Polyploid speciation inHedera (Araliaceae): Phylogenetic and biogeographic insights based on chromosome counts and ITS sequences

Variation in chromosome number and internal transcribed sequences (ITS) of nrDNA is used to infer phylogenetic relationships of a wide range ofHedera species. Polyploidy was found to be frequent inHedera, with diploid, tetraploid, hexaploid and octoploid populations being detected. Nucleotide additivity occurs in the ITS sequences of one tetraploid (H. hibernica) and two hexaploid species (H. maderensis, H. pastuchovii), suggesting that all three species originated by allopolyploidisation. ITS sequence polymorphism and nucleotide characters may indicate the presence of an ancient genome persistent only in some allopolyploid species. Phylogenetic analyses of ITS sequence data reveal two lineages ofHedera: one containing all sequences belonging to extant diploids plus the tetraploidH. algeriensis, and a second that includes this ancient ITS type and others exclusive to several polyploid species. The origin of the polyploids is evaluated on the basis of morphology, chromosome counts, ITS sequence polymorphism, and phylogenetic analyses. Reconstruction of reticulate evolution inHedera agrees with two allopolyploid areas on both sides of the Mediterranean basin. Morphological, molecular and cytological evidence also suggests an active dispersal ofHedera populations that may account for three independent introductions in Macaronesia.     

A chromosome-specific sequence common to the B genome of polyploid wheat and Aegilops searsii

A low-copy, non-coding chromosome-specific DNA sequence, isolated from common wheat, was physically mapped to the distal 19% region of the long arm of chromosome 3B (3BL) of common wheat. This sequence, designated WPG118, was then characterized by Southern hybridization, PCR amplification and sequence comparison using a large collection of polyploid wheats and diploid Triticum and Aegilops species. The data show that the sequence exists in all polyploid wheats containing the B genome and absent from those containing the G genome. At the diploid level, it exists only in Ae. searsii, a diploid species of section Sitopsis, and not in other diploids including Ae. speltoides, the closest extant relative to the donor of the B genome of polyploid wheat. This finding may support the hypothesis that the B-genome of polyploid wheat is of a polyphyletic origin, i.e. it is a recombined genome derived from two or more diploid Aegilops species.     

Does polyploidy occur in central European species of the family Sphaeriidae (Mollusca: Bivalvia)?

Some representatives of the bivalve family Sphaeriidae are assumed to be polyploid. In this study, 11 sphaeriid species (nine of the genus Pisidium, one of Musculium, and one of Sphaerium) inhabiting central Europe were studied karyologically, 10 of them for the first time. Analysis revealed high chromosome numbers (from 140 to 240). To elucidate the origin of high chromosome numbers, DNA contents were measured by flow cytometry in 5 of the studied species and, for comparison, in S. corneum and S. nucleus, which are known to be diploid (2n=30). Species with high chromosome counts yielded very similar DNA contents that are not higher than in the related species with low diploid numbers. This finding contradicts a possible origin of these species by recent polyploidization or hybridization of related species. Chromosome complements of the investigated species with high chromosome numbers differ from those with low 2n in their small chromosome size and the high proportion of subtelo- or acrocentric chromosomes. This indicates their possible origin either by an ancient polyplodization event followed by chromosomal rearrangements or by multiple chromosome fissions.     

Genome evolution in a Mediterranean species complex: phylogeny and cytogenetics of Helictotrichon (Poaceae) allopolyploids based on nuclear DNA sequences (rDNA,topoisomerase gene) and FISH

We examined the molecular phylogeny and chromosomal features of European Helictotrichon species to explore the relationships within the genus and to investigate the origin of several polyploids. Using both approaches, molecular and cytogenetic, revealed the strong impact of allopolyploidization on genome organization from chromosome structure to sequence level. Our research focused on Mediterranean and endemic species of the Alps. Altogether, the molecular phylogenetic analyses include a sample of 17 Helictotrichon species and subspecies, used DNA sequences from the nuclear ribosomal (nr) internal transcribed spacer region (ITS) and the single copy gene topoisomerase 6 (Topo6), and were analysed by maximum parsimony and Bayesian methods. Karyotype structures were investigated by fluorescence in situ hybridization (FISH) and fluorochrome banding. Cytogenetic characters were mapped on the combined phylogenetic tree. The absence or comparatively rare occurrence of different ITS sequence types in some (allo-) polyploid species of Helictotrichon suggests frequent intergenomic homogenization of ribosomal DNA (rDNA) loci due to the phenomenon of concerted evolution. This result implies that the ITS region is not an ideal marker to study polyploid evolution of these grasses. The phylogenetic analysis of the Topo6 region revealed three major clades that concur with three different copy types (termed SAR, SET, PAR), representing the major genome groups in Helictotrichon. A comparison of the molecular phylogenetic trees with the chromosome and karyotype structure supports allopolyploidy of several Helictotrichon species and identifies potential genome donors. A correlation between molecular phylogenetic/cytogenetic results and geographic distribution is expressed by a west-east disjunction, in the narrower or wider sense, of the analysed species. While SAR represents a geographically narrowly distributed southwest Mediterranean genome group, PAR and SET are very widespread (Mediterranean to Asia) and encompass several instances of west-east disjunctions.     

The origin of polyploid genomes of bluegrasses <Emphasis Type="Italic">Poa</Emphasis> L. and Gene flow between northern pacific and sub-Antarctic Islands

The involvement of present-day diploid bluegrass species in the formation of polyploid genomes was investigated using comparison of sequences of internal transcribed spacers ITS1 and ITS2, and the 5.8S rRNA sequence. It was demonstrated that highly polyploid New Zealand bluegrasses, P. cita (2n = 84; ca. 96 to 100), P. chathamica (2n = 112), and P. litorosa (2n 263–266) formed separate highly supported clade together with tetraploids (2n = 28) P. intrusa, P. anceps, and P. triodioides (Austrofestuca littoralis). Among the diploid species (2n = 14), the closest relatives of these species, as well as of the polyploid species of section Poa, are the genomes of Eurasian species P. remota, P. chaixii (sect. Homalopoa), P. densa (sect. Bolbophorum), and P. sibirica (sect. Macropoa). Nuclear genomes of polyploid Stenopoa, Tichopoa, Oreinos, and Secundae are definitely related to the genome of Arctic species P. pseudoabbreviata (sect. Abbreviatae). On the contrary, judging by the genes for nuclear 45S rRNA, genomes of diploid P. trivialis (sect. Pandemos), P. annua, and P. supina (sect. Ochlopoa both) are only remotely related to the genomes of highly polyploid species (p-distances between them and other bluegrass species from different sections of subgenus Poa constitute 6–10% and 11–15%, respectively). The conclusion on the relationships between highly polyploid and diploid bluegrass species was tested using analysis of synapomorphic mutations in the 5.8S rRNA gene. It was demonstrated that genomes of Poa eminens (2n = 42) and P. schischkinii (2n = 70) (sect. Arctopoa both) were noticeably different in ITS regions from the genomes of the members of the type subgenus Poa. A comparison of the Arctopoa ITS regions showed that the differences between them constituted only 0.2%. At the same time, p-distances between the Arctopoa ITS and those from the species belonging to other sections of the genus Poa varied from 5 to 14%. South American species P. chonotica (sect. Andinae) (= Nicoraepoa chonotica) (2n = 42) was found to be related to Arctagrostis, Festucella, and Hookerochloa, being at the same time quite distant from the other species of the genus Poa. Polymorphic in chromosome number highly polyploid species of Northern Hemisphere, P. arctica (2n = 42 to 106), P. turneri (2n = 42, 63 to 64), and P. smirnowii (2n = 42, 70) (sect. Malacanthae) are relative to a large group of tetraploid (2n = 28) endemic bluegrass species from New Zealand and sub-Antarctic islands (P. novaezelandiae and allied species).     

A bicontinental origin of polyploid Australian/New Zealand Lepidium species (Brassicaceae)? Evidence from genomic in situ hybridization

Background and Aims

Incongruence between chloroplast and nuclear DNA phylogenies, and single additive nucleotide positions in internal transcribed spacer (ITS) sequences of polyploid Australian/New Zealand (NZ) Lepidium species have been used to suggest a bicontinental hybrid origin. This pattern was explained by two trans-oceanic dispersals of Lepidium species from California and Africa and subsequent hybridization followed by homogenization of the ribosomal DNA sequence either to the Californian (C-clade) or to the African ITS-type (A-clade) in two different ITS-lineages of Australian/NZ Lepidium polyploids.

Methods

Genomic in situ hybridization (GISH) was used to unravel the genomic origin of polyploid Australian/NZ Lepidium species. Fluorescence in situ hybridization (FISH) with ribosomal DNA (rDNA) probes was applied to test the purported ITS evolution, and to facilitate chromosome counting in high-numbered polyploids.

Key Results

In Australian/NZ A-clade Lepidium polyploids, GISH identified African and Australian/NZ C-clade species as putative ancestral genomes. Neither the African nor the Californian genome were detected in Australian/NZ C-clade species and the Californian genome was not detected in Australian/NZ A-clade species. Five of the eight polyploid species (from 7x to 11x) displayed a diploid-like set of rDNA loci. Even the undecaploid species Lepidium muelleriferdinandi (2n = 11x = 88) showed only one pair of each rDNA repeat. In A-clade allopolyploids, in situ rDNA localization combined with GISH corroborated the presence of the African ITS-type.

Conclusions

The nuclear genomes of African and Australian/NZ C-clade species were detected by GISH in allopolyploid Australian/NZ Lepidium species of the A-clade, supporting their hybrid origin. The presumed hybrid origin of Australian/NZ C-clade taxa could not be confirmed. Hence, it is assumed that Californian ancestral taxa experienced rapid radiation in Australia/NZ into extant C-clade polyploid taxa followed by hybridization with African species. As a result, A-clade allopolyploid Lepidium species share the Californian chloroplast type and the African ITS-type with the C-clade Australian/NZ polyploid and African diploid species, respectively.Key words: Lepidium, Brassicaceae, FISH, GISH, hybridization, polyploidy, long-distance dispersal, ITS, rDNA, Australia, New Zealand     

CHROMOSOME RACES IN CLAYTONIA LANCEOLATA (PORTULACACEAE)

Chromosome numbers of n = 8, 12, and 16 were determined for 11 populations of Claytonia lanceolata occurring in the southwestern Rocky Mountains of Utah. No evidence of the wide infra-populational variation of chromosome numbers known in the related eastern species, C. virginica, was observed. The chromosome numbers in C. lanceolata probably evolved from a base number of x = 8. Diploids(n = 8) apparently produced tetraploids (n = 16) of putative autoploid origin. Pairing relationships, including the presence of univalents, bivalents, and trivalents, suggest the chromosome numbers of n = 12 are triploids derived from natural hybridization between diploids and tetraploids. Higher chromosome numbers previously reported in C. lanceolata from Colorado, and presumably based on x = 12, can be explained by subsequent polyploid increases in the triploids. The diploid and tetraploid populations analyzed in this study occupy different ecological habitats. The diploids occur at lower elevations along the foothills, whereas the tetraploids are restricted to higher montane and sub-alpine elevations. The triploids were discovered at intermediate elevations.     

Chromosome number variation and polyploidy in the genus Echinocereus (Cactaceae)

Analyses of meiotic and mitotic chromosomes were undertaken in 16 taxa of Echinocereus belonging to 12 species and all seven taxonomic sections (sensu Taylor). Chromosome numbers are reported for the first time for eight taxa, and previously published chromosome counts are confirmed for the remaining eight. Both diploid and polyploid counts were obtained. Eleven (69%) of the taxa surveyed were diploid (2n = 22); the five varieties of E. engelmannii were polyploid (2n = 44). Overall, chromosome counts are available for 23 of the 48 proposed species (sensu Taylor). Of these, 19 (82%) are diploid, and four (18%) are polyploid. Polyploid cytotypes are most common in the primitive sections, e.g., sections Erecti and Triglochidiatus, which suggests that polyploidy is probably a derived condition in Echinocereus. Polyploid taxa range from medium to high latitudes and elevations relative to the overall distribution of the genus. Polyploidy, hybridization, and cryptic chromosomal rearrangements are thought to be the major causes of the speciation events of the genus.     

Chloroplast DNA variation in diploid and polyploid species of Bromus (Poaceae) subgenera Festucaria and Ceratochloa

Summary Chloroplast DNA (cpDNA) restriction endonuclease patterns are used to examine phylogenetic relationships between Bromus subgenera Festucaria and Ceratochloa. Festucaria is considered monophyletic based on the L genome, while Ceratochloa encompasses two species complexes: the B. catharticus complex, which evolved by combining three different genomes, and the B. carinatus complex, which is thought to have originated from hybridization between polyploid species of B. catharticus and diploid members of Festucaria. All species of subgenus Ceratochloa (hexaploids and octoploids) were identical in chloroplast DNA sequences. Similarly, polyploid species of subgenus Festucaria, except for B. auleticus, were identical in cpDNA sequences. In contrast, diploid species of subgenus Festucaria showed various degrees of nucleotide sequence divergence. Species of subgenus Ceratochloa appeared monophyletic and phylogenetically closely related to the diploid B. anomalus and B. auleticus of subgenus Festucaria. The remaining diploid and polyploid species of subgenus Festucaria appeared in a distinct grouping. The study suggests that the B. catharticus complex must have been the maternal parent in the proposed hybrid origin of B. carinatus complex. Although there is no direct evidence for the paternal parent of the latter complex, the cpDNA study shows the complex to be phylogenetically very related to the diploid B. anomalus of subgenus Festucaria.     

Quantitative DNA variation between and within chromosome complements of Vigna species (Fabaceae)

Cytogenetical investigations, so far, on the organisation and evolution of the genomes of Vigna species have proved difficult due to small chromosome size, large chromosome number and uniformity in chromosome shape and size within and between the complements. In this investigation the nature and extent of DNA variation between thirteen diploid and one polyploid species have been estimated. The DNA variation between diploid species was small and species clustered around a mean value of 2.7 pg. The polyploid species had a greater DNA value of 4.95 pg. No significant variation in 2C DNA content was found between accessions of V. radiata. A comparison of the distribution of DNA among the chromosomes within complements has shown that the excess DNA acquired in evolution was distributed evenly in all chromosomes despite significant differences in chromosome size. The relative changes in chromatin area and DNA density which accompany evolutionary DNA variation was also compared.     

Phylogenetic relationships and character evolution in Primula L.: The usefulness of ITS sequence data

ABSTRACT

The main goals of this research were to reconstruct the infrageneric phylogeny of the genus Primula based on both nuclear and chloroplast DNA sequences, and to use the resulting phylogenies to elucidate the evolution of breeding systems, morphological characters, chromosome number, and biogeographic distribution in the genus. In this paper, the results of a pilot study based on the nuclear ribosomal Internal Transcribed Spacer (ITS) region are described. ITS sequences from 21 taxa produced a number of variable characters sufficient to resolve relationships among sections. The resulting phylogeny confirmed the monophyly of sections Auricula and Aleuritia. Sections Armerina, Proliferae, Crystallophlomis, Parryi, and Auricula, with a base chromosome number of x = 11, and sect. Aleuritia, with a base chromosome number of x = 9, formed two well supported clades. The ITS topology also suggested that leaves with revolute vernation, previously believed to be a derived state, might represent the ancestral condition in Primula, with later reversals to the involute condition. Finally, this initial ITS tree provides preliminary support to the proposed role of the widespread, diploid and heterostylous P. mistassinica as having given origin to the polyploid, homostylous P. incana and P. laurentiana.     

Biosystematics and evolutionary studies in Indian Drimia species

Abstract The taxonomic position and genetic relationship within Indian Drimia species is controversial due to their morphological similarities and genomic complexities. The present work gives an insight on the genetic relationship between Indian Drimia species on the basis of their karyotype, pollen morphology, flower opening characteristics, hybridization behavior, and by use of DNA sequence of two molecular markers (internal transcribed spacers [ITS] and maturase K [matK]). The karyotypic studies of Indian Drimia species revealed various polyploid forms making their identification and delimitation more difficult. The five species of Indian Drimia are grouped into two complexes, indica complex and wightii complex on the basis of their pollen morphology, karyotype, and hybridization behavior. These two groups were found to be evolving separately. The cytomorphological studies of wightii complex revealed that it is evolving through polyploid and chromosome repatterning, while indica complex have adapted polyploid as well as hybridization for evolution. Phylogeny obtained from DNA sequences of molecular markers (ITS and matK) confirmed that the indica complex and wightii complex are evolving parallely, by grouping them in two clusters. Thus, a combination of conventional and molecular methods proved to be of great use for delimiting a small but complex group of Indian Drimia species.     

Relationships betweenNor-loci from differentTriticeae species

TheNor-loci of polyploid wheats and their putative diploid progenitor species were assayed by probing isolated nuclear DNA with ribosomal DNA spacer sequences (spacer rDNA sequences, isolated by cloning), from theNor-loci of genomes B (Triticum aestivum), G (T. timopheevi), B (syn. S,T. speltoides), A (T. monococcum) and V (Dasypyrum villosum). DNA samples for analysis were digested with the restriction endonuclease Taq 1 and assayed by DNA-DNA hybridization under standard (37°C) and high stringency (64°C) conditions. The assay procedure emphasized differences between the divergent spacer sequences of the polyploid species and allowed relative homologies to the respective sequences in diploid species to be established. — The studies indicated thatT. timopheevi andT. speltoides contain different sets of spacer rDNA sequences which were readily distinguishable and, in the case ofT. timopheevi, assigned toNor-loci on different chromosomes. This contrast with the spacer rDNA sequences of the majorNor-loci on chromosomes 1 B and 6 B inT. aestivum, which were difficult to distinguish and were deduced to contain very similar sequences. Among the diploid progenitor species only the spacer rDNA fromT. speltoides shared close homology with polyploid wheat species. OneNor-locus inT. timopheevi (on chromosome 6 G) did not show close homology with any of the rDNA spacer probes available. — The data suggestsT. speltoides was the origin of someNor-loci for both theT. timopheevi andT. turgidum lines of tetraploid wheats. The possibility that the 6GNor-locus inT. timopheevi may have derived from an unknown diploid species by introgressive hybridization is discussed. The spacer rDNA sequence probe fromT. monococcum shared good homology with some accessions ofD. villosum and a line ofT. dicoccoides; the implications of this finding for evolution of present-day wheats are discussed.     

Centromeric repetitive DNA sequences in the genus Brassica

Representatives of two major repetitive DNA sequence families from the diploid Brassica species B. campestris and B. oleracea were isolated, sequenced and localized to chromosomes by in situ hybridization. Both sequences were located near the centromeres of many chromosome pairs in both diploid species, but major sites of the two probes were all on different chromosome pairs. Such chromosome specificity is unusual for plant paracentromeric repetitive DNA. Reduction of stringency of hybridization gave centromeric hybridization sites on more chromosomes, indicating that there are divergent sequences present on other chromosomes. In tetraploid species derived from the diploids, the number of hybridization sites was different from the sum of the diploid ancestors, and some chromosomes had both sequences, indicating relatively rapid homogenization and copy number evolution since the origin of the tetraploid species.     

Molecular evidence for polyploid origins in Saxifraga (Saxifragaceae): the narrow arctic endemic S. svalbardensis and its widespread allies

The recently described polyploid Saxifraga svalbardensis is endemic to the arctic archipelago of Svalbard. We investigated relationships among four closely related species of Saxifraga in Svalbard and tested three previously proposed hypotheses for the origin of S. svalbardensis: (1) differentiation from the morphologically and chromosomally variable polyploid S. cernua; (2) hybridization between the diploid S. hyperborea and S. cernua; and (3) hybridization between the tetraploid S. rivularis and S. cernua. Fifteen populations were analyzed using random amplified polymorphic DNAs (RAPDs) and nucleotide sequences of the chloroplast gene matK and the internal transcribed spacers (ITS) of nuclear ribosomal DNA (rDNA). RAPD and matK data suggest that S. svalbardensis has originated from a hybrid with S. rivularis as the maternal parent and S. cernua as the paternal parent, possibly a single time, whereas ITS data could not be used to discriminate among the hypotheses. The data also suggest that the diploid S. hyperborea is a progenitor of the tetraploid S. rivularis. The four populations examined of S. svalbardensis were virtually identical for RAPD and ITS markers, whereas S. cernua showed high levels of variation, suggesting that the latter polyploid either has formed recurrently or has undergone considerable differentiation since its origin.     

In situ hybridization in Actinidia using repeat DNA and genomic probes

 In situ hybridization has been used to probe chromosome spreads of hexaploid Actinidia deliciosa (kiwifruit; 2n=6x=174) and tetraploid A. chinensis (2n=4x=116). When a species-specific repeat sequence, pKIWI516, was used, six hybridization sites were observed in some accessions of tetraploid A. chinensis and all of A. deliciosa. Southern analysis with the pKIWI516 probe revealed that there are two types of tetraploid A. chinensis. Genomic probes from diploid A. chinensis (2n=2x=58) did not differentiate the genomes of hexaploid A. deliciosa and tetraploid A. chinensis, irrespective of the presence or absence of blocking DNA. The results indicate that the genomes of polyploid Actinidia species are similar but not identical. The origin of A. deliciosa is discussed. Received: 29 June 1996 / Accepted: 5 July 1996     

Molecular phylogenetic analyses of nuclear and plastid DNA sequences support dysploid and polyploid chromosome number changes and reticulate evolution in the diversification of Melampodium (Millerieae, Asteraceae)

Chromosome evolution (including polyploidy, dysploidy, and structural changes) as well as hybridization and introgression are recognized as important aspects in plant speciation. A suitable group for investigating the evolutionary role of chromosome number changes and reticulation is the medium-sized genus Melampodium (Millerieae, Asteraceae), which contains several chromosome base numbers (x = 9, 10, 11, 12, 14) and a number of polyploid species, including putative allopolyploids. A molecular phylogenetic analysis employing both nuclear (ITS) and plastid (matK) DNA sequences, and including all species of the genus, suggests that chromosome base numbers are predictive of evolutionary lineages within Melampodium. Dysploidy, therefore, has clearly been important during evolution of the group. Reticulate evolution is evident with allopolyploids, which prevail over autopolyploids and several of which are confirmed here for the first time, and also (but less often) on the diploid level. Within sect. Melampodium, the complex pattern of bifurcating phylogenetic structure among diploid taxa overlain by reticulate relationships from allopolyploids has non-trivial implications for intrasectional classification.     

Physical Mapping of rRNA Gene Loci and Inter-specific Relationships in Wild <Emphasis Type="Italic">Lilium</Emphasis> Distributed in Korea

Molecular cytogenetic analyses using fluorescence in situ hybridization (FISH) and genomic in situ hybridization (GISH) were carried out to elucidate inter-specific relationships among wild Lilium species distributed in Korea. FISH revealed four to eight 45S rRNA gene loci, which are located on chromosomes 1–7, 10, and 11 among the different species. In contrast, the 5S rRNA gene locus was conserved on the long arm of chromosome 3, occasionally with two adjacent sites on the same chromosome arm in a few species. The 5S rDNA site was located adjacent to the 45S rDNA site in only three species, Lilium distichum, Lilium hansonii, and Lilium tsingtauense. GISH analysis using genomic DNA probes detected strong hybridization of genomes between diploid and triploid Lilium lancifolium species, demonstrating that triploid plants were derived from diploid L. lancifolium and not from Lilium maximowiczii. Phylogenetic analysis of the ITS and NTS sequences supported the cytogenetic data as well as Comber’s classification of the genus Lilium.     

Identification of C-genome chromosomes involved in intergenomic translocations in Avena sativa L., using cloned repetitive DNA sequences

Four anonymous non-coding sequences were isolated from an Avena strigosa (A genome) genomic library and subsequently characterized. These sequences, designated As14, As121, As93 and As111, were 639, 730, 668, and 619 bp long respectively, and showed different patterns of distribution in diploid and polyploid Avena species. Southern hybridization showed that sequences with homology to sequences As14 and As121 were dispersed throughout the genome of diploid (A genome), tetraploid (AC genomes) and hexaploid (ACD genomes) Avena species but were absent in the C-genome diploid species. In contrast, sequences homologous to sequences As93 and As111 were found in diploid (A and C genomes), tetraploid (AC genomes) and hexaploid (ACD genomes) species. The chromosomal locations of the 4 sequences in hexaploid oat species were determined by fluorescent in situ hybridization and found to be distributed over the length of the 28 chromosomes (except in the telomeric regions) of the A and D genomes. Furthermore, 2 C-genome chromosome pairs with the As14 sequence, and 4 with As121, were discovered to beinvolved in intergenomic translocations. These chromosomes were identified as 1C, 2C, 4C and 16C by combining the As14 or As121 sequences with two ribosomal sequences and a C-genome-specific sequence as probes in fluorescence in situ hybridization. These sequences offer new tools for analyzing possible intergenomic translocations in other hexaploid oat species. Received: 8 April 1999 / Accepted: 30 July 1999     

Recent and recurrent polyploidy in Tragopogon (Asteraceae): cytogenetic, genomic and genetic comparisons

Tragopogon 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. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 485–501.