Homologous chromosome pairing in meiosis of higher eukaryotes-still an enigma?

Meiosis is the basis of the generative reproduction of eukaryotes. The crucial first step is homologous chromosome pairing. In higher eukaryotes, micrometer-scale chromosomes, micrometer distances apart, are brought together by nanometer DNA sequences, at least a factor of 1000 size difference.

Incomplete pole orientation of kinetochores in complex meiotic metaphase I configurations delays metaphase-anaphase transition in Secale.

To prevent unbalanced chromosome segregation, meiotic metaphase I - anaphase I transition is carefully regulated by delaying anaphase until all kinetochores are well oriented (anaphase checkpoint) in mammals and insects. In plants this has not yet been established. In heterozygotes of two reciprocal translocations of Secale cereale, with one chromosome replaced by its two telocentric arms, anaphase delay was correlated with the orientation of the kinetochores of the complex of five chromosomes.

Can epigenetic control explain pronounced within-plant heterogeneity of meiosis in a translocation trisome of Secale L.?

Meiotic metaphase I configuration frequencies were determined in different tillers of genetically related plants of rye ( Secale cereale L.) heterozygous for reciprocal translocation T248W (between chromosome arms 1RS and 6RS) and with an additional (telocentric) arm 1RS. Seventeen different configurations could be recognized, grouped into three categories.

Trisomy greatly enhances interstitial crossing over in a translocation heterozygote of Secale.

Chromosomal rearrangements, including reciprocal translocations, may prevent recombinational transfer of genes from a donor genotype to a recipient, especially when the gene is located in an interstitial segment. The effect of trisomy of chromosome arm 1RS on recombination was studied in translocation heterozygote T248W of rye ( Secale cereale ), involving chromosome arms 1RS and 6RS. (Pro)metaphase I configuration frequencies were analyzed.

Chromosome pairing and chiasma formation in autopolyploids of different Lathyrus species.

Chromosome pairing and chiasma formation were studied in natural and induced tetraploids (2n = 28) of Lathyrus odoratus (induced), Lathyrus pratensis (natural and induced), Lathyrus sativus (induced), and Lathyrus venosus (natural), as well as in triploids of L. pratensis and diploids of L. odoratus, L.

Epigenetic control may explain large within-plant heterogeneity of meiotic behavior in telocentric trisomics of rye.

In telocentric trisomics (telotrisomics) of organisms in which the chromosomes normally have two distinct arms, a single chromosome arm with a centromere is present in addition to a complete diploid set of chromosomes. It is the simplest form of polysomy and suitable for analyzing meiotic pairing and recombination patterns in situations where chromosomes compete for pairing. When no suitable meiotic chromosome markers are available, four metaphase I configurations can be distinguished.

Estimating meiotic chromosome pairing and recombination parameters in telocentric trisomics.

Telocentric trisomics (telotrisomics; one arm of a metacentric chromosome present in addition to two complete genomes) are used in theoretical studies of pairing affinities and chiasma formation in competitive situations and applied in genome analysis, gene localization, gene transfer, and breakage of close linkages. These applications require knowledge of the recombination characteristics of telotrisomics. Appropriate cytological and molecular markers and favorable chromosome morphology are not always available or applicable for quantitative analyses.

The relation between pairing preference and chiasma frequency in tetrasomics of rye.

The association pattern of marked tetrasomes of Secale chromosome 1R at meiotic first metaphase was analyzed. Two of the four chromosomes were identical with terminal C-bands at both arms; the other two were also identical but lacked C-bands and were homologous or homeologous with the first two. Four different types of heterozygotes for 1R were studied: (i).

What makes homologous chromosomes find each other in meiosis? A review and an hypothesis.

The conditions re reviewed that must be met by any model of long distance attraction and transport of homologous chromosomes to the points of intimate DNA synapsis. A proposal for possible mechanisms is presented. It includes transcription and repair factors acting on coding sequences as a preparatory step toward pairing, and the attachment of specific pairing proteins to these sequences.

Chromosome pairing affinity and quadrivalent formation in polyploids: do segmental allopolyploids exist?

When polyploid hybrids with closely related genomes are propagated by selfing or sib-breeding, the meiotic behaviour will turn into essentially autopolyploid behaviour as soon as the affinity between the genomes is sufficient to permit occasional homoeologous pairing. An allopolyploid will only be formed when the initial differentiation is sufficient to completely prevent homoeologous pairing (in some cases enhanced by specific genes), or when segregational dysgenesis prevents transmission of recombined chromosomes. A new polyploid hybrid may be considered a segmental allopolyploid and may show reduced multivalent formation as a result of preferential pairing between the least differentiated genomes.

Recombination and chiasmata: few but intriguing discrepancies.

The paradigm that meiotic recombination and chiasmata have the same basis has been challenged, primarily for plants. High resolution genetic mapping frequently results in maps with lengths far exceeding those based on chiasma counts. In addition, recombination between specific homoeologous chromosomes derived from interspecific hybrids is sometimes much higher than can be explained by meiotic chiasma frequencies.

Cytogenetics of Lathyrus palustris, a natural autohexaploid.

American as well as British forms of the perennial plant Lathyrus palustris have 2n = 42 chromosomes with one group of 6 long submetacentric chromosomes, two groups of 6 medium-long subacrocentric chromosomes, three groups of 6 medium-long to medium-short submetacentric to subacrocentric chromosomes, and one group of 6 medium-short metacentric chromosomes. One haploid complement measures 45.8 μm at somatic metaphase.

Preferential pairing estimates from multivalent frequencies in tetraploids.

Mathematical models are presented for estimating preferential pairing and chiasma parameters in amphidiploids and autotetraploids on the basis of diakinesis or metaphase I configuration frequencies and are compared with other approaches of estimating affinity. With a preferential pairing factor p, estimated from quadrivalent and trivalent frequencies, and estimated chiasmate association factors for the two arms in quadrivalents (a(qu) and b(qu) for arms A and B, respectively) and in bivalents (a(bi) and b(bi)) a perfect fit between observed and predicted configuration frequencies can often be obtained in amphidiploids of several plant species, including Solanaceae and Gramineae. Since several proven autotetraploids give very similar apparent preferential pairing estimates, the biological significance of such parameters as preferential pairing and affinity factors is considered limited.

Pachytene pairing and metaphase I configurations in a tetraploid somatic Lycopersicon esculentum x L. peruvianum hybrid.

In the tetraploid somatic hybrid between the diploid Lycopersicon species L. esculentum (tomato) and L. peruvianum, synaptonemal complexes formed quadrivalents in 73 of the 120 sets of four chromosomes (60.

Generative transmission of the extra chromosome in a rye tertiary trisomic and its relation with inbreeding depression and pollen quality.

Transmission of the extra (translocated) chromosome of tertiary trisomic T282W of rye (Secale cereale L.) upon seifing, through the male and/or the female, ranged from 0% to 36% in different inbred lines. Tetrasomics arising from simultaneous male and female transmission were not recovered and thus apparently not viable.

Mapping seed storage protein loci Sec-1 and Sec-3 in relation to five chromosomal rearrangements in rye (Secale cereale L.).

Linkage relationships were established between the secalin loci, Sec 1 (40-K gamma and omega secalins, homologous to the wheat gliadins) and Sec 3 (HMW = high-molecular-weight secalins, homologous to the wheat HMW glutenin subunits), and five chromosomal rearrangements involving chromosome 1R of rye (Secale cereale L.). These were: interchanges T273W (1RL/5RS), T306W (1RS/5RL), and T850W (1RS/ 4RL), Robertsonian centromere split Rb1RW and the interchanged Robertsonian split Rb2R/248W.

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

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.

Meiotic multivalent orientation and cell developmental delay.

In the heterozygote for the combination of an interchange (662W;3R/6R) and a Robertsonian split (3R) of rye, one type of adjacent orientation leads to trisomy in the progeny. Pollen mother cells with adjacent orientation of the translocation quinquivalent or with a trivalent and a bivalent were delayed in their development and appeared at prometaphase and metaphase later than cells with alternate quinquivalents. Delay in cell development is ascribed to unfavorable (early) prophase positioning of chromosomes.

In vitro culture of cell aggregates of rye (Secale cereale L.) at low densities.

The growth of cell aggregates from a rye suspension culture was tested at low density in three culture systems. Mass seeding was the most supportive system, followed by agarose droplets. Microdroplet culture using Cuprak dishes was the least effective system.

Identification of the chromosomes of the rye translocation tester set.

Intercrossing the Wageningen translocation tester set of rye and the series of 'Imperial' rye additions to 'Chinese Spring' wheat of Sears yielded 29 chromosome disomic translocation hybrids. Observation of trivalents led to the identification of the chromosomes of the tester set in terms of the terminology system used in the Triticinae. The analysis was complicated by very low chiasma frequencies in some short chromosome segments in the hybrids.

The taxonomy of multivalent orientation: six modes of alternate or one?

Unlike adjacent I and II, alternate I and II orientations of interchange and tetrasome quadrivalents can be considered extremes within one particular population of orientations (alternate) and thus lack sufficient distinction to justify a separate taxonomic status. Within the population up to six types may be distinguished, but the biological significance of this distinction is small.

Genetic manipulation in plant breeding: somatic versus generative.

A comparison is made between molecular/ in vitro/somatic and plant-level/generative approaches in the reconstruction of genotypes and reproductive systems. Although classical methods will remain the basis of plant breeding, a number of new somatic as well as generative genetic manipulation techniques are definitely applicable in several special situations. The first are technically more demanding, the latter are often conceptually more difficult, and both are laborious.

No 5-B compensation by rye B-chromosomes.

Chinese Spring mono-5B wheat was crossed with rye plants, with and without B-chromosomes, to produce polyhaploids with and without 5B and with and without rye B-chromosomes. As expected, absence of 5B resulted in a strong increase of homoeologous pairing. It was accompanied by a decrease in chiasmata in the rye B-chromosomes.

Identification of rye chromosomes: the Giemsa banding pattern and the translocation tester set.

The Giemsa banding pattern is given for eleven reciprocal translocations of rye, Secale cereale L., together involving all chromosomes at least once, and one telocentric substitution. It is possible to correlate the identification system based on the Giemsa pattern with that based on the translocation tester set.