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doi: 10.1111/j.1095-8312.2004.00328.xpmid: N/A
AbstractIt is timely to re-examine the phenomenon of polyploidy in plants. Indeed, the power of modern molecular technology to provide new insights, and the impetus of genomics, make polyploidy a fit, fashionable and futuristic topic for review. Some historical perspective is essential to understand the meaning of the terms, to recognize what is already known and what is dogma, and to frame incisive questions for future research. Polyploidy is important because life on earth is predominantly a polyploid phenomenon. Moreover, civilization is mainly powered by polyploid food – notably cereal endosperm. Ongoing uncertainty about the origin of triploid endosperm epitomizes our ignorance about somatic polyploidy. New molecular information makes it timely to reconsider how to identity polyploids and what is a polyploid state. A functional definition in terms of a minimal genome may be helpful. Genes are known that can raise or lower ploidy level. Molecular studies can test if, contrary to dogma, the relationship between diploids and polyploids is a dynamic two-way system. We still need to understand the mechanisms and roles of key genes controlling ploidy level and disomic inheritance. New evidence for genome duplications should be compared with old ideas about cryptopolyploidy, and new views of meiosis should not ignore premeiotic genome separation. In practice, new knowledge about polyploidy will be most useful only when it reliably predicts which crops can be usefully improved as stable autopolyploids and which genomes combined to create successful new allopolyloids.
FURLONG, REBECCA F.; HOLLAND, PETER W. H.
doi: 10.1111/j.1095-8312.2004.00329.xpmid: N/A
AbstractOver 30 years ago, Susumu Ohno proposed that two rounds of polyploidy occurred early in vertebrate evolution. We re-examine this proposal using three recent lines of evidence. First, total gene number estimates from completely sequenced genomes suggest an increase in total gene number somewhere along the vertebrate or prevertebrate lineage, compatible with Ohno's model. Second, analyses of homeobox and other genes from amphioxus reveal very extensive gene duplication specifically on the vertebrate lineage. This refines the timing of putative polyploidy to after the divergence of amphioxus and vertebrates. Third, the existence of four-fold paralogy regions in the human genome is suggestive of two rounds of polyploidy, although other explanations are possible. We propose an experimental test, based on chromosomal localization of genes in amphioxus, that should resolve whether paralogy regions are indeed remnants of duplication in vertebrate ancestry.
COMBER, STEVEN C. LE; SMITH, CARL
doi: 10.1111/j.1095-8312.2004.00330.xpmid: N/A
AbstractFishes are the most speciose group of vertebrates, with more than 24 000 species. They are characterized by great diversity in ecology, morphology, life history, behaviour and physiology. Here, the phylogenetic patterns of orders in which polyploidy has been recorded are considered, with special reference to patterns of species richness and hybridization: these orders include such phylogenetically diverse taxa as the Lepidosireniformes (lungfish) and the Perciformes (perch). Examples, predominantly drawn from the Cyprinidae and Salmonidae, are used to illustrate attributes of polyploidy in fishes. It is concluded (i) that polyploidy may have been of considerable importance in the evolution of fishes, and (ii) that fishes, with their diverse life histories, represent a useful model system with which to test theories relating to the origin and consequences of polyploidy that have been derived from work on plants.
GALLARDO, MILTON H.; KAUSEL, G.; JIMÉNEZ, A.; BACQUET, C.; GONZÁLEZ, C.; FIGUEROA, J.; KÖHLER, N.; OJEDA, R.
doi: 10.1111/j.1095-8312.2004.00331.xpmid: N/A
AbstractThe discovery of tetraploidy in the red viscacha rat, Tympanoctomys barrerae (4n = 102) has emphasized the evolutionary role of genome duplication in mammals. The tetraploid status of this species is corroborated here by in situ PCR and Southern blot analysis of a single-copy gene. The species meiotic configuration strongly suggests a hybrid derivation. To investigate the origin of T. barrerae further, the recently described Pipanacoctomys aureus was studied. This 92-chromosome species also has a duplicated genome size, redundant gene copy number and diploid-like meiotic pairing, consistent with an event of allotetraploidization. Phylogenetic analysis of mitochondrial sequences indicates sister-group relationships between these two tetraploid rodents. The new karyotypic data and the phylogenetic relationships suggest the participation of the ancestral lineages of Octomys mimax in the genesis of P. aureus. The high overall DNA similarity and shared band homology revealed by genomic Southern hybridization as well as matching chromosome numbers between O. mimax and the descendant tetraploid species support the notion of introgressive hybridization between these taxa.
doi: 10.1111/j.1095-8312.2004.00332.xpmid: N/A
AbstractAlthough polyploidy has been involved in speciation in both animals and plants, the general perception is often that it is too rare to have been a significant factor in animal evolution and its role in plant diversification has been questioned. These views have resulted in a bias towards explanations for what deters polyploidy, rather than the somewhat more interesting question of the mechanisms by which polyploidy arises and becomes established in both plants and animals. The evidence for and against some of the traditional views on polyploidy is reviewed, with an attempt to synthesize factors promoting evolution through genome duplication in both groups. It is predicted that polyploidy should be more common in temperate than in tropical breeders because environmental fluctuations may promote unreduced gamete formation, it should be most common in organisms with sufficient numbers of gametes that random meiotic problems can be overcome, and it should be more frequent when mechanisms to promote assortative mating are a direct byproduct of genome duplication.
ABBOTT, RICHARD J.; LOWE, ANDREW J.
doi: 10.1111/j.1095-8312.2004.00333.xpmid: N/A
AbstractTwo new polyploid species of Senecio have originated in the British Isles in recent times following hybridization between native S. vulgaris (2n = 40) and introduced S. squalidus (2n = 20). One of these is the allohexaploid S. cambrensis (2n = 60), the other is the recombinant tetraploid S. eboracensis (2n = 40). We review what is known about when and how each species originated, and their reproductive isolation from parents due to high selfing rates. We also review evidence that suggests S. cambrensis may have undergone rapid genome evolution since its origin, and comment on the risks of extinction to each species due to chance factors operating during the early establishment phase. The discovery of both species soon after their origin provides an unparalleled opportunity to examine two different but related forms of speciation following hybridization between the same parent species. Further detailed study of the ecology and genomics of S. cambrensis and S. eboracensis will help improve our understanding of the process of polyploid speciation in plants.
AINOUCHE, MALIKA L.; BAUMEL, ALEX; SALMON, ARMEL
doi: 10.1111/j.1095-8312.2004.00334.xpmid: N/A
AbstractSpartina anglica arose during the end of the 19th century in England by hybridization between the indigenous Spartina maritima and the introduced East American Spartina alterniflora and following genome duplication of the hybrid (S. × townsendii). This system allows investigations of the early evolutionary changes that accompany stabilization of a new allopolyploid species in natural populations. Various molecular data indicate that S. anglica has resulted from a unique parental genotype. This young species contains two distinctly divergent homoeologous genomes that have not undergone extensive change since their reunion. No burst of retroelements has been encountered in the F1 hybrid or in the allopolyploid, suggesting a ‘structural genomic stasis’ rather than ‘rapid genomic changes’. However, modifications of the methylation patterns in the genomes of S. × townsendii and S. anglica indicate that in this system, epigenetic changes have followed both hybridization and polyploidization.
SOLTIS, DOUGLAS E.; SOLTIS, PAMELA S.; PIRES, J. CHRIS; KOVARIK, ALES; TATE, JENNIFER A.; MAVRODIEV, EVGENY
doi: 10.1111/j.1095-8312.2004.00335.xpmid: N/A
AbstractTragopogon mirus Ownbey and T. miscellus Ownbey are allopolyploids that formed repeatedly during the past 80 years following the introduction of three diploids (T. dubius Scop., T. pratensis L. and T. porrifolius L.) from Europe to western North America. These polyploid species of known parentage are useful for studying the consequences of recent and recurrent polyploidization. We summarize recent analyses of the cytogenetic, genomic and genetic consequences of polyploidy in Tragopogon. Analyses of rDNA ITS (internal transcribed spacer) + ETS (external transcribed spacer) sequence data indicate that the parental diploids are phylogenetically well separated within Tragopogon (a genus of perhaps 150 species), in agreement with isozymic and cpDNA data. Using Southern blot and cloning experiments on tissue from early herbarium collections of T. mirus and T. miscellus (from 1949) to represent the rDNA repeat condition closer to the time of polyploidization than samples collected today, we have demonstrated concerted evolution of rDNA. Concerted evolution is ongoing, but has not proceeded to completion in any polyploid population examined; rDNA repeats of the diploid T. dubius are typically lost or converted in both allopolyploids, including populations of independent origin. Molecular cytogenetic studies employing rDNA probes, as well as centromeric and subtelomeric repeats isolated from Tragopogon, distinguished all chromosomes among the diploid progenitors (2n = 12). The diploid chromosome complements are additive in both allopolyploids (2n = 24); there is no evidence of major chromosomal rearrangements in populations of either T. mirus or T. miscellus. cDNA-AFLP display revealed differences in gene expression between T. miscellus and its diploid parents, as well as between populations of T. miscellus of reciprocal origin. Approximately 5% of the genes examined in the allopolyploid populations have been silenced, and an additional 4% exhibit novel gene expression relative to their diploid parents. Some of the differences in gene expression represent maternal or paternal effects. Multiple origins of a polyploid species not only affect patterns of genetic variation in natural populations, but also contribute to differential patterns of gene expression and may therefore play a major role in the long-term evolution of polyploids.
DAVID, JACQUES L.; BENAVENTE, ELENA; BRÈS-PATRY, CÉCILE; DUSAUTOIR, JEAN-CLAUDE; ECHAIDE, MERCEDES
doi: 10.1111/j.1095-8312.2004.00336.xpmid: N/A
AbstractSpontaneous hybridization between durum wheat (Triticum turgidum durum) and Aegilops ovata is regularly observed in nature. The frequency of spontaneous amphiploidy in sympatric populations was estimated at 10−6 (direct in situ observations and germinated seed collected from A. ovata plants). In nursery conditions some genotype combinations gave frequencies that were much higher at 10−3. Genomic in situ hybridization revealed that fertile amphiploids had arisen through unreduced gametes, and that some of them carried wheat − A. ovata recombinant chromosomes. The frequency of production of unreduced gametes is probably genetically inherited. Amphiploids provide a route for gene flow, including that of transgenes, to the wild. Gene flow could potentially be minimized through the choice of wheat cultivars that produce a low frequency of unreduced gametes.
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