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.

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

Cell cycle regulation is of pivotal importance for plant growth and development. Although plant cell division shares basic mechanisms with all eukaryotes, plants have evolved novel molecules orchestrating the cell cycle. Some regulatory proteins, such as cyclins and inhibitors of cyclin-dependent kinases, are particularly numerous in plants, possibly reflecting the remarkable ability of plants to modulate their postembryonic development. Many plant cells also can continue DNA replication in the absence of mitosis, a process known as endoreduplication, causing polyploidy. Here, we review the molecular mechanisms that regulate cell division and endoreduplication and we discuss our understanding, albeit very limited, on how the cell cycle is integrated with plant development.

Cell cycle regulation in plant development.

Genetics and Conservation of Rare Plants

Somaclonal variation is manifested as cytological abnormalities, frequent qualitative and quantitative phenotypic mutation, sequence change, and gene activation and silencing. Activation of quiescent transposable elements and retrotransposons indicate that epigenetic changes occur through the culture process. Epigenetic activation of DNA elements further suggests that epigenetic changes may also be involved in cytogenetic instability through modification of heterochromatin, and as a basis of phenotypic variation through the modulation of gene function. The observation that DNA methylation patterns are highly variable among regenerated plants and their progeny provides evidence that DNA modifications are less stable in culture than in seed-grown plants. Future research will determine the relative importance of epigenetic versus sequence or chromosome variation in conditioning somaclonal variation in plants.

Epigenetic aspects of somaclonal variation in plants.

summary
The production of 2n gametes in plants, i.e. gametes with a somatic chromosome number, is considered to be the dominant process involved in the origin of polyploid plants. In this review, we provide a synthesis of current knowledge concerning the production of 2n gametes. Firstly, we describe the different methods used to detect and quantify the production of 2n gametes in plants, which include morphological and flow cytometry screening of the occurrence of 2n pollen, the analysis of crosses among diploid and tetraploid parents and the instigation of micro-and mega-sporogenesis. Secondly, the high level of inter- and infra-specific variation in 2n gametes production is described. Thirdly, the various cytological anomalies responsible for the production of 2n gametes are reviewed, with particular reference to the relative genetic consequences of the first and second restitution divisions that give rise to 2n gametes. Fourthly, the significance of 2n gametes in crop plant improvement is discussed, in relation to somatic chromosome doubling to obtain new polyploid varieties. In particular, we compare the genetic and yield consequences of methods based on unilateral and bilateral sexual polyploidization. Finally, we outline how knowledge of the variety of mechanisms involved in 2n gamete production have increased our understanding of the evolutionary significance of polyploidy and the population biology of polyploid plants.

Gametes with the somatic chromosome number: mechanisms of their formation and role in the evolution of autopolyploid plants.

After a short introduction, the cytogenetics of plant cell and tissue cultures and their regenerates will be discussed. In the first section discussion will focus on cytogenetic conditions “in vivo”, i.e., in the original explant: (I) widespread ocurrence of polysomaty as a consequence of endoreduplication; (2) aneusomaty, an important, though rare, cause of chromosome number variation in vivo; (3) occurrence of chromosome structural changes in differentiated tissues, especially in association with aging; (4) mixoploidy and/or gene mutations, either nuclear or organellar, present as mosaics or periclinal chimeras, especially in vegetatively propagated plants. In section two the discussion will follow with nuclear processes at and during callus induction: (1) mitosis induction in diploid (haploid) and endoreduplicated cells and initiation of cell lines with different ploidy levels; (2) chromosome endoreduplication prior to mitosis induction as a mechanism of polyploidization; (3) nuclear fragmentation (ami...

Cytogenetics of plant cell and tissue cultures and their regenerates

SUMMARYThe actually available data on the occurrence of polyploidy in plant tissues and cells have been critically reviewed. The incidence of polyploidy in the plant body, the cytological mechanisms by which the polyploid condition is produced, the physiological and general significance of polyploidy in the « soma » of plant organisms and related questions have been considered.The most common mechanism of polyploidization acting in plant tissues and cells appears to be supernumerary chromosome reproduction. This is responsible for polysomaty in root and shoot meristems as well as for polyploidy in any type of differentiated tissues and for nuclear growth in general. During the development of ephemeral tissues, such as the anther tapetum, supernumerary chromosome reproduction has been sometimes found to occur intermingled with monochromosome mitoses and/or typical endomitosis of the Gerris-type.Abortive mitoses due to spindle disturbances and chromosome stickiness or both are a very common polyploidization...

https://www.tandfonline.com/doi/pdf/10.1080/00087114.1952.10797544

Polyploidy in the Differentiation and Function of Tissues and Cells in Plants

RIASSUNTOVengono riassunte le attuali conoscenze sulla distribuzione e sul modo di origine della endopoliploidia nei tessuti vegetali e viene discusso il possibile significato della endopoliploidia nella differenziazione istologica.

https://www.tandfonline.com/doi/pdf/10.1080/00087114.1964.10796115

Endopolyploidy as a Factor in Plant Tissue Development

https://www.tandfonline.com/doi/pdf/10.1080/00087114.1989.10796966

Polyploidy in Cell Differentiation

Summary The authors have studied growth responses and induction of laterals in onion roots subjected to various experimental treatments. The following reactions have been observed: C-tumours, coiling and bending responses, basal thickening, root hair formation and browning of roots. Fasciation, sometimes present in onion roots, is brought to greater evidence by many chemical treatments; none of these seems, however, to enhance its frequency. A more or less complete inhibition of growth and induction of lateral roots are generally concomitant phenomena; an exception to this rule are sulfathiazole and the threshold concentrations of 2,4-D (2,4-dichlorophenoxyacetic acid) and its sodium salt (see Table I). No relation has been found between degree of macroscopical growth responses and intensity of lateral root formation: treatments inducing the most intense growth reactions may show very poor « root-forming » effects (for ex., nicotinic acid and moist air). For most of the chemicals tested the threshold valu...

Francesco D'Amato

Papers

Cytogenetics of plant cell and tissue cultures and their regenerates

Polyploidy in the Differentiation and Function of Tissues and Cells in Plants

Endopolyploidy as a Factor in Plant Tissue Development

Polyploidy in Cell Differentiation

Reazioni di natura auxinica ed effetti rizogeni in Allium Cepa L.