Polyploid Formation Research Articles

Gene flow contributes to genetic diversity of tetraploid populations of the North American plant genusHoustonia(Rubiaceae)

Polyploids are thought to harbor more genetic diversity than diploids, despite the genetic bottleneck they experience upon formation. Identifying the means by which polyploids become more genetically variable can advance our understanding of the evolutionary trajectories of polyploid lineages. Gene flow is one such mechanism that can contribute to polyploid genetic diversity. Here, we used microsatellite markers to measure genetic diversity and to estimate gene flow among closely related diploid and tetraploid species of the North American plant genus Houstonia section Amphiotis. We found that tetraploid populations were more genetically variable when compared to diploid populations and that gene flow (admixture) occurred more often among tetraploid populations than among diploids. In addition, we provide evidence to support previous reports that autopolyploidy is the likely mode of polyploid formation in this group. We suggest that gene flow among closely related species, particularly among tetraploid populations, has likely contributed to the genetic diversity levels of tetraploid lineages of Houstonia species.

Environmental correlates of cytotype distribution in Andropogon gerardii (Poaceae).

• Information about geographic distribution of cytotypes can provide insight into the origin and maintenance of autopolyploid complexes and builds a foundation for understanding cytotype differentiation and the dynamics of mixed-ploidy populations. Here, we investigate environmental correlates of the geographic distributions of 6x and 9x individuals in the ecologically dominant grass Andropogon gerardii to examine the role of climate in shaping patterns of cytotype distribution in this species.• Flow cytometry was used to estimate ploidy level in 352 individuals from 32 populations across North America. Ecological differentiation of cytotypes was tested by relating BIOCLIM variables to cytotype distribution using principal components analysis and partial linear regression.• Broad geographic sampling confirmed two primary cytotypes-6x (hexaploid) and 9x (enneaploid)-and revealed that 9x plants are more common than previously thought. Enneaploids occur frequently in the southern portions of the range, with hexaploids dominating in northern regions. Mixed-ploidy populations were common (46.9%). Principal components analysis and partial linear regression indicated that reduced summer precipitation and increased variation in diurnal and seasonal temperature range were significant predictors of the frequency of 9x plants in a population.• Results indicate that (1) geographic distribution of 6x and 9x individuals is nonrandom; (2) environmental variables are associated with cytotype distribution in A. gerardii; and (3) nearly half of populations surveyed include both 6x and 9x individuals. The persistence of mixed-ploidy populations may reflect a combination of recurrent polyploid formation and the prevalence of clonal reproduction.

Asymmetric epigenetic modification and elimination of rDNA sequences by polyploidization in wheat.

rRNA genes consist of long tandem repeats clustered on chromosomes, and their products are important functional components of the ribosome. In common wheat (Triticum aestivum), rDNA loci from the A and D genomes were largely lost during the evolutionary process. This biased DNA elimination may be related to asymmetric transcription and epigenetic modifications caused by the polyploid formation. Here, we observed both sets of parental nucleolus organizing regions (NORs) were expressed after hybridization, but asymmetric silencing of one parental NOR was immediately induced by chromosome doubling, and reversing the ploidy status could not reactivate silenced NORs. Furthermore, increased CHG and CHH DNA methylation on promoters was accompanied by asymmetric silencing of NORs. Enrichment of H3K27me3 and H3K9me2 modifications was also observed to be a direct response to increased DNA methylation and transcriptional inactivation of NOR loci. Both A and D genome NOR loci with these modifications started to disappear in the S4 generation and were completely eliminated by the S7 generation in synthetic tetraploid wheat. Our results indicated that asymmetric epigenetic modification and elimination of rDNA sequences between different donor genomes may lead to stable allopolyploid wheat with increased differentiation and diversity.

A lineage of diploid platelet-forming cells precedes polyploid megakaryocyte formation in the mouse embryo

AbstractIn this study, we test the assumption that the hematopoietic progenitor/colony-forming cells of the embryonic yolk sac (YS), which are endowed with megakaryocytic potential, differentiate into the first platelet-forming cells in vivo. We demonstrate that from embryonic day (E) 8.5 all megakaryocyte (MK) colony-forming cells belong to the conventional hematopoietic progenitor cell (HPC) compartment. Although these cells are indeed capable of generating polyploid MKs, they are not the source of the first platelet-forming cells. We show that proplatelet formation first occurs in a unique and previously unrecognized lineage of diploid platelet-forming cells, which develop within the YS in parallel to HPCs but can be specified in the E8.5 Runx1-null embryo despite the absence of the progenitor cell lineage.

Chromosome changes after polyploidization in Triticeae

AbstractChromosome changes are common in Triticeae, and they occur widely in natural and resynthesized polyploidy. Two important factors, nucleocytoplasmic interaction (internal) and the environment (external), can significantly influence chromosome changes after polyploidization. And chromosomal DNA changes play key roles during the initial formation, gradual stabilization, and establishment of polyploids. Hybrid breeding between common wheat and related wild species of Triticeae is an example of polyploidization, and many of the chromosome changes occurring after hybridization could be useful for improving wheat varieties. The famous chromosomal translocation 1BL/1RS that occurred after ancestral hybridization between wheat and rye is distributed widely among modern wheat varieties and makes a big contribution to wheat breeding; xiaoyan 6 is a similarly distant hybridization between wheat and Agropyron elongatum (Host) P. Beauv. in China. This chromosome translocation line was cultivated as the main variety in Shaanxi Province for 16 years and has also been used as a core parent for wheat breeding in China during the past 20 years because of its outstanding merits.

Polyploid formation shapes flowering plant diversity.

Polyploidy, or whole genome duplication, has been an important feature of eukaryotic evolution. This is especially true in flowering plants, where all extant angiosperms have descended from polyploid species. Here we present a broad comparative analysis of the effect of polyploidy on flowering plant diversity. We examine the widely held hypothesis that polyploid flowering plants generate more diversity than their diploid counterparts, by fitting stochastic birth/death models to observed ploidal frequency data from 60 extant angiosperm genera. Our results suggest the opposite, that diploids speciate at higher rates than polyploids, through a combination of simple diploid speciation and tetraploidy. Importantly, the estimated diploid advantage stemmed primarily from a higher rate of polyploidization in diploids than polyploids. Our model is also able to account for the empirically observed correlation between polyploidy and species richness without assuming that polyploids have a speciation advantage over diploids.

Advance in Researching Polyploid Induction on Heavy Metal Hyperaccumulator

Using phytoremediation technique to remove contaminants from soil had become a topic in current research. This article expounded the domestic and oversea research progress of plants on polyploid techniques and was systematically elaborated from many aspects, including the formation of polyploid, induction methods, and application, etc. The direction of phytoremediation is put forward that the biomass and heavy metal accumulation quantity of hyperaccumulators would be raised through polyploid induction.

Polyploidy in the Arabidopsis genus.

Whole genome duplication (WGD), which gives rise to polyploids, is a unique type of mutation that duplicates all the genetic material in a genome. WGD provides an evolutionary opportunity by generating abundant genetic "raw material," and has been implicated in diversification, speciation, adaptive radiation, and invasiveness, and has also played an important role in crop breeding. However, WGD at least initially challenges basic biological functions by increasing cell size, altering relationships between cell volume and DNA content, and doubling the number of homologous chromosome copies that must be sorted during cell division. Newly polyploid lineages often have extensive changes in gene regulation, genome structure, and may suffer meiotic or mitotic chromosome mis-segregation. The abundance of species that persist in nature as polyploids shows that these problems are surmountable and/or that advantages of WGD might outweigh drawbacks. The molecularly especially tractable Arabidopsis genus has several ancient polyploidy events in its history and contains several independent more recent polyploids. This genus can thus provide important insights into molecular aspects of polyploid formation, establishment, and genome evolution. The ability to integrate ecological and evolutionary questions with molecular and genetic understanding makes comparative analyses in this genus particularly attractive and holds promise for advancing our general understanding of polyploid biology. Here, we highlight some of the findings from Arabidopsis that have given us insights into the origin and evolution of polyploids.

Multiple hybridization events in Cardamine (Brassicaceae) during the last 150 years: revisiting a textbook example of neoallopolyploidy

Recently formed allopolyploid species represent excellent subjects for exploring early stages of polyploid evolution. The hexaploid Cardamine schulzii was regarded as one of the few nascent allopolyploid species formed within the past ∼150 years that presumably arose by autopolyploidization of a triploid hybrid, C. × insueta; however, the most recent investigations have shown that it is a trigenomic hybrid. The aims of this study were to explore the efficiency of progenitor-specific microsatellite markers in detecting the hybrid origins and genome composition of these two allopolyploids, to estimate the frequency of polyploid formation events, and to outline their evolutionary potential for long-term persistence and speciation. Flow-cytometric ploidy-level screening and genotyping by progenitor-specific microsatellite markers (20 microsatellite loci) were carried out on samples focused on hybridizing populations at Urnerboden, Switzerland, but also including comparative material of the parental species from other sites in the Alps and more distant areas. It was confirmed that hybridization between the diploids C. amara and C. rivularis auct. gave rise to triploid C. × insueta, and it is inferred that this has occurred repeatedly. Evidence is provided that C. schulzii comprises three parental genomes and supports its origin from hybridization events between C. × insueta and the locally co-occurring hypotetraploid C. pratensis, leading to two cytotypes of C. schulzii: hypopentaploid and hypohexaploid. Each cytotype of C. schulzii is genetically uniform, suggesting their single origins. Persistence of C. schulzii has presumably been achieved only by perennial growth and clonal reproduction. This contrasts with C. × insueta, in which multiple origins and occasional sexual reproduction have generated sufficient genetic variation for long-term survival and evolutionary success. This study illustrates a complex case of recurrent hybridization and polyploidization events, and highlights the role of triploids that promoted the origin of trigenomic hybrids.

Interspecific in vitro hybridization in genus Centaurium and identification of hybrids via flow cytometry, RAPD, and secondary metabolite profiles

Most of the species from the genus Centaurium Hill readily hybridize between themselves in natural habitats. Artificial interspecific hybrid plants were obtained under in vitro conditions using reciprocal crosses between 4 species: Centaurium erythraea, C. littorale, C. maritimum, and C. pulchellum. Among 76 crossing experiments performed, 3 types of crosses produced seeds and gave 20 interspecific hybrid plants. Nuclear DNA quantification, fingerprinting of molecular markers, and determination of variation in main secondary metabolite content were employed to confirm their hybrid nature. All 3 approaches were capable of detecting hybrid plants. RAPD molecular markers proved to be the most reliable, always placing the hybrids between their parents in dendrograms, while secoiridoid glycoside content also showed intermediate profiles in the hybrid plants in relation to the parents. Hence, each of the techniques employed can be recommended for elucidating the mechanisms of interspecific hybridization in natural populations of Centaurium species in further studies. This could help to comprehend their breeding system and recognize interspecific hybridization as one of the crucial steps in polyploid formation and speciation in this genus.

Sodium cacodylate as antimitotic agent

The effect of pure sodium cacodylate on dividing cells was studied. The root meristematic cells of <em>Allium cepa</em> L. (the roots were squashed in acetoorcein) and endosperm cells of <em>Haemanthus katherinae</em> Bak. (<em>in vitro</em> observations) were used. Serious disturbances in karyokinesis and cytokinesis were found that led most often to the formation of polyploid or multinucleate (<em>A. cepa</em>) cells. These results point to damage of the mitotic spindle and phragmoplast. Careful use of cacodylate buffer in ultrastructural studies of microtubules is advised.

Aberrant Classopollis pollen reveals evidence for unreduced (2n) pollen in the conifer family Cheirolepidiaceae during the Triassic-Jurassic transition.

Polyploidy (or whole-genome doubling) is a key mechanism for plant speciation leading to new evolutionary lineages. Several lines of evidence show that most species among flowering plants had polyploidy ancestry, but it is virtually unknown for conifers. Here, we study variability in pollen tetrad morphology and the size of the conifer pollen type Classopollis extracted from sediments of the Triassic-Jurassic transition, 200 Ma. Classopollis producing Cheirolepidiaceae were one of the most dominant and diverse groups of conifers during the Mesozoic. We show that aberrant pollen Classopollis tetrads, triads and dyads, and the large variation in pollen size indicates the presence of unreduced (2n) pollen, which is one of the main mechanisms in modern polyploid formation. Polyploid speciation may explain the high variability of growth forms and adaptation of these conifers to different environments and their resistance to extreme growth conditions. We suggest that polyploidy may have also reduced the extinction risk of these conifers during the End-Triassic biotic crisis.

English

Polyploidy plays an important role in plant evolution and constitutes an important mechanism of diversification and creation of genetic variability. Artificial polyploidy can be induced using the colchicine. The aim of this study was to determine the Trigonella foenum-greacum (2n= 16) population which resists better to colchicine treatment and which has the highest rate of polyploid formation among 38 populations collected from different Tunisian regions. The comparative study between treated and control plants showed significant increases (p < 0.05) for the parameters survival rate, rate of malformed leaves appearance, stem height and seeds number by pod. The 38 treated populations showed significant variations (p < 0.05) between them for the parameters survival rate, branches number, stem height and seeds number by pod. Results also showed that treatment of the shoot meristem by colchicine allowed to obtain diploid (2n= 16) and mixoploid plants having mixoploid branches (4n = 32 and 2n= 16) and branches entirely tetraploid (4n = 32). The obtained rate of mixoploidy was 65.79% of treated plants. Among the 38 analyzed fenugreek populations, the population 15 of Menzel Temime presented the highest rate of mixoploids formation and survival rate to colchicine treatment.

Hybridization and long-distance colonization at different time scales: towards resolution of long-term controversies in the sweet vernal grasses (Anthoxanthum).

Repeated hybridization and/or polyploidization confound classification and phylogenetic inference, and multiple colonizations at different time scales complicate biogeographical reconstructions. This study investigates whether such processes can explain long-term controversies in Anthoxanthum, and in particular its debated relationship to the genus Hierochloë, the evolution of its conspicuously diverse floral morphology, and the origins of its strikingly disjunct occurrences. A hypothesis for recurrent polyploid formation is proposed. Three plastid (trnH-psbA, trnT-L and trnL-F) and two nuclear (ITS, ETS) DNA regions were sequenced in 57 accessions of 17 taxa (including 161 ETS clones) and Bayesian phylogenetic analyses were conducted. Divergence times were inferred in *BEAST using a strict molecular clock. Anthoxanthum was inferred as monophyletic and sister to one species of Hierochloë based on the plastid data, whereas the nuclear data suggested that one section (Anthoxanthum section Anthoxanthum) is sister to a clade including the other section (Anthoxanthum section Ataxia) as sister to the genus Hierochloë. This could explain the variation in floral morphology; the aberrant characters in Ataxia seem to result from a Miocene hybridization event between one lineage with one fertile and two sterile florets (the Anthoxanthum lineage) and one which probably had three fertile florets as in extant Hierochloë. The distinct diploid A. gracile lineage originated in the Miocene; all other speciation events, many of them involving polyploidy, were dated to the Late Pliocene to Late Pleistocene. Africa was apparently colonized twice in the Late Pliocene (from the north to afro-alpine eastern Africa, and from south-east Asia to southern Africa), whereas Macaronesia was colonized much later (Late Pleistocene) by a diploid Mediterranean lineage. The widespread European tetraploid A. odoratum originated at least twice. Many of the controversies in Anthoxanthum can be explained by recurring hybridization and/or polyploidization on time scales ranging from the Miocene to the Late Pleistocene. All but one of the extant species shared most recent common ancestors in the Late Pliocene to the Late Pleistocene. The disjunct occurrences in Africa originated in the Late Pliocene via independent immigrations, whereas Macaronesia was colonized in the Late Pleistocene.

Small-Molecule Inhibitor BMS-777607 Induces Breast Cancer Cell Polyploidy with Increased Resistance to Cytotoxic Chemotherapy Agents

The RON receptor tyrosine kinase is a therapeutic target for cancer treatment. Here, we report therapeutic effect and phenotypic change of breast cancer cells in response to BMS-777607, a RON tyrosine kinase inhibitor. Treatment of breast cancer cells with BMS-777607 at therapeutic doses inhibited cancerous clonogenic growth but had only minimal effect on cell apoptosis. Significantly, BMS-777607 induced extensive polyploidy with multiple sets of chromosomes in cancer cells. This effect is independent of RON expression. Knockdown of RON in T-47D and ZR-75-1 cells by specific siRNA did not prevent polyploid formation. Immunofluorescent analysis of α-tubulin and γ-tubulin expression in polyploid cells revealed that BMS-777607 disrupts bipolar spindle formation and causes multipolar-like microtubule assembly. Also, both metaphase equatorial alignment and chromosomal segregation were absent in polyploid cells. These results suggest that cellular mitosis arrests at prophase/pro-metaphase and fails to undergo cytokinesis. By analyzing kinase-inhibitory profiles, aurora kinase B was identified as the target molecule inhibited by BMS-777607. In BMS-777607-treated cells, aurora kinase B was inhibited followed by protein degradation. Moreover, BMS-777607 inhibited Ser10 phosphorylation of histone H3, a substrate of aurora kinase B. Chemosensitivity analysis indicated the resistance of polyploid cells toward chemotherapeutics. Treatment with doxorubicin, bleomycin, methotrexate, and paclitaxel significantly increased cellular IC50 values. These findings highlight the theory that BMS-777607 acts as a multikinase inhibitor at therapeutic doses and is capable of inducing polyploidy by inhibiting aurora kinase B. Increased resistance of polyploid cells to cytotoxic chemotherapeutics could have a negative impact on targeted cancer therapy using BMS-777607.

Abstract 921: Small-molecule inhibitor BMS-777607 induces breast cancer cell polyploidy with increased resistance to cytotoxic chemotherapy agents.

Abstract The RON receptor tyrosine kinase is a therapeutic target for cancer treatment. Here we report therapeutic effect and phenotypic change of breast cancer cells in response to BMS-777607, a RON tyrosine kinase inhibitor. Treatment of breast cancer cells with BMS-777607 at therapeutic doses inhibited cancerous clonogenic growth but had only minimal effect on cell apoptosis. Significantly, BMS-777607 induced extensive polyploidy with multiple sets of chromosomes in cancer cells. This effect is independent of RON expression. Knockdown of RON in T-47D and ZR-75-1 cells by specific siRNA did not prevent polyploid formation. Immunofluorescent analysis of α-tubulin and γ-tubulin expression in polyploid cells revealed that BMS-777607 disrupts bi-polar spindle formation and causes multi-polar-like microtubule assembly. Also, both metaphase equatorial alignment and chromosomal segregation were absent in polyploid cells. These results suggest that cellular mitosis arrests at prophase/pro-metaphase and fails to undergo cytokinesis. By analyzing kinase-inhibitory profiles, aurora kinase B was identified as the target molecule inhibited by BMS-777607. In BMS-777607 treated cells, aurora kinase B was inhibited followed by protein degradation. Moreover, BMS-777607 inhibited Ser10 phosphorylation of histone H3, a substrate of aurora kinase B. Chemosensitivity analysis indicated the resistance of polyploid cells towards chemotherapeutics. Treatment with doxorubicin, bleomycin,cisplatin, methotrexate, and paclitaxel significantly increased cellular IC50 values. These findings highlight the theory that BMS-777607 acts as a multi-kinase inhibitor at therapeutic doses and is capable of inducing polyploidy by inhibiting aurora kinase B. Increased resistance of polyploid cells to cytotoxic chemotherapeutics could have a negative impact on targeted cancer therapy using BMS-777607. Citation Format: Sharad Sharma, Jun-Yin Zeng, Chunmei Zhunag, Ruiwen Zhang, Ming-hai Wang. Small-molecule inhibitor BMS-777607 induces breast cancer cell polyploidy with increased resistance to cytotoxic chemotherapy agents. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 921. doi:10.1158/1538-7445.AM2013-921

Consequences of colchicine induced intermeiocyte connections inHelianthus annuus

During microsporogenesis sometimes the chromatin materials migrate from one cell to another by intermeiocyte connections. In the present study, during induction of tetraploidy in sunflower, changes in the normal behaviour of chromosomes was detected. Besides chromosomal aberrations, cytomictic connections were frequently recorded. Cytomixis has been reported by many researchers in many diploid or tetraploid plants. In sunflower, cytomixis in colchiploids has not been recorded. In the current study high numbers of cytoplasmic connections were observed in the plants failing to reach higher ploidy level. Formation of cytoplasmic connections was very high, leading to many hyperploids, hypoploids and even empty pollen mother cells (PMCs). Cytomixis occurred frequently, with two to many meiocytes engaged in exchange of chromatin through one or multiple cytoplasmic connections. Few rare polyploid PMCs with cytoplasmic connections were observed, which favours formation of aneuploids and polyploids. Besides variou...

Ploidy frequencies in plants with ploidy heterogeneity: fitting a general gametic model to empirical population data.

Genome duplication (polyploidy) is a recurrent evolutionary process in plants, often conferring instant reproductive isolation and thus potentially leading to speciation. Outcome of the process is often seen in the field as different cytotypes co-occur in many plant populations. Failure of meiotic reduction during gametogenesis is widely acknowledged to be the main mode of polyploid formation. To get insight into its role in the dynamics of polyploidy generation under natural conditions, and coexistence of several ploidy levels, we developed a general gametic model for diploid-polyploid systems. This model predicts equilibrium ploidy frequencies as functions of several parameters, namely the unreduced gamete proportions and fertilities of higher ploidy plants. We used data on field ploidy frequencies for 39 presumably autopolyploid plant species/populations to infer numerical values of the model parameters (either analytically or using an optimization procedure). With the exception of a few species, the model fit was very high. The estimated proportions of unreduced gametes (median of 0.0089) matched published estimates well. Our results imply that conditions for cytotype coexistence in natural populations are likely to be less restrictive than previously assumed. In addition, rather simple models show sufficiently rich behaviour to explain the prevalence of polyploids among flowering plants.

Absence of gene flow between diploids and hexaploids of Aster amellus at multiple spatial scales

The potential for gene exchange across ploidy levels has long been recognized, but only a few studies have explored the rate of gene flow among different cytotypes. In addition, most of the existing knowledge comes from contact zones between diploids and tetraploids. The purpose of this paper was to investigate relationships between diploid and hexaploid individuals within the Aster amellus aggregate. A. amellus is known to occur in diploid and hexaploid cytotypes in Europe, with a complex contact zone in central Europe. Patterns of genetic diversity were investigated using seven microsatellite loci at three different spatial scales: (1) in the single known mixed-ploidy population; (2) in populations at the contact zone and (3) in a wider range of populations across Europe. The results show clear separation of the cytotypes at all three spatial scales. In addition, analysis of molecular variance strongly supported a model predicting a single origin of the hexaploids, with no or very limited gene flow between the cytotypes. Some hexaploid individuals found in the mixed-ploidy population, however, fell into the diploid cluster. This could suggest recurrent polyploid formation or occasional cross-pollination between cytotypes; however, there are strong post-zygotic breeding barriers between the two cytotypes, making the latter less plausible. Overall, the results suggest that the cytotypes could represent two cryptic species. Nevertheless, their formal separation is difficult as they cannot be distinguished morphologically, occupy very similar habitat conditions and have largely overlapping distribution ranges. These results show that polyploid complexes must be treated with caution as they can hide biological diversity and can have different adaptation potentials, evolving independently.

Molecular tools for exploring polyploid genomes in plants.

Polyploidy is a very common phenomenon in the plant kingdom, where even diploid species are often described as paleopolyploids. The polyploid condition may bring about several advantages compared to the diploid state. Polyploids often show phenotypes that are not present in their diploid progenitors or exceed the range of the contributing species. Some of these traits may play a role in heterosis or could favor adaptation to new ecological niches. Advances in genomics and sequencing technology may create unprecedented opportunities for discovering and monitoring the molecular effects of polyploidization. Through this review, we provide an overview of technologies and strategies that may allow an in-depth analysis of polyploid genomes. After introducing some basic aspects on the origin and genetics of polyploids, we highlight the main tools available for genome and gene expression analysis and summarize major findings. In the last part of this review, the implications of next generation sequencing are briefly discussed. The accumulation of knowledge on polyploid formation, maintenance, and divergence at whole-genome and subgenome levels will not only help plant biologists to understand how plants have evolved and diversified, but also assist plant breeders in designing new strategies for crop improvement.