Polyploidy is widely acknowledged as a major mechanism of adaptation and speciation in plants. The stages in polyploid evolution include frequent fertility bottlenecks and infrequent events such as gametic nonreduction and interspecific hybridization, yet little is known about how these and other factors influence overall rates of polyploid formation. Here we review the literature regarding polyploid origins, and quantify parameter values for each of the steps involved in the principal pathways. In contrast to the common claim that triploids are sterile, our results indicate that the triploid bridge pathway can contribute significantly to autopolyploid formation regardless of the mating system, and to allopolyploid formation in outcrossing taxa. We estimate that the total rate of autotetraploid formation is of the same order as the genic mutation rate (10 i5 ), and that a high frequency of interspecific hybridization (0.2% for selfing taxa, 2.7% for outcrossing taxa) is required for the rate of tetraploid formation via allopolyploidy to equal that by autopolyploidy. We conclude that the rate of autopolyploid formation may often be higher than the rate of allopolyploid formation. Further progress toward understanding polyploid origins requires studies in natural populations that quantify: (a) the frequency of unreduced gametes, (b) the effectiveness of triploid bridge pathways, and (c) the rates of interspecific hybridization.

/pdf/pathways-mechanisms-and-rates-of-polyploid-formation-in-45lhl9s528.pdf

Pathways, mechanisms, and rates of polyploid formation in flowering plants

https://www.cell.com/cell/pdf/0092-8674(88)90494-1.pdf

Isolation of a higher eukaryotic telomere from Arabidopsis thaliana

The intrachromosomal distribution of non-telomeric sites of the (TTAGGG)n telomeric repeat was determined for 100 vertebrate species. The most common non-telomeric location of this sequence was in the pericentric regions of chromosomes. A variety of species showed relatively large amounts of this sequence present within regions of constitutive heterochromatin. We discuss possible relationships between the non-telomeric distribution of the (TTAGGG)n sequence and the process of karyotype evolution, during which these sites may provide potential new telomeres.

Distribution of non-telomeric sites of the (TTAGGG)n telomeric sequence in vertebrate chromosomes.

1)

Almost all polyploids arise by way of unreduced gametes; other mechanisms occur, but are negligible.




2)

The most widespread and common event is (2n+n) reproduction.




3)

Triploids so produced frequently yield 4X plants on backcrossing and 6X plants on selfing.




4)

Spontaneous polyploids appear repeatedly, but their fate depends on vigor and competitive ability.




5)

The polyploids most frequently produced are probably strict autoploids, but they often lack competitive ability and may not become successfully established.




6)

Wide crosses occur in nature but not very often; if the hybrid is vigorous, it may yield a stable, fertile polyploid in the second or later generations via unreduced gametes.




7)

Wide crosses by way of (2n+n) reproduction are probably more common and successful than the above (6) pathway.




8)

The most likely pathway to a successful polyploid in nature involves neither wide crosses nor strict autoploidy but crosses between races, ecotypes and cytotypes within a biological species.




9)

The more or less autoploids so produced may generate genuine alloploids by interspecific hybridization at polyploid levels.




10)

The classic explanation of alloploidyvia “chromosome doubling” of a sterile diploid interspecific hybrid is misleading in most cases, and this route is probably negligible in the evolution of polyploid systems.

https://link.springer.com/content/pdf/10.1007/BF02860830.pdf

On Ö. Winge and a Prayer: The origins of polyploidy

A detailed survey is given of the types and classification of primary structural changes that can be induced in chromosomes and observed at the first metaphase after the initial damage. Comments upon identification and scoring are given for the benefit of new workers. The annotation concludes with a brief discussion of the potential relationships between the primary types, and the secondary or derived types encountered in clinical studies.

https://jmg.bmj.com/content/jmedgenet/13/2/103.full.pdf

Classification and relationships of induced chromosomal structual changes.

1. Fragments pair with major chromosomes inTradescantia by lateral chiasmata whose chromatid structure is revealed by pre-treatment. They do not then develop the usual major spiral. 2. Less contracted chromosomes in mutant cells have more coils. Spiralization therefore consists in reducing the number of coils. 3. Interlocked chromosomes, like multivalents, may have linear, twisted, or discordant orientations according to the method of interlocking. 4. The type of distribution of the bivalents on the metaphase plate inT. bracteata depends on the numbers of their chiasmata and on the radial or tangential adjustment of the individual bivalents. 5. The central chromosomes of the metaphase plate are pushed to the edge of the group during anaphase in some forms and species. This change may take place asymmetrically both at mitosis and meiosis. It shows that the repulsion from the poles is still acting on the chromosomes while they are moving towards the poles. Thus the anaphase in some forms comes to correspond with the hollow metaphase in others. 6.Rhoeo discolor and diploid, triploid and tetraploid species ofTradescantia show the results of inversion crossing-over under normal conditions. Fragments are small inRhoeo and in the tetraploids, showing that the assumption of pairing and crossing-over being restricted to the end regions is justified.

Chromosome behaviour and structural hybridity in the tradescantiae. II.

C~Ro~oso~ES are made up of str ingsof particles, one of which, the centromere, can Mways be distinguished from the rest, the genes. Centromeres and genes agree in one property: they reproduce their like by forming pairs of similar particles which lie side by side in similar strings in the same linear order. It is to this thai Shsy owe their common maintenance as parts of chromosomes. In all other respects they behave &ifferently. The centromeres are active in moving the chromd~somes on the @indle. The genes are. largely inactive. The centromeres of different chromosomes are all alike in their :form and behaviouz. The genes are largely differentiated from one another. The centromeres all repel one a.nother indifferently, while the genes a,gtraet one another specifically and usually in }?airs. Finally, the cen.tromeres divide at metaphase of mitosis but the genes div.ide in She preceding resting s~a,ge. A{ meiosis, owing to the preeocRy of the sxternaI phases, both divide later in rela$ion to these phases, the centromeres at the second[ metaphase ~nd ~he genes daring the first proph~se. In spite o f the timing alteration of meiosis i~ will be seen. that the distinctive conditions psrsis~ for each : the genes req~dre the protection of the nucleus, the cen.tromeres the freedom of the cytoplasm, for bheir division.. These conditions are distinct ~nd in fact usnally incompatible.

Misdivision and the genetics of the centromere

1. The determination of the relationship of crossing-over to chiasmata and of chiasmata to segregation has made it possible to define the conditions of variation of pairing chromosomes in complex-heterozygotes and sex-heterozygotes and to analyse their behaviour in genetic terms. The object of the present studies is, first, to test the predictions made with regard to sex-chromosomes deducing their genetic structure from cytological observations and, secondly, to note those respects in which the behaviour of the sex-chromosomes throws light on the problems of chromosome mechanics.

The genetical and mechanical properties of the sex-chromosomes

Irradiation affects the time scale of development by which we measure its effects in the breakage of chromosomes. This explains why single-time comparisons have hitherto shown discordant results. Our comparative breakage-time series enable us to measure both effects and to show direct proportionality of dose and breakage in the pollen grain.

Chromosome breakage and the nucleic acid cycle

1

The pollen mother-cell divisions in triploid tulips are described At prophase only pairs of homologous chromosomes associate at any particular point, but different pairs associate at different points The fact that three threads lie side by side proves that each corresponds to a whole chromosome rather than to half a chromosome; the parasynaptic interpretation is thus substantiated




2

After the formation of diplotene chiasmata, either all three corresponding chromosomes are left attached as trivalents, or one may come to lie entirely free




3

Variation occurs in the number of trivalents at the first metaphase, and in the number of chiasmata joining the associated chromosomes




4

The association of the chromosomes agrees with the stated requirements of the chromosome theory if we regard the genetical evidence fromDrosophila as applicable, and pairing as a condition of crossing-over




5

InTulipa Clusiana DC associations of three, four or five homologous chromosomes may occur at the first metaphase as a result of earlier random association amongst homologous chromosomes

C. D. Darlington

Papers

Chromosome behaviour and structural hybridity in the tradescantiae. II.

Misdivision and the genetics of the centromere

The genetical and mechanical properties of the sex-chromosomes

Chromosome breakage and the nucleic acid cycle

Meiosis in polyploids