The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of M I configurations. IV. Interchange heterozygotes

The quadrivalent of an interchange heterozygote can be divided into six regions, each with its own probability of being bound by at least one chiasma. This gives rise to 2⁶ or 64 different possible combinations of bound and not bound regions and as many M I configurations, several of which are isomorph. Sixteen different types of configuration occur, 10 of which involve more than two chromosomes and can thus be distinguished from the non-interchange bivalents. As each combination of bound and not bound regions has a certain probability of occurring depending on the probability of each region to be bound or not, a frequency expectation value can be estimated for each configuration. Also, when the relative frequencies of the different configurations are known, the probabilities of being bound can be estimated for each of the six regions of a quadrivalent, and from these the crossing-over potentials. One translocation heterozygote was studied, involving the long arm of the satellite chromosome of rye (Secale cereale). Crossing-over was reduced, perhaps mainly by partner exchange. The terminal segments retained a relatively high chiasma frequency. The interstitial regions had low chiasma frequencies even when relatively long.     

Meiotic behaviour of telo-tertiary compensating trisomics of rye: evaluation for use in hybrid varieties

Summary Meiosis of four telocentric-tertiary compensating trisomics of rye (Secale cereale L.) was studied with the purpose of evaluating their suitability for use in maintaining genic male-sterile lines applied in hybrid varieties. They had been constructed from four different reciprocal translocations and three different telocentrics. In one trisomic a slight, but significant tendency was demonstrated for preferential pairing of the two normal chromosomes associated with the compensating complex. This promotes the desired segregation into one normal and one compensating karyotype. In all trisomies, however, too high a frequency of failure of chiasma formation in a critical segment of the complex was evident. This is correlated with the ease of recovery of the trisomics, but results in undesired segregational products. Interstitial chiasmata leading to the formation of branched configurations were also present, more in some trisomics than in others. These also result in undesired segregations. The behaviour at meiosis was so closely correlated with the length of the chromosome segments involved that a prediction of the most favourable combination of telocentric and translocation can be made. The telocentric should be large, the corresponding translocated segment large and the interstitial segment small. The non-translocated arm of the translocated chromosome should be large and the second translocated segment small. The combinations of translocations and telocentrics had not been selected for these criteria and did not meet the requirements for practical application.     

The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of MI configurations. V. Interchange trisomics

Information obtained previously and presently on chromosome pairing and chiasma formation in trisomics and in interchange heterozygotes has been applied in newly constructed models for calculating expected MI configuration frequencies in interchange trisomics. Good fit betwen calculated and observed frequencies in some and poor fit in other cases confirmed the expectation of genetic variation in the crossing-over potentials of some or all chromosome regions. If conclusions in respect of chromosome pairing pattern are to be based on relative frequencies of MI configurations, valid values for crossing-over potentials are required. These can only be obtained from genetically comparable material. A few more disturbing factors are recognised. Environmental effects are one of these factors but may have a relatively simple character. Good agreement between expected and observed frequencies of configurations was taken to indicate the validity of the assumption that homologous chromosome end segments have equal probability of being involved in pairing, irrespective of the length of the segment. This conclusion was confirmed by the segregation of chromosomal types in the progenies of interchange trisomics: the excess chromosome was combined as frequently with the interchange set and with the normal set respectively, as expected on basis of the same models, assuming 60–80% viability of trisomes compared to diploids.     

Meiotic behaviour of single-arm tetrasomic combinations of isochromosomes,telocentrics and normal chromosomes in rye (Secale cereale L.)

The isochromosome studied was derived from the short arm of the satellite chromosome of rye (Secale cereale, 2n=14); the telocentrics represent both the short and long arms of the same chromosome. Three different combinations, tetrasomic for the short arm, have been composed and studied: I: 2 isochromosomes (short arm) + 2 telocentrics (long arm) + 6 normal pairs. II: 1 isochromosome + 2 telocentrics (short arm) + 2 telocentrics (long arm) + 6 normal pairs. III: 1 isochromosome + 1 telocentric (short arm) + 1 normal satellite chromosome + 1 telocentric (long arm) + 6 normal pairs. — Over 20,000 cells were analysed. Simple mathematical models describing the frequencies of the different types of MI configurations in terms of frequency of chiasmata in the different pairing combinations of the polysomic arms, and of the frequency of multivalent pairing of this arm, were developed. They were used to derive estimates for chiasma frequencies and multivalent pairing frequencies in the different chromosome constitutions from the observations on configuration frequencies. Variation between plants and within plants was studied, and it was concluded that much of the within plant heterogeneity was due to regulatory variation expressed independently in different chromosomal segments. There was also a significant genetic component. Analysis of the reasons for the models to fail under certain conditions led to suggestions for extension of the models.     

The calculation of the map length of interchange segments from meiotic configuration frequencies

Standard mapping functions permit the calculation of map length from the frequency by which a chromosome segment has at least one chiasma. In interchange heterozygotes, approximately exact estimates of such frequencies can be obtained from the relative frequencies of specific groups of meiotic metaphase I configurations. Examples are given for two interchanges inSecale cereale.     

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.     

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.     

Isolation of telotertiary compensating trisomics from telocentric translocation trisomics and telo-substituted translocation heterozygotes of rye (Secale cereale L.)

Telotertiary compensating trisomics (CTs) of rye (Secale cereale L.), in which the absence of one normal chromosome is compensated by the presence of a telocentric and a translocation chromosome, were isolated in progenies of telocentric translocation trisomics, and telo-substituted translocation heterozygotes, respectively. These two sources were obtained from crosses between five interchanges of the Wageningen translocation tester set, and telocentric normal trisomics (for IRS, IRL and 5RS), or telocentric substitutions (for IR and 3R), respectively. In test crosses with normal male plants, CTs were identified using either critical meiotic configurations, the segregation of karyotypes in selfed trisomic progenies, or the segregation of a marker located on the compensated chromosome. CT yields ranged from 0.0–6.3%. These frequencies were concluded to be determined mainly by the frequency of the exchanged segment of the translocation chromosome involved in the CT complex being associated at first meiotic metaphase (MI) in the source plants. The lower association frequencies result in the higher CT yields. The correlation between high association frequency of this segment and low CT yield suggests that infrequent adjacent orientation of one critical segment is also responsible for the origin of CTs. This agrees with cytogenetic theory.     

Load dependence of simulated central tremor

Previous studies have argued that tremors of central versus peripheral origin can be distinguished based on their load dependence: the frequency of peripheral tremor decreases when a weight is added to the tremulous limb, while the frequency of central tremors remains unchanged. The present study scrutinizes the latter statement. We simulated central tremor using a simple network of coupled neural oscillators, which receives proprioceptive feedback from the motor periphery. The network produced a self-sustained, stable oscillation. When the gain of proprioceptive feedback was high, oscillation frequency decreased in the presence of an inertial load. When the gain was low, the oscillation frequency was load independent. We conclude that load dependence is not an exclusive property of peripheral tremors but may be found in tremors of central origin as well. Therefore, the load test is not sufficient to reject a central tremor origin. Received: 1 October 1998 / Accepted in revised form: 28 January 1999     

A genetic basis for the origin of six different isolectins in hexaploid wheat

Wheat (Triticum aestivum) germ agglutinin represents a complex mixture of multiple isolectin forms. Upon ion exchange chromatography at pH 3.8, three isolectins can be separated, each of which is composed of two identical subunits. At pH 5.0, however, three additional isolectins can be distinguished, which are built up of two different subunits (heteromeric lectins). Evidence is presented that these heterodimers are normal constituents of the wheat embryo cells. Analyses of the isolectin patterns in extracts from Triticum monococcum, Triticum turgidum dicoccum and Triticum aestivum, provide evidence that each genome, either in simple or complex (polyploid) genomes, directs the synthesis of a single lectin subunit species. In addition, a comparison of the isolectin pattern in these wheat species of increasing ploidy level, made it possible to determine unequivocally the genome by which the individual lectin subunits in polyploid species are coded for. The possible use of lectins in studies on the origin of individual genoms in polyploid species is discussed.Abbreviations CL cereal lectin - PBS phosphate buffered saline - SP Sephadex sulfopropyl Sephadex - WGA wheat germ agglutinin     

Multiple chromosomal interchanges in pearl millet

Summary Intercrossing and irradiation were successfully used in pearl millet (Pennisetum typhoides) to develop multiple interchanges involving up to the total complement of all the chromosomes in one complex. In interchange heterozygotes showing 12 + 1 II and 14, 90.9 and 87.8 per cent of the cells, respectively, had chromosome configurations other than that of 12 and 14 chromosomes. In general, the frequency of such cells resulting from breakdown of the expected complex configuration increased with the increase in the number of translocated chromosomes in the complex. The higher the number of chromosomes involved in the interchange ring, the higher were the pollen and ovule sterility. The results indicated that meiotic instability, deficiency-duplication gametes, and unequal distribution of chromosomes account for increased sterility of multiple interchange heterozygotes. Even though interchanges in pearl millet predominantly show the alternate type of segregation, sterility seems to be the major barrier for the exploitation of the multiple interchange method for gamete selection and the establishment of homozygous lines in this plant species.     

Somatic association of telocentric chromosomes carrying homologous centromeres in common wheat

Summary Measurements of distances between telocentric chromosomes, either homologous or representing the opposite arms of a metacentric chromosome (complementary telocentrics), were made at metaphase in root tip cells of common wheat carrying two homologous pairs of complementary telocentrics of chromosome 1 B or 6 B (double ditelosomic 1 B or 6 B). The aim was to elucidate the relative locations of the telocentric chromosomes within the cell. The data obtained strongly suggest that all four telocentrics of chromosome 1 B or 6 B are spacially and simultaneously co-associated. In plants carrying two complementary (6 B ^S and 6 B ^L) and a non-related (5 B ^L) telocentric, only the complementary chromosomes were found to be somatically associated. It is thought, therefore, that the somatic association of chromosomes may involve more than two chromosomes in the same association and, since complementary telocentrics are as much associated as homologous, that the homology between centromeres (probably the only homologous region that exists between complementary telocentrics) is a very important condition for somatic association of chromosomes. The spacial arrangement of chromosomes was studied at anaphase and prophase and the polar orientation of chromosomes at prophase was found to resemble anaphase orientation. This was taken as good evidence for the maintenance of the chromosome arrangement — the Rabl orientation — and of the peripheral location of the centromere and its association with the nuclear membrane. Within this general arrangement homologous telocentric chromosomes were frequently seen to have their centromeres associated or directed towards each other. The role of the centromere in somatic association as a spindle fibre attachment and chromosome binder is discussed. It is suggested that for non-homologous chromosomes to become associated in root tips, the only requirement needed should be the homology of centromeres such as exists between complementary telocentrics, or, as a possible alternative, common repeated sequences of DNA molecules around the centromere region.Dedicated to Professor Dr. Marcus M. Rhoades on his 70th birthday.     

Study of a translocation in tetraploid rye

Colchicine induced tetraploids (4x=28) from diploidSecale cereale heterozygous for a translocation showed a strong tendency of non-preferential pairing for the interchanged chromosomes. The normal chromosomes associated in configurations up to quadrivalents, and the translocation complex formed multivalents up to octavalents. Most of the interchanged chromosome associations were characterized by their heteromorphic nature. The percentage of the chromosomes in the interchange complex forming multivalent associations was far higher than that of the remaining twenty chromosomes. Abnormalities were observed at anaphase I and II in the pollen mother cells. The tetraploids appeared to be completely sterile. It is suggested that the high frequency of multivalent formation may be explained on the basis that the interchange might have involved a region of localized chiasmata. The absence of polyploidy in the genusSecale as against its widespread occurrence in the related grass genera may be accounted for, in part, on the basis of non-preferential pairing.     

Synaptonemal complex pairing and metaphase I association in a telo-substituted telotrisomic of rye (Secale cereale L.)

The synaptonemal complexes in pollen mother cells (PMCs) of rye in which one chromosome 1R was replaced by the two corresponding telocentrics, and where one additional telocentric 1RS was present, showed approximately the expected 21 ratio of 1R-1RL-1RS trivalent with 1RS univalent versus heteromorphic 1R-1RL bivalent with 1RS bivalent. In addition, however, many cells with a partner exchange were found, several even including bivalents other than 1R. At metaphase I1R-1RL-1RS trivalents predominated, cells with two univalent telocentrics were relatively frequent but partner exchange configurations were extremely rare. It is concluded that the almost consistent failure to form chiasmata in the interstitial region of 1RS after partner exchange, combined with much more frequent chiasma formation in the terminal segment, is the main reason for the unexpected metaphase I configuration frequencies. Possible causes are discussed. The shift observed does not yet explain the erratic variation in relative frequencies of metaphase I configurations reported earlier in similar material. Frequent pairing partner exchange may play a role there also.     

High recombination between the breakpoint of a reciprocal translocation in rye (Secale cereale L.) and an interstitially located gene

Summary The recombination fraction between the interstitially located gene an and interchange 303 of rye was found to be 0.244±0.038 in a test cross using the translocation as the male parent. In first metaphase translocation configurations in pollen mother cells of the same plant, the chiasma frequency between an and the translocation breakpoint was found to be significantly more than twice the recombination fraction. Recombination was concluded to be masked by a difference in the alternate frequency between configurations without interstitial chiasmata and configurations with interstitial chiasmata, the effect of the first type being of major importance. Random centromere orientation of translocation multivalents with interstitial chiasmata was concluded to be a realistic assumption. The exceptionally high recombination between an and translocation 303 is discussed. Consideration is also given to the use of interchanges in the establishment of a marker's chromosomal position, and to the use of translocation chromosomes in balanced systems for hybrid breeding purposes.     

A methanogen hosted the origin of the genetic code

A comparison is made between orthologous proteins from a methanogen (Methanopyrus kandleri) and from a non-methanogen (Pyrococcus abyssi) in order to determine the amino acid substitution pattern. This analysis makes it possible to establish which amino acids are significantly and asymmetrically utilised by these two organisms. A methanophily index (MI) based on this asymmetry makes it possible for any protein to be associated with a numerical value which, when calculated for the same orthologous protein from methanogenic and non-methanogenic organisms, turns out to have the power to discriminate between these two groups of organisms, even if only for about 20% of the analysed proteins. The MI can also be associated to the genetic code under the assumption that the frequency of synonymous codons specifying the amino acids in the genetic code also reflects the frequency with which amino acids appeared in ancestral proteins. Finally a t-test shows that the MI value associated to the genetic code is not different from the mean value of the MI deriving from methanogen proteins, but it differs from the mean MI of non-methanogen proteins. This might indicate that the genetic code evolved in a methanogenic ‘organism’.     

Phylogenetic relationships among populations of the Nacobbus aberrans (Nematoda,Pratylenchidae) complex reveal the existence of cryptic species

The plant‐parasitic nematode Nacobbus aberrans sensu lato is an agricultural pest of quarantine importance. Due to the morphometric, physiological and genetic variability observed within the species, there is no agreement on the taxonomy of this nematode. The objective of this study was to analyse the ITS rDNA region and the D2–D3 expansion segments of 28S rDNA in 10 Argentine populations and one from Ecuador and to establish their phylogenetic relationship with other known sequences from South and North America. Phylogenetic trees of the ITS gene showed seven statistically well‐supported clades; the high and significant F_st values obtained among these groups confirmed this partitioning. The Argentine populations here considered were separated into three clades: one comprising a population from the Andean region and two grouping nematodes from lower altitudes. Three other clades were distinguished for South American populations, which included known sequences of individuals from Peru, Bolivia and north of Argentina. The other clade included sequences from Mexico, Ecuador and two Argentine populations of unknown origin. The important degree of genetic divergence observed among Andean populations suggests that the Andes may have played a crucial role in speciation of Nacobbus, which would have originated in this region. Although D2–D3 segments exhibited lower variation, they were useful for establishing phylogenetic relationships among the Argentine populations considered in this work. As there are no other GenBank sequences available for these segments, it was not possible to make comparisons with other populations from South and North America. The considerable genetic differentiation observed in ITS rDNA region among Nacobbus populations showed evidence of cryptic species within the N. aberrans s.l. complex. Integration of morphological and morphometric studies and molecular analyses considering other genes may aid in the identification of species and their phylogenetic relationships within this genus.     

An interchange which raises chiasma frequency

In the grasshopper Cibolacris parviceps a chromosomal interchange is described which considerably raises the chiasma frequency in all the chromosomes, including the interchange itself. Both of the chromosomes involved in the interchange have pronounced heterochromatic regions and the possible role of heterochromatin in altering recombination is discussed.     

Spontaneous chromosome mutations in Truxaline grasshoppers

Three distinct mutant conditions are described in single male individuals from three species of short horn grasshopper. Of these, one is an entire germ line mutant of Myrmeleotettix maculatus, heterozygous for a centric fusion between single M₄ and M₅ telocentric chromosomes. In contrast, the remaining two mutants are present in mosaic form. One is heterozygous for an L₁-M₄ interchange in Omocestus viridulus, the other tetrasomic for the M₄ chromosome in Chorthippus parallelus which in addition is characterised by the inclusion of a supernumerary heterochromatic segment on one S₈ homologue. Centric fusion in Myrmehotettix maculatus has neither disturbed the chiasma potential of the elements constituting the fusion multiple nor, has it apparently influenced the production of balanced gametes. The pattern of chiasma formation in the L₁-M₄ interchange multiple lends support to the contention that the process of chiasma formation originates near the distal end of chromosome arms in Omocestus viridulus. There is no interaction between the two mutant conditions of tetrasomy and the presence of supernumerary segments in Chorthippus parallelus. Moreover, because of the precocious nature of two of the four M₄ homologues there is little tendency to form multivalents. The two M₄ bivalents share a similar mean chiasma frequency.     

The effect of vanadium on nitrate reductase of Chlorella vulgaris

Added vanadate ions inhibit purified nitrate reductase from Chlorella vulgaris by reacting with the enzyme in a manner rather similar to that of HCN. Thus vanadate, like HCN, forms an inactive complex with the reduced enzyme, and this inactivated enzyme can be reactivated rapidly by adding ferricyanide. The inactive vanadate enzyme complex is less stable than the inactive HCN complex, and the two can be distinguished by the fact that EDTA causes a partial reactivation of the former, but not of the latter. Vanadate can also cause an increase in HCN formation by intact Chlorella vulgaris cells. When these cells were incubated with vanadate, their nitrate reductase was reversibly inactivated, and all of this inactive enzyme could be shown to be the HCN complex rather than the vanadate complex. When HCN and vanadate are both present, the HCN-inactivated enzyme, being more stable, will be formed in preference to the vanadate-inactivated enzyme.Abbreviation EDTA ethylenediamine tetraacetate