Polyploids — organisms that have multiple sets of chromosomes — are common in certain plant and animal taxa, and can be surprisingly stable. The evidence that has emerged from genome analyses also indicates that many other eukaryotic genomes have a polyploid ancestry, suggesting that both humans and most other eukaryotes have either benefited from or endured polyploidy. Studies of polyploids soon after their formation have revealed genetic and epigenetic interactions between redundant genes. These interactions can be related to the phenotypes and evolutionary fates of polyploids. Here, I consider the advantages and challenges of polyploidy, and its evolutionary potential.

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The advantages and disadvantages of being polyploid

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

http://www.biologie.uni-freiburg.de/data/bio3/laux/publications_files/Mayer1998.pdf

Role of WUSCHEL in Regulating Stem Cell Fate in the Arabidopsis Shoot Meristem

The anatomy of the developing root of Arabidopsis is described using conventional histological techniques, scanning and transmission electron microscopy. The root meristem is derived from cells of the hypophysis and adjacent cells of the embryo proper. The postembryonic organization of the root is apparent in the mature embryo and is maintained in the growing primary root after germination. Cell number and location is relatively invariant in the primary root, with 8 cortical and endodermal cell files but more variable numbers of pericycle and epidermal cells. The organisation of cells in lateral roots is similar to that of the primary root but with more variability in the numbers of cell files in each layer. [3H]thymidine labeling of actively growing roots indicates that a quiescent centre of four central cells (derived from the hypophysis) is located between the root cap columella and the stele. This plate of four cells is surrounded by three groups of cells in, proximal, distal and lateral positions. The labeling patterns of these cells suggest that they are the initials for the files of cells that comprise the root. They give rise to four sets of cell files: the stele, the cortex and endodermis, the epidermis and lateral root-cap and the columella. A model of meristem activity is proposed based on these data. This description of Arabidopsis root structure underpins future work on the developmental genetics of root morphogenesis.

/pdf/cellular-organisation-of-the-arabidopsis-thaliana-root-57dirkhooj.pdf

Cellular organisation of the Arabidopsis thaliana root

We have investigated the effects on plant development of mutations in the Arabidopsis thaliana CLAVATA1 gene. In clavata1 plants, vegetative, inflorescence and floral meristems are all enlarged relative to wild type. The apical meristem can fasciate in the more severe mutant alleles, and this fasciation can occur prior to the transition to flowering. Flowers of clavata1 plants can have increased numbers of organs in all four whorls, and can also have additional whorls not present in wild-type flowers. Double mutant combinations of clavata1 with agamous, apetala2, apetala3 and pistillata indicate that CLAVATA1 controls the underlying floral meristem structure upon which these homeotic genes act. Double mutant combinations of clavata1 with apetala1 and leafy indicate CLAVATA1 plays a role in establishing and maintaining floral meristem identity, in addition to its role in controlling meristem size. In support of this, RNA expression patterns of AGAMOUS and APETALA1 are altered in clavata1 flowers.

/pdf/clavata1-a-regulator-of-meristem-and-flower-development-in-3yon9zybez.pdf

CLAVATA1, a regulator of meristem and flower development in Arabidopsis

Demonstration of the three germ layers in the shoot apex of Datura by means of induced polyploidy in periclinal chimeras

Periclinal chimeras in datura stramonium in relation to development of leaf and flower

1. Dyad formation in sporogenesis in Datura, owing to a recessive gene, was found among the F2 offspring from radium-treated pollen. 2. During megasporogenesis the first meiotic division is present, but the second meiotic division is regularly absent. As a result only two megaspores (dyads) are formed. 3. One of the megaspores disintegrates; the other undergoes three successive mitotic divisions. Its nucleus contains 12 pairs of chromosomes instead of 12 single chromosomes. 4. An embryo sac is formed containing eight nuclei, each with 24 chromosomes. 5. During microsporogenesis the same meiotic abnormality is found in the pollen mother cell. There is a failure of the second division, and after the first division a thick cell wall develops between the two nuclei, forming a dyad of microspores. 6. Each microspore, containing 12 pairs of chromosomes, develops into the male gametophyte. 7. A small percentage (about 1 to 5 per cent) of megasporocytes and microsporocytes in dyad plants develops normally. Since ...

Sophia Alexandrovna Satina

Papers

Demonstration of the three germ layers in the shoot apex of Datura by means of induced polyploidy in periclinal chimeras

Periclinal chimeras in datura stramonium in relation to development of leaf and flower

Cytological Effects of a Gene in Datura which Causes Dyad Formation in Sporogenesis

Chromosome behavior in triploids of datura stramonium. i. the male gametophyte

Disintegration of ovules in tetraploid-diploid and in incompatible species crosses in Datura