Chromosomal DNA variation,genomic constraints and recombination in Lathyrus

There is approximately a doubling of the total nuclear DNA between the 8 Lathyrus species and there are significant differences in the amounts of DNA in euchromatin and heterochromatin. Between the 8 species chiasma frequency and total nuclear DNA are not correlated but within complements it is positively correlated with the amount of DNA in the chromosomes. There is no significant correlation between chiasma frequency and euchromatin DNA nor between chiasma frequency and heterochromatin DNA among species, but among chromosomes, as with total DNA, it is positively correlated with euchromatin DNA and heterochromatin DNA. Results show that despite the large differences in DNA amounts between species there are genomic constraints underlying the frequency and distribution of chiasmata in the chromosome complements.     

Molecular organisation of the plant genome: Its relation to structure,recombination and evolution of chromosomes

Large scale changes in nuclear DNA amount accompany the evolution of species of higher plants. Much of the nuclear DNA accrued during the evolution of species does not encode genetic information and is selectively neutral. Nonetheless, the pattern of distribution of the excess DNA within and between chromosome complements suggests that there are rigid constraints underlying evolutionary changes in genome organisation. A five-fold increase in the amount of nuclear DNA has occurred in the evolution ofLathyrus species. Not withstanding this massive DNA variation, species show consistently similar patterns in base sequence proliferation, divergence and DNA distribution within and between chromosome complements. Within chromosome complements, the excess DNA is distributed evenly in all chromosomes irrespective of the large differences in chromosome size and, between complements, DNA distribution is discontinuous; species cluster into DNA groups with remarkably regular intervals. Similar constraints govern the frequency and distribution of chiasmata in the chromosome complements. Between species chiasma frequency and nuclear DNA amounts are not correlated but within complements it is positively correlated with the amount of DNA contained in each chromosome.     

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.     

The structure,meiotic behaviour and effects of B chromosomes in Briza humilis Bieb. (Gramineae)

A single population of Briza humilis contained two types of B chromosome, one a large (B^L) and the other a small (B^S) acrocentric. DNA measurements show that the B^L chromosome contains approximately twice as much DNA per unit length as the members of the regular complement. The meiotic pairing behaviour of the Bs is variable and B^L and B^S are seen to pair in some cells. The presence of B^L depresses the chiasma frequency of the regular complement but the chiasma frequency of A and B chromosomes does not appear to be related. The transmission rate of the B chromosomes is variable and the B^L shows a non-disjunction mechanism during microsporogenesis that is absent during megasporogenesis. For the B^S chromosome the transmission rate is very low and there is no evidence of a non-disjunction mechanism. In general seeds containing B^L chromosomes germinate more slowly than those without B chromosomes.     

The effect of supernumerary chromosomes on meiosis in Puschkinia libanotica (Liliaceae)

The presence of the partly heterochromatic supernumerary chromosomes in pollen mother cells of Puschkinia libanotica raises the chiasma frequency in the A chromosome bivalents. The pattern of chiasma distribution along each of the five A bivalents was related to the DNA labelling pattern of mitotic chromosomes. Regions that showed heavy labelling at the end of the DNA synthetic phase had fewer chiasmata than lightly labelled regions. As this relation is the opposite to that found by Rees and Evans in another species we regard any correlation between labelling pattern and chiasma distribution as fortuitous.     

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.     

The supernumerary segment system of Stethophyma

Three species of the genus Stethophyma have been cytologically examined and all three show variation both for supernumerary heterochromatic segments and for the distribution of standard heterochromatin among the autosomes. The European species, S. grossum, for example, shows considerable interpopulation variation for standard heterochromatin while two of the populations, from Spain and Austria, show supernumerary segment polymorphism. The segments are located interstitially on the S₁₁ chromosome but occupy different positions in the different populations. — In all species, the presence of the extra heterochromatic segments increases the mean chiasma frequency. Moreover, the influence of the segments upon mean chiasma frequency is different in different populations and in different species. In the Spanish population, the increase is both intra- and interchromosomal whereas in Austria the influence of the segment is completely interchromosomal. — In the American species, S. gracile and S. lineatum, where supernumerary heterochromatic segments are carried on both S₁₀ and S₁₁ chromosomes, the effect on chiasma frequency shows a dosage relationship, an increase in the number of segments per individual being correlated with an increase in mean chiasma frequency. It is suggested that the interstitial segments found in all species have originated by direct duplication of chromosome material. By contrast the terminal segments in S. lineatum and S. gracile may be derived by translocation from a B-chromosome since such a chromosome has been found in one individual of the former species. — The variation in segment structure and the distribution of standard heterochromatin, among the European species of S. grossum suggests that these systems have evolved independently in different populations.On educational leave from the Forest Research Laboratory, Fredericton, N. B. Canada.     

Chromosome pairing in tetraploid hybrids between Lolium perenne and L. multiflorum

Summary Chromosome association at first meiotic metaphase in tetraploid hybrids between Lolium perenne and L. multiflorum was compared with that in autotetraploid L. perenne. The hybrids were found to have significantly higher levels of bivalent frequency, and lower levels of multivalent and chiasma frequency. A significant increase in multivalent frequency with increasing chiasma formation was found in both groups, but the increase was much less in the hybrids. These differences in chromosome associations between the two groups must therefore reflect differences in chiasma distribution and it is suggested that the results indicate a significant degree of preferential bivalent pairing in the hybrids.     

Regulation of R-protein and endonuclease activities in meiocytes by homologous chromosome pairing

Achiasmate meiocytes of Lilium cv. Black Beauty do not undergo DNA nicking at pachytene, nor do they acquire the high level of R-protein during zygotene-pachytene that is characteristic of chiasmate types. Induction of tetraploidy in such meiocytes with its accompanying increase in chiasma frequency results in a pachytene nicking pattern that is indistinguishable from what occurs in chiasmate plants. R-protein also increases to a level that on a per diploid genome basis is at least as high, and probably higher, than the level of chiasmate diploids. The absence of nicking in the achiasmate diploid is not due to a lack of appropriate enzyme activity but seems to result from the fact that unpaired or ineffectively paired chromosome regions are not accessible to the enzyme. The regulation of R-protein level by homologous pairing is examined but remains poorly understood.     

Fertility and meiotic chromosome behaviour in autotetraploid pearl millet

Summary Tetraploidy was induced in outbred pearl millet and selection for high and low seed set was started in the C₁ generation. Segregation in the C₃ generation was observed for fertility and also for meiotic features: per cent seed set in selfed earhead, chiasma frequency, chromosome association and chromosome distribution in pollen mother cells were all affected. However, variation in seed set was observed even between samples not differing in meiotic features. It is apparent that factors regulating seed set in autotetraploid pearl millet were genic as well as chromosomal.A high frequency of univalents and trivalents was the main cause of sterility; quadrivalent misdisjunction was not a significant factor. As univalency decreased with increased chiasma formation, the gain was in the form of quadrivalents. However, individuals not differing in chiasma frequency did differ in chromosome association frequencies, indicating that the dependence of chromosome pairing behaviour on chiasmata was subject to genotypic influence.     

Genetics of speciation in the Aedes (Stegomyia) scutellaris group (Diptera: Culicidae) V. Chromosomal relationships among five species

A comparison was made of karyotypes of 5 species in the Aedes scutellaris group and their hybrids. All species had 3 distinct pairs of metacentric chromosomes (2n=6). These were of similar lengths in Ae: malayensis and Ae. alcasidi, and in Ae. polynesiensis and Ae. pseudoscutellaris. However, chromosome 1 in Ae. polynesiensis and Ae. kesseli, and chromosome 2 in Ae. pseudoscutellaris and Ae. kesseli were of unequal lengths. Meiotic analyses revealed that chromosome asynapsis was frequently seen in species hybrids. There was a significant variation in chiasma frequencies between species and their hybrids. However, the mean chiasma frequency was species specific. In addition, the mean chiasma frequency of species hybrids and the extent of chromosomal asynapsis provided a measure of genetic homology between species. Based on the assumption that a dicentric bridge and an acentric fragment were due to a single crossover within a paracentric inversion loop the following conclusions are made. Ae. malayensis and Ae. alcasidi are polymorphic for one paracentric inversion in chromosome 1. Ae. polynesiensis and Ae. pseudoscutellaris, and Ae. pseudoscutellaris and Ae. kesseli are fixed for one paracentric inversion in chromosome 2. Similarly, Ae. polynesiensis and Ae. kesseli are fixed for one paracentric inversion in chromosome 1. These chromosomal differences between species are discussed with respect to hybrid fertility data reported earlier.     

The influence of parental genotype on the chromosome behaviour of Hordeum vulgare X H. bulbosum diploid hybrids

Summary The PMCs of 74 diploid hybrids involving ten H. vulgare varieties and three H. bulbosum lines were analysed at metaphase I and chromosome number and chiasma frequency recorded. There were differences between parental combinations and between plants within those combinations for both chromosome and chiasma number. It is suggested that these characters are controlled by both parents and that differences between plants within families reflect the heterozygosity of the H. bulbosum parents. Chromosomally stable, high pairing lines have been identified for use in a backcrossing programme to introduce H. bulbosum characters to the H. vulgare germplasm.     

Unusual chromosome pairing and B chromosomes inBriza spicata (Poaceae)

One plant from a population ofBriza spicata (Poaceae) was found to have highly irregular meiotic behaviour. It is characterized by having a reduced chiasma frequency, a large between cell variance in chiasma frequency and the formation of multivalents involving pairs of A chromosomes. The B chromosome present in this plant also forms multivalents with a pair of A chromosomes. It is suggested that the normal control of strict bivalent pairing has broken down and homoeologous chromosomes are associating as multivalents. Furthermore, the partial homology of the B chromosome with a pair of A chromosomes is revealed.     

Asynapsis in Lolium perenne

Eighteen plants displaying varying degrees of asynapsis ranging from weak to very strong were found among four out of six populations of Lolium perenne L. (2n=14) which had been subjected to three cycles of directional phenotypic selection for productivity of green material. No plants were found displaying univalents in the original generation but the incidence increased with cycles of selection, indicating the genetic control and differential distribution of asynaptic genes among these populations. — The analysis of univalents and chiasma frequency of pollen mother cells (PMC) of six partially asynaptic plants chosen for detailed study revealed that univalents occurred throughout all PMC chiasma classes irrespective of chiasma frequency, but the higher the chiasma frequency of any PMC the less the likelihood of univalents occurring. The relationship between chiasma frequency and univalent frequency per PMC per plant was negative. — Mean chiasma frequency per bivalent increased for the asynaptic cells in comparison with the normal in both the weak and medium asynaptic groups which was explained by the availability of additional chiasmata for redistribution.     

RING-POSITION AND FREQUENCY OF CHIASMA FAILURE IN RHOEO SPATHACEA

The mean chiasma number per cell was 11.44 ± 0.04 in 489 cells. Due to chiasma failures, the ring-of-twelve chromosomes may be broken into two or three chains. Cells with four or more chains were not observed. All six possible two-chain situations and eight different threechain situations were found. All possible lengths of chains from one to all twelve chromosomes were found, with “chain”-of-one inordinately frequent. The overall mean number of chromosomes in 273 chains in 188 cells is 8.26 ± 0.31 and 5.38 ± 0.31 among 154 chains in the 69 cells that had two or more chains. The mean number of chains per cell among these 188 was 1.45 ± 0.13. In 73 cells, 113 chiasma failures were found to be distributed at random among the twelve chromosome arm positions. The absence of association either between length of arm or between presence-absence of secondary constriction and frequency of chiasma failure support the generally accepted theory that, in Rhoeo, synapsis and crossing over are restricted to small terminal segments on all chromosomes.     

Histone hyperacetylation affects meiotic recombination and chromosome segregation in Arabidopsis

In this study, the meiotic role of MEIOTIC CONTROL OF CROSSOVERS1 (MCC1), a GCN5‐related histone N‐acetyltransferase, is described in Arabidopsis. Analysis of the over‐expression mutant obtained by enhancer activation tagging revealed that acetylation of histone H3 increased in male prophase I. MCC1 appeared to be required in meiosis for normal chiasma number and distribution and for chromosome segregation. Overall, elevated MCC1 did not affect crossover number per cell, but has a differential effect on individual chromosomes elevating COs for chromosome 4, in which there is also a shift in chiasma distribution, and reducing COs for chromosome 1 and 2. For the latter there is a loss of the obligate CO/chiasma in 8% of the male meiocytes. The meiotic defects led to abortion in about half of the male and female gametes in the mutant. In wild type, the treatment with trichostatin A, an inhibitor of histone deacetylases, phenocopies MCC1 over‐expression in meiosis. Our results provide evidence that histone hyperacetylation has a significant impact on the plant meiosis.     

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.     

Supernumerary chromosomes in Tradescantia I. Meiotic behavior of three homologous isochromosomes

The meiotic behavior of three homologous iso-supernumerary chromosomes in an individual of Tradescantia ohiensis Raf. has revealed sevealed several interesting features. There is a high frequency of pairing-partner exchange within bivalents and trivalents. The mean chiasma frequency per chromosome is relatively high (1.046) for supernumerary chromosomes and is not significantly different from that of the much larger normal (A) chromosomes. The high frequency of trivalent and bivalent configurations would seem to suggest that the isosupernumerary chromosomes are premeiotically dispositioned to allow for full random pairing among all six homologous arms.     

Karyotype, DNA Content and Meiotic Behaviour in Five South American Species ofVicia(Fabaceae)

This paper presents the karyotype, DNA content and meiotic behaviourof five species ofViciafrom Argentina (V. macrogramineaBurk.,V.gramineaSM.,V. epetiolarisBurk.,V. pampicolaBurk. andV. nanaVog.).All the species have the same chromosome number and karyotypeformula (2n=14; 6m+4st+4t). Each species, however, displaysa characteristic number and position of the nucleolar organizerregion (NOR) and different sizes of the respective satellites,confirmed by Ag-NOR banding. Moreover, significant differenceswere found in the total chromosome volume (TCV) and DNA contentof the species. Positive correlations between DNA content andTCV, and between DNA content and type of life cycle were alsofound. TCV and DNA content are lower inV. nana(annual) and higherinV. macrograminea(biennial–perennial). The material displayedmarked karyotypic orthoselection, with similar karyotypes inall studied species, even when the overall chromosome size varied.Evolutionary changes in DNA amount are proportional to the relativelength of each chromosome arm, maintaining karyotypic uniformity.Significant differences were found between the meiotic behaviourofV. gramineaand that of the other species.V. gramineahas alower frequency of ring bivalents and chiasmata per cell, andalso has a lower interstitial chiasma frequency. In general,the results are congruent with the morphological data reportedfor these species.Copyright 1998 Annals of Botany Company. Viciaspecies, karyotype, orthoselection, nuclear DNA content, NOR banding, meiotic behaviour.     

Frequency of telomere repeat units in the Drosophila miranda genome

Cloned DNA fragments of Drosophila miranda which label all chromosome ends show a basic tandem repeat unit of 4.4 kb. The D. miranda telomere specific tandem repeats do not cross-hybridize with genomic D. melanogaster DNA which itself contains telomere repeat units of 3 kb. For a more detailed analysis of the functional criteria of telomere specific sequences we determined the repetition frequency of the tandem repeat units. As a low estimate we found a repetition frequency of 20 for female D. miranda DNA. This is on average equivalent to 2 telomere repeat units per chromosome end in the female D. miranda karyotype. However, a variable number of tandem repeat units per chromosome end would describe more closely the obtained differences in the labeling intensity between the individual chromosomes (X¹L-5). For the D. miranda male DNA we determined a repetition frequency of 90. The frequency difference of 70 copies between male and female DNA must be due to the Y-chromosome.