency due toBs is closely analogous to the increase in crossing-over carlier attributed toBs between marker genes located on chromosome III. Together the evidence confirm the role ofBs in controlling genetic recombination at meiosis.     

Heterosis for chiasma frequency and quantitative traits in common beans (Phaseolus vulgaris L.)

Summary Ten genotypes, including inbreds, hybrids, and advanced populations, were examined in order to elucidate the relationship between position and frequency distribution of chiasmata and quantitative traits, including yield heterosis in common beans. The hybrid and advanced population groups were determined to possess 83% and 54% increased chiasma frequency, respectively in contrast to inbred lines. The increase in chiasma frequency of these populations was further manifested in a high number of interstitial chiasmata. The regular and superior chromosome behaviour of the hybrids was found to be positively associated with quantitative measures on bean yield, harvest index and bean yield efficiency. The results were discussed from the point of view that: a) increased interstitial chiasmata may provide an effective mechanism for maintaining genetic diversity and heterosis in hybrid populations; and b) heterosis for chiasma frequency and quantitative traits may be due to dispersed genes on the chromosomes having combined intra-and interallelic interactions. The data provide evidence for the existence of positive associations between interstitially localized chiasmata with its recombination potential and regular chromosome behaviour to bean yield heterosis. The role of enhanced interstitial chiasmata to promote higher levels of genetic variation and heterozygous advantage is discussed.     

Double insertion of homogeneously staining regions in chromosome 1 of wild Mus musculus musculus: effects on chromosome pairing and recombination

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. The region delineated by the proximal breakpoint of Is(HSR;1C5) 1Icg and the distal breakpoint of Is(HSR;1E3)2Icg is desynapsed during the early pachytene stage and heterosynapsed at the midpachytene, as shown by electron microscopic analysis of synaptonemal complexes. The HSRs have no effect on the segregation of chromosome 1 in heterozygous mice. The lack of homosynapsis in the region under study causes chiasmata redistribution in heteromorphic bivalents. In normal males, single chiasmata are located in the medial part of the chromosome. In heterozygotes, this segment is heterosynapsed and unavailable for recombination. This leads to a significant decrease in the frequency of bivalents bearing single chiasmata. The total number of chiasmata per bivalent is much higher in heterozygous males than in normal ones. The recombination frequency between proximal markers fz and In also is higher in heterozygous animals. The increase in the total chiasma number in the heteromorphic bivalent is due to the addition of double chiasmata located mostly at precentromeric and pretelomeric regions of the chromosome.     

The effect of colchicine on meiosis in Lilium speciosum cv. “Rosemede”

Chromosome pairing and chiasma frequency were studied in meiocytes at diakinesis of Lilium speciosum cv. Rosemede fixed up to 21 days after the start of either continuous or 3 day pulse colchicine treatment. The two treatments gave similar results. In pulse treated pollen mother cells (PMCs) the mean chiasma frequency per cell fell from 26.4 in controls to 8.5 after fourteen days while the mean number of univalents per cell increased from 0.05 to 17.58. There was a negative correlation between mean chiasma frequency per bivalent and per PMC in colchicine treated buds; univalents were preferentially induced in bivalents with one chiasma, and preferentially excluded in bivalents with 4 chiasmata. Some chiasmata were redistributed to surviving bivalents despite the concurrent reduction in chiasma frequency per meiocyte. — Colchicine sensitivity began in premeiotic interphase and extended to mid or late zygotene in PMCs; ongoing synapsis was unaffected. However, susceptibility to univalency was asynchronous between bivalents occurring at zygotene in short chromosomes but at late premeiotic interphase in the longest chromosomes. The number of chiasmata per bivalent could be altered by colchicine without inducing univalents, but the ultimate effect was to reduce the number of chiasmata per bivalent (or per chromosome arm) directly to zero. The major factors determining the order and extent of reduced pairing and chiasma number were total chromosome length and arm length. Pairing and chiasma formation in embryo sac mother cells were less sensitive to colchicine than in PMCs, but their behavior was otherwise similar.     

A mathematical framework for examining whether a minimum number of chiasmata is required per metacentric chromosome or chromosome arm in human

We introduce a piecewise linear regression called "hockey stick regression" to model the relationship between genetic and physical lengths of chromosomes in a genome. This piecewise linear regression is an extension of the two-parameter linear regression we proposed earlier [W. Li and J. Freudenberg, Two-parameter characterization of chromosome-scale recombination rate, Genome Res., 19 (2009) 2300-2307]. We use this, as well as the one-piece regression with a fixed y-intercept, to compare the two competing hypotheses concerning the minimum number of required chiasmata for meiosis: minimum one chiasma per chromosome (PC) and per chromosome arm (PA). Using statistical model selection and testing, we show that for human genome data, one-piece PC (PC1) is often in a statistical tie with two-piece PA model (PA2). If an upper bound for the segmentation point in two-piece regression is imposed, PC is usually the preferred model. This indicates that a presence of more than one chiasmata is rather caused by the relationship between chromosome size and chiasma formation than by cytogenetic constraints.     

Comparative chiasma analysis using a computerised optical digitiser

A new computerised technique has been devised for measuring the distribution of chiasmata along diplotene bivalents. The method involves the introduction into the field of view of the microscope, of a fine light spot which can be accurately manipulated along the chromosomes of each bivalent. The data recorded include (a) the positions of the chiasmata along the bivalent in terms of their relative distances from the centromere and (b) the individual bivalent and cellular chiasma frequencies. — The method has been applied to the analysis of chiasma distribution patterns in the two known species of the genus Caledia, C. species nova 1 and C. captiva and in two chromosomal races of the latter. Statistical tests indicate that within bivalents at least 40% of the comparative distribution patterns of chiasmata between races and species are significantly different. Similar comparisons between populations within races reveal only 18% significant differences. — The observed distribution patterns of chiasmata in this genus suggest that chiasma formation is sequential from centromere to telomere. — The variation in the frequency and distribution of chiasmata between races and species suggests that the interference distances between successive chiasmata are, at least partially, independent of chiasma frequency and position. — The interracial and interspecific differences in chromosome structure are correlated with changes in chiasma pattern.     

Effects of centric-shift polymorphisms on chiasma conditions inTrimerotropis pallidipennis (Oedipodinae:Acrididae)

Two populations ofTrimerotropis pallidipennis from Mendoza (MA) and Uspallata (UA) (Argentina), polymorphic with respect to centric shifts and B-chromosomes, were cytologically analysed. The medium-sized chromosomes from both populations were polymorphic for 7 (MA) and 5 (UA) inversions respectively. A clear tendency towards chiasma localisation was evident in both populations although they differ significantly with respect to mean frequencies of both total and interstitial chiasmata. The analysis of these frequencies in relation to the polymorphisms showed that in UA, the inversions do not affect chiasma formation while in MA, total chiasma frequency is negatively correlated in a significant fashion with the number of heteromorphic bivalents. This decrease is related, in non-B-carriers, to a significant increase of interstitial chiasmata. Therefore a redistribution of chiasmata, which could increase genetic recombination, is evident. The successful maintenance of polymorphisms in each population might depend, in part, on their effects on recombination, which could be related with the local requirements of genetic variability.     

Studies on chiasma distribution and chiasma movement in Zoniopoda tarsata (Orthoptera: Romaleidae)

A sample of 27 males of Zoniopoda tarsata from Argentina was studied cytologically. The three largest autosomal pairs and the X were characterized by the presence of interstitial C-bands. Chiasma position relative to the bands was analyzed at diplotene and diakinesis. The frequency of interstitial, terminal and total chiasmata per cell was studied for the whole autosomal bivalent set, analysing the variations between stages and among individuals. The comparison of interstitial chiasma frequencies between stages and among individuals and the study of chiasma position relative to the bands in pairs 1, 2 and 3 indicated that chiasma distribution varied from diplotene to diakinesis. Therefore, terminalization does exist in this species and the movement may occur towards the centromere. The frequency of terminal associations at diplotene showed a high negative correlation (r=-0.89; p<10^-5) with the number of interstitial chiasmata. This correlation would not be expected if the two kinds of association were produced by different (independent) mechanisms. Consequently, terminal associations were considered genuine chiasmata. The correlation between interstitial and total chiasmata was very much lower then the former (r=0.39; p=0.04). This fact, besides the relatively low variation for chiasma number, observed among individuals suggests that in this species the number of interestitial chiasmata, which are the most important in controlling the genetical recombination, is mainly regulated by changes in chiasma distribution, while variations in total chiasma frequency are of much lower magnitude.Member of Carrera del Investigador del Consejo Nacional de Investigaciones Cientificas y Técnicas (CONICET)     

Chiasma studies in structural hybrids IX. Crossing-over in pericentric inversion ofScilla scilloides

InScilla scilloides (Lindle) Druce, the heterozygotes for a pericentric inversion were found to be predominant in a small natural population consisting of cytogenetic type BB (2n=18). Pericentric inversion may include about half the length of the original subtelocentric chromosome, changing it to submetacentric. The 9II were always formed in these heterozygotes as well as in normal plants at MI in PMCs. A single chiasma was formed in the shorter one of two inverted segments divided by the kinetochore at MI, while one or two inversion chiasmata were observed in the longer segment. The AI separation was always regular. Since both arms of a normal chromosome and those of an inverted one were clearly distinguishable from one another at AI and AII, two kinds of crossover chromatids could be identified. Both sides of the single inversion chiasma always opened out reductionally. The frequency of bivalent without inversion chiasma agreed statistically with that of half-bivalent at AI or chromatid structure at AII, which resulted from non crossing-over within the inverted segment. Likewise, no statistical difference was found between the frequency of a single chiasma and that of a single crossing-over product in a longer inverted segment. These findings have clearly proved that the chiasma is a consequence of genetic crossing-over. The average proportion of good pollen grains in the inversion heterozygotes, 53.6%, amounted to about half that of normal plants, 97.7%.     

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.     

Near-normal chiasma formation and maintenance in a short distal translocated segment in maize

Study of successful crossover pairing and chiasma formation is informatively extended to a very short translocated segment. Contrary to previous suggestion it now seems likely that the extreme distal region of the long arm of maize chromosome 1 is not deficient in intrinsic capacity for the initiation of crossover pairing. In addition, chiasmata formed in this short region appear to be efficiently maintained.     

Chiasma distribution in Truxaline grasshoppers

Similar patterns of chiasma distribution are found within the individual arms of the chromosome complement in four species of Truxaline grasshopper. There is a linear relationship between chiasma frequency and chromosome arm length although the telocentric elements have a consistently higher mean number of chiasmata per unit of arm length. The positions of successive chiasmata can be defined in terms of residual (r.c. and r.t.) and interference (T) distances which vary in value according to both arm length and chiasma frequency. There is a tendency for one chiasma to lie in a distal position which is accentuated when additional chiasmata form. Supernumerary B chromosomes do not appear to influence the overall control mechanism of chiasma distribution. There is no indication that bivalents within a nucleus compete for chiasmata nor does the chiasma distribution in one arm of the metacentric members influence that in the other. It is suggested that the control of chiasma formation is determined mainly by interference factors.     

Fitting of a binomial distribution to the number of chiasmata per bivalent

Statistical tests on the distribution of the number of chiasmata per chromosome, collected from literatures, showed that they can be approximated by binomial distributions with one obligatory chiasma, i.e., B(N-1, p). N is proportional to the average number of chiasmata, while p is nearly constant for the species tested.     

Simultaneous negative and positive chiasma interference across the break point in interchange heterozygotes

The impossibility to obtain real roots from equations published earlier for estimating chiasma frequencies in the two translocated segments from configuration frequencies in interchange heterozygotes, was shown to be a result of lack of independence of chiasma formation. This is interpreted as negative interference. Similarly, negative interference could be shown to operate between the two interstitial segments. In all cases where a sufficient number of bivalents was formed by the interchange complex, chiasma frequency in the interstitial segments was strikingly higher in bivalents (having no chiasmata in the translocated segments) than in multivalents (with chiasmata in one or both translocated segments). This indicates strong positive interference between the interstitial and translocated segments.Negative interference between opposite-and positive interference between adjacent segments across the break point of the interchange occurred simultaneously in the cell populations. The phenomenon was attributed to complications in effective chromosome pairing at the point of partner exchange which in interchanges is determined by the breakpoint.The material was Secale cereale where five interchanges were analysed in a total of 12000 PMC's from 14 plants.     

Spatiotemporal Asymmetry of the Meiotic Program Underlies the Predominantly Distal Distribution of Meiotic Crossovers in Barley

Meiosis involves reciprocal exchange of genetic information between homologous chromosomes to generate new allelic combinations. In cereals, the distribution of genetic crossovers, cytologically visible as chiasmata, is skewed toward the distal regions of the chromosomes. However, many genes are known to lie within interstitial/proximal regions of low recombination, creating a limitation for breeders. We investigated the factors underlying the pattern of chiasma formation in barley (Hordeum vulgare) and show that chiasma distribution reflects polarization in the spatiotemporal initiation of recombination, chromosome pairing, and synapsis. Consequently, meiotic progression in distal chromosomal regions occurs in coordination with the chromatin cycles that are a conserved feature of the meiotic program. Recombination initiation in interstitial and proximal regions occurs later than distal events, is not coordinated with the cycles, and rarely progresses to form chiasmata. Early recombination initiation is spatially associated with early replicating, euchromatic DNA, which is predominately found in distal regions. We demonstrate that a modest temperature shift is sufficient to alter meiotic progression in relation to the chromosome cycles. The polarization of the meiotic processes is reduced and is accompanied by a shift in chiasma distribution with an increase in interstitial and proximal chiasmata, suggesting a potential route to modify recombination in cereals.     

Cyclic variation of chiasma frequency and distribution in Podarcis sicula (Reptilia: Lacertidae)

The seasonal variations of chiasma frequency and distribution have been studied in the lizard: Podareis sicula. In this species, as in Phyllodactylus (King & Hayman, Chromosoma 69: 131–154, 1978), chiasma frequencies vary following a definite annual cycle, and clearly different trends are shown by interstitial and terminal chiasmata.A comparison between these seasonal chiasma frequency variations and those of environmental temperature shows the existence of a clear correlation between these two parameters. However, this correlation is different in the two types of chiasmata, and may be different within the same type of chiasma depending on the period of the year.A more significant correlation is observed between chiasma cycles and annual variations of the haematic levels of sexual steroid hormones. In particular we observe a highly significant correlation between interstitial chiasma frequencies and testosterone concentration. A less precise correlation between terminal chiasma frequencies and estradiol concentration is also observed.In Podarcis, as in Phyllodactylus, the sperm that will be used for fertilization derive from the spermatocytes showing the highest rate of interstitial chiasmata. This supports the hypothesis that the cyclic variations in interstitial chiasma frequencies represent a mechanism to ensure an adequate level of variability in a given population. The above mentioned correlation between chiasma frequencies and steroia hormone concentrations suggests that the seasonal chiasma cycles are controlled by the same environmental and hormonal factors regulating the spermatogenetic cycle.     

The effect of accessory chromatin on chiasma distribution in maize.

From an analysis of metaphase I bivalent configurations in Zea mays L. it was possible to determine the effects of two supernumerary elements on chiasma formation. Both the B chromosome and abnormal chromosome 10 increased chiasma frequency. In addition to enhancing total exchanges, both elements caused a redistribution of chiasmata from distal to more proximal locations.