Increase In Chromosome Number Research Articles

Triploid and aneuploid hybrids obtained from hybridization between Eriobotrya japonica and E. cavaleriei

A short ripening stage, multiple large seeds per fruit, and anthracnose disease seriously restrict the loquat (Eriobotrya japonica) industry. To obtain breeding germplasm of loquat with multiple ideal traits, a wild Eriobotrya species, E. cavaleriei (2n=2x=34), which flowers in late spring and early summer and whose fruits ripen in autumn, was used as the male parent to produce triploid plants from crosses with loquat tetraploid (2n=4x=68) lines (EjK474, EjB433 and EjB456). Fifteen progenies were obtained from three hybridizations, and 13 plants were identified as hybrids according to insertion-deletion (InDel) markers. All 13 hybrids were roughly identified as triploids by flow cytometry. Therein, 4 plants were identified as aneuploids with decreased or increased chromosome numbers based on sequencing depth according to reduced-representation genome sequencing. The sequencing depth on some chromosomes of these 4 hybrids was approximately 2/3 times or 1.41 times the average depth of all detected InDel loci. Frequencies of the loquat parent-specific alleles on increased or decreased chromosomes were approximately 50% or 75%. This verified that increased or decreased alleles in the 4 hybrids were all from tetraploid parents. The fruit flesh color genotypes of all hybrids were detected; there were two red-flesh alleles and one white-flesh allele in all hybrids. Leaves of E. cavaleriei were highly resistant to anthracnose in an in vitro assay, and the leaves of loquat B433 were sensitive. Disease spot diameters on the leaves of hybrids were significantly smaller (p < 0.01) than those of B433 after infection by spores of C. gloeosporioides. Therefore, the resistance of the hybrids was greater than that of the female parent B433. All these results showed that allotriploids can be obtained via hybridization between loquat tetraploids and E. cavaleriei, with some aneuploids as secondary products. Different flowering phases and fruit ripening stages, seedlessness and resistance to anthracnose might be combined in allotriploids. This study verified that resequencing can be used to identify the molecular karyotype of interspecific hybrids.

Genome balance and dosage effect drive allopolyploid formation in Brassica

Polyploidy is a major evolutionary force that has shaped plant diversity. However, the various pathways toward polyploid formation and interploidy gene flow remain poorly understood. Here, we demonstrated that the immediate progeny of allotriploid AAC Brassica (obtained by crossing allotetraploid Brassica napus and diploid Brassica rapa) was predominantly aneuploids with ploidal levels ranging from near-triploidy to near-hexaploidy, and their chromosome numbers deviated from the theoretical distribution toward increasing chromosome numbers, suggesting that they underwent selection. Karyotype and phenotype analyses showed that aneuploid individuals containing fewer imbalanced chromosomes had higher viability and fertility. Within three generations of self-fertilization, allotriploids mainly developed into near or complete allotetraploids similar to B. napus via gradually increasing chromosome numbers and fertility, suggesting that allotriploids could act as a bridge in polyploid formation, with aneuploids as intermediates. Self-fertilized interploidy hybrids ultimately generated new allopolyploids carrying different chromosome combinations, which may create a reproductive barrier preventing allotetraploidy back to diploidy and promote gene flow from diploids to allotetraploids. These results suggest that the maintenance of a proper genome balance and dosage drove the recurrent conversion of allotriploids to allotetraploids, which may contribute to the formation and evolution of polyploids.

Artificial induction of chromosome aneuploidy in CHO cells alters their function as host cells.

Chinese hamster ovary (CHO) cells are major host cells for biopharmaceuticals. During culture, the chromosome number of CHO cells alters spontaneously. Here, we investigated the effects of artificial changes in the chromosome number on productivity. When cell fusion between antibody-producing CHO-K1-derived cells was induced, we observed a wide range of aneuploidy that was not detected in controls. In particular, antibody productivities were high in clone-derived cell populations that retained a diverse chromosome number distribution. We also induced aneuploid cells using 3-aminobenzamide that causes chromosome non-disjunction. After induction of aneuploidy by 3-aminobenzamide, cells with an increased chromosome number were isolated, but cells with a decreased chromosome number could not be isolated. When antibody expression vectors were introduced into these isolated clones, productivity tended to increase in cells with an increased chromosome number. Further analysis was carried out by focusing on clone 5E8 with an average chromosome number of 37. When 5E8 cells were used as host, the productivity of multiple antibodies, including difficult-to-express antibodies, was improved compared with CHO-K1 cells. The copies of exogenous genes integrated into the genome were significantly increased in 5E8 cells. These findings expand the possibilities for host cell selection and contribute to the efficient construction of cell lines for recombinant protein production.

Plant mortality on ultramafic soils after an extreme heat and drought event in the Mediterranean area

AimsPlant mortality associated with the growing frequency and intensity of heatwaves and dry spells is increasingly occurring in various ecosystems worldwide. However, the impacts of extreme events on plant communities of ultramafic soils are still unknown. Here, we describe a first case of plant mortality on serpentine outcrops in Italy following a heat and drought event that occurred between autumn 2016 and summer 2017.MethodsMortality of perennial plants was assessed in thirty plots representing ten major serpentine areas along a SW-NE gradient, with the sparse vegetation of ultramafic soils. Statistical modelling was based on temperature and precipitation data and local site conditions.ResultsResponses of single species were largely different and not related to their taxonomic position, growth form or level of edaphic specialization for serpentine soils. Notably, obligate serpentine endemics were not less impacted than non-endemics. As expected, species mortality decreased with increasing chromosome number, suggesting higher tolerance and adaptive capacity in taxa of likely polyploid origin. Mortality increased with decreasing number of rainy days and duration of the heatwave and decreased with increasing distance from the Tyrrhenian coastline, reaching 60% in the southern and western areas with a Mediterranean climate. Local site conditions such as increasing soil depth and north-facing aspect significantly increased plant survivorship.ConclusionsOur findings show that extreme drought and heat can have a previously unrealized impact on ultramafic plant communities in the short term. Further studies should examine the recovery capacity and resilience of serpentine plants, together with the long term effects.

Zanthoxylum-specific whole genome duplication and recent activity of transposable elements in the highly repetitive paleotetraploid Z. bungeanum genome

Zanthoxylum bungeanum is an important spice and medicinal plant that is unique for its accumulation of abundant secondary metabolites, which create a characteristic aroma and tingling sensation in the mouth. Owing to the high proportion of repetitive sequences, high heterozygosity, and increased chromosome number of Z. bungeanum, the assembly of its chromosomal pseudomolecules is extremely challenging. Here, we present a genome sequence for Z. bungeanum, with a dramatically expanded size of 4.23 Gb, assembled into 68 chromosomes. This genome is approximately tenfold larger than that of its close relative Citrus sinensis. After the divergence of Zanthoxylum and Citrus, the lineage-specific whole-genome duplication event η-WGD approximately 26.8 million years ago (MYA) and the recent transposable element (TE) burst ~6.41 MYA account for the substantial genome expansion in Z. bungeanum. The independent Zanthoxylum-specific WGD event was followed by numerous fusion/fission events that shaped the genomic architecture. Integrative genomic and transcriptomic analyses suggested that prominent species-specific gene family expansions and changes in gene expression have shaped the biosynthesis of sanshools, terpenoids, and anthocyanins, which contribute to the special flavor and appearance of Z. bungeanum. In summary, the reference genome provides a valuable model for studying the impact of WGDs with recent TE activity on gene gain and loss and genome reconstruction and provides resources to accelerate Zanthoxylum improvement.

Ploidy state of aquatic macrophytes: Global distribution and drivers

Despite the increasing evidence that ploidy is a factor of importance for environmental adaptation, little work has hitherto been done about cytotype patterns in aquatic plants. We tested a set of hypotheses concerning the drivers of ploidy in macrophyte communities at worldwide scale, and also how ploidy state may be related to other attributes of aquatic plants such as community diversity, species range size and invasiveness. We built a global ploidy database with 1574 species (representing 45.0 % of the known total global pool) and allocated ploidy state to species occurring in 238 10 × 10° gridcells worldwide. We used Boosted Regression Trees Analysis to analyse the relationship of 16 global-scale spatial, landscape, environmental and biotic variables with ploidy state. Our results demonstrate that (1) Polyploid and mixed ploidy species occupied larger geographical ranges than haploid/diploid species. (2) There were contrasting latitudinal patterns between haploid/diploid and polyploid species with polyploidy increasing in importance with distance from the equator, while species richness measures also influenced ploidy state. (3) Climatic factors, especially temperature and evapotranspiration, are congruent with macrophyte species ploidy state, across the planet. Our data support the assertion that increased chromosome number is an adaptation associated with plant survival of harsh environmental conditions, especially in environmentally-stressed high latitudes. Our findings increase the generality of knowledge of the large-scale factors that drive cytotype patterns in vascular plant species at global scale. The ploidy database provides a resource to support further studies about the effects of varying environmental conditions on ploidy state in aquatic plants, at differing scales.

Assessment of morphological and anatomical variability in Triticum species, Aegilops species, interspecific and intergeneric hybrids

Wheat species and wild relatives offer promising resources for wheat improvement and research in the current period of the genetic narrowing of modern wheat cultivars. The present study was performed to evaluate the morphological and anatomical traits of 20 diverse genotypes including Triticum and Aegilops species with intergeneric and interspecific wheat hybrids, which were compared with modern bread and durum wheat cultivars locally adapted to rainfed and irrigated conditions. The study showed that stomata density and size ranged from 55.3 to 108.6 stomata/mm2 and 401.4 to 1296 µm2, respectively, in the selected genotypes. Moving tetraploid to hexaploid genotypes, increased chromosome numbers yielded lower densities of large stomata in wheat species and hybrids. In this regard, the stomatal patterns of two hexaploid wheat hybrids and a wheat species including ‘Agrotriticum’, ‘Aegilotriticum’, and T. compactum, were of low density and large size stomata compared to T. durum cv. ‘Kunduru 1149’ with high density and small size stomata. Interestingly, the wild progenitor of the bread wheat D genome, Ae. tauschii, had a high density of the smallest stomata among the studied genotypes. The study further indicated that morphological parameters decreased under rainfed conditions compared to those under irrigated conditions, with levels varying among the genotypes. The rainfed flag leaf area and 1000-grain weight varied from 0.9 to 23.7 cm2 and from 7.3 to 61.9 g, respectively under rainfed conditions, while they ranged from 1.2 to 35.7 cm2 and 11.5 to 69.9 g under irrigated conditions. The flag leaf area had a significant and strong association with 1000-grain weight under rainfed (r2= 0.79) and irrigated (r2 = 0.77) conditions. T. turanicum and T. polonicum were characterized by the significantly highest 1000-grain weight in both rainfed and irrigated conditions. This study suggests that these wheat species with high 1000-grain weight might have promising alleles to be transferred into durum wheat to increase grain yield.

A chromosome-scale genome assembly of Antheraea pernyi (Saturniidae, Lepidoptera).

Antheraea pernyi is a semi-domesticated lepidopteran insect species valuable to the silk industry, human health, and ecological tourism. Owing to its economic influence and developmental properties, it serves as an ideal model for investigating divergence of the Bombycoidea super family. However, studies on the karyotype evolution and functional genomics of A.pernyi are limited by scarce genomic resource. Here, we applied PacBio sequencing and chromosome structure capture technique to assemble the first high-quality A.pernyi genome from a single male individual. The genome is 720.67Mb long with 49 chromosomes and a 13.77-Mb scaffold N50. Approximately 441.75Mb, accounting for 60.74% of the genome, was identified as repeats. The genome comprises 21,431 protein-coding genes, 85.22% of which were functionally annotated. Comparative genomics analysis suggested that A.pernyi diverged from its common ancestor with A.yamamai ~30.3 million years ago, and that chromosome fission contributed to the increased chromosome number. The genome assembled in this work will not only facilitate future research on A.pernyi and related species but also help to progress comparative genomics analyses in Lepidoptera.

Individual and interactive effects of herbivory on plant fitness: endopolyploidy as a driver of genetic variation in tolerance and resistance.

Previous studies have shown a causal link between mammalian herbivory, tolerance, and chemical defense in Arabidopsis thaliana, driven by the process of endoreduplication (replication of the genome without mitosis). Removal of the apical meristem by mammalian herbivores lowers auxin, which triggers entry into the endocycle. Increasing chromosome number through endoreduplication, and therefore gene copy number, provides a means of increasing gene expression promoting rapid regrowth rates, higher defensive chemistry and enhanced fitness. Here, we assess whether insect leaf-feeding elicits the same compensatory response as the removal of apical dominance. Insect feeding has been shown to downregulate auxin production, which should trigger endoreduplication. Results here support this contention; insect leaf-feeding by Trichoplusia ni elicited a compensatory response similar to that of mammalian herbivores-an ecotype-specific response consistent with the level of endoreduplication. The interactive effects of mammalian and insect herbivory were also assessed to determine whether interactions were additive (pairwise) or non-additive (diffuse) on tolerance (fitness). Specifically, results indicate that herbivory is either diffuse (a significant clipping × T. ni interaction) or pairwise (no significant interaction between clipping and T. ni herbivory), dependent upon plant genotype and compensatory ability. In general, herbivore-induced changes in plant quality appear to be responsible for the observed differences in herbivory and fitness compensation. We discuss the importance of evaluating endoreduplication among plants within a population to avoid masking the association between tolerance and resistance and the fitness consequences of multi-herbivore interactions.

Overexpression of hsa-miR-186 induces chromosomal instability in arsenic-exposed human keratinocytes

The mechanism of arsenic-induced skin carcinogenesis is not yet fully understood. Chromosomal instability contributes to aneuploidy and is a driving force in carcinogenesis. Arsenic causes mitotic arrest and induces aneuploidy. hsa-miR-186 overexpression is associated with metastatic cancers as well as arsenic-induced squamous cell carcinoma and is reported to target several mitotic regulators. Decreased levels of these proteins can dysregulate chromatid segregation contributing to aneuploidy. This work investigates the potential aneuploidogenic role of hsa-miR-186 in arsenic carcinogenesis. Clones of immortalized human keratinocytes (HaCaT) stably transfected with a hsa-miR-186 expression or empty vector were isolated. Three clones with high and low hsa-miR-186 expression determined by RT-qPCR were selected for further analysis and cultured with 0 or 100 nM NaAsO2 for 8 weeks. Analysis of mitoses revealed that chromosome number and structural abnormalities increased in cells overexpressing hsa-miR-186 and were further increased by arsenite exposure. Double minutes were the dominant structural aberrations. The peak number of chromosomes also increased. Cells with >220 to >270 chromosomes appeared after 2 months in hsa-miR-186 overexpressing cells, indicating multiple rounds of endomitosis had occurred. The fraction of cells with increased chromosome number or structural abnormalities did not increase in passage matched control cells. Levels of selected target proteins were determined by western blot. Expression of BUB1, a predicted hsa-miR-186 target was suppressed in hsa-miR-186 overexpressing clones, but increased with arsenite exposure. CDC27 remained constant under all conditions. These results suggest that overexpression of miR-186 in arsenic exposed tissues likely induces aneuploidy contributing to arsenic-induced carcinogenesis.

Mechanisms of karyotype evolution in the Brazilian scorpions of the subfamily Centruroidinae (Buthidae).

The recently-revised subfamily Centruroidinae is part of the New World clade of buthid scorpions. In this study, we analyzed the cytogenetic characteristics of nine of the 10 Brazilian centruroidines, and one undescribed species of the genus Ischnotelson, using a phylogenetic approach to determine the chromosomal rearrangements responsible for the differentiation of karyotypes among the species. The cytogenetic data recorded in the present study supported the new taxonomic arrangement of the Centruroidinae, with all the species of the same genus sharing the same or similar diploid numbers, i.e., 2n = 20 or 22 in Troglorhopalurus lacrau and T. translucidus, 2n = 25 or 26 in Ischnotelson sp., I. guanambiensis and I. peruassu, and 2n = 28 in Jaguajir agamemnon, J. pintoi and J. rochae. The karyotype modelling in the ChromEvol software indicated 2n = 18 as the ancestral diploid number of the Centruroidinae. The differentiation of karyotypes among the centruroidine genera was based on increasing chromosome numbers resulting from progressive fission events. These changes probably occurred prior to the diversification of the genera Ischnotelson, Jaguajir, Physoctonus and Rhopalurus, and appear to have played a more important role in karyotype evolution at the intergeneric level than the interspecific one. However, the observed increase in diploid numbers was not accompanied by changes in the number or location of ribosomal genes or telomeric sequences. The identification of meiotic cells in female specimens also allowed us to discuss the mechanisms of achiasmatic meiosis in scorpions.

Molecular constraints on resistance-tolerance trade-offs.

Plants have numerous mechanisms to cope with the negative effects of herbivory, including plant resistance, structural and chemical traits that reduce damage, and plant tolerance, the ability to compensate for tissues lost. It has been argued that resistance and tolerance represent alternate strategies and thus there should be a trade-off between resistance and tolerance. However, resistance and tolerance are controlled via the same molecular pathway, the oxidative pentose phosphate pathway and the process of endoreduplication. Endoreduplication is the replication of the genome without mitosis, which leads to an increase in cellular chromosome number. Increasing chromosome number and therefore gene copy number provides a means of increasing gene expression that has been shown to enhance compensation following herbivory. By measuring glucosinolate levels and seed production following the removal of apical dominance in genotypes of Arabidopsis thaliana we show that there is a positive association between tolerance and induced chemical defense. Similarly, the direct association between tolerance and resistance is demonstrated by genetically manipulating the endoreduplication pathway. By overexpressing ILP1, a positive regulator of endoreduplication, and thus compensation, we experimentally increased glucosinolate production and tolerance in the Col-0 genotype. We suggest that many herbaceous plants that endoreduplicate (~90%) would show a positive relationship between compensation and chemical defense, given that the molecular pathways are shared in common. We discuss these findings in light of contrasting results on measures of tolerance and resistance, given that the true relationship can be masked by ignoring genetic variation in endoreduplication and the timing of chemical measurement.

Prognostic significance of relative 1p/19q codeletion in oligodendroglial tumors.

1p/19q codeletion is a favorable prognostic marker for oligodendroglial tumors (OT). Compare outcome in OT with simple deletions of 1p or 19q to those with relative deletions defined as the presence of both increased copy number (polysomy) and 1p/19q codeletion. 525 cases were examined by fluorescence in situ hybridization (FISH) using dual color probes to determine the deletion status of chromosome arms 1p and 19q. Categories included simple deletions defined as a proportion of either 1p32 or 19q13 FISH signals compared to 1q42 or 19p13 signals less than 0.80 and relative deletions (1p or 19q) defined as the combination of a <0.80 proportion with >30 % of nuclei showing increased chromosome number (based on enumeration of 1q25 or 19p13). 464 (80 %) were WHO Grade II or III OT of which 209 (48 %) had both 1p and 19q deleted (codeletion). 72 (16 %) had relative deletions for either one or both 1p and 19q of which 28 (6 %) had relative deletions of 1p and 19q (relative codeletion). Overall survival in WHO Grade II OT was 13 + years when 1p/19q codeleted (n = 156); 5 + years in uni- or nondeleted (n = 86); 6 + years in relative deletion for either 1p or 19q (n = 41); and 6 + years in relative 1p/19q codeletion (n = 15). Similarly in WHO Grade III OT (n = 168) overall survival was 11 + years in 1p/19q codeleted (n = 54) OT; 2.5 years in uni- or nondeleted (n = 70); 3 years in relative deletion for one or both 1p or 19q (n = 31); and 4 + years in relative 1p/19q codeletion (n = 13). Survival for OT regardless of grade with relative codeletion of 1p/19q was approximately one half that of 1p/19q codeleted tumors. The presence of relative 1p/19q codeletion is of prognostic significance.

Chromosome evolution of some Palaearctic and Nearctic species of Monochamus (Coleoptera: Cerambycidae: Lamiinae)

SummaryMitotic and meiotic C-banded chromosomes of four species of Monochamus Dejean 1821, including two Palaearctic, M. sartor (Fabricius 1787) (2n = 24) and M. sutor (L. 1758) (2n = 20) and two Nearctic species, M. scutellatus (Say 1824) and M. notatus (Drury 1773) (2n = 21 for both) were studied and compared. Considering published data on other species and personal unpublished data, a chromosomal phylogeny is proposed. The karyotypes of M. sutor and M. saltuarius (Gebler 1830) remained close to that of the ancestor. From this starting point, the karyotype of other Palaearctic species evolved by fissions, increasing chromosome number up to 24, as in M. urussovi (Fischer 1806)/M. sartor. The karyotype of Nearctic species underwent the pericentric inversion of at least two (M. scutellatus) to four (M. notatus) chromosomes and acquired one or two dispensable B chromosomes, previously interpreted as multiple sex chromosomes.

Palaeoploidization and adaptation: An evolutionary strategy among pteridophytes with a reference to Ophioglossum L

Homosporous pteridophytes have extremely high chromosome numbers relative to flowering plants, but the species with the lowest recorded chromosome numbers show gene expression patterns of diploid organisms, suggesting that these probable ancient polyploids are now completely diploidized. Assuming palaeopolyploidy in the present-day diploids, their genomes have seemingly undergone wholesale diploidization over vast intervals of geological time This evolutionary strategy of high chromosome numbers and genetic diploidy has been briefly reviewed on two points: First, these plants are ancient polyploids that have undergone extensive gene silencing to produce ‘genetic diploids’, and second, they may have increased chromosome numbers also through chromosomal aberrations. For this accomplishment it is hypothesized that the allopoloyloidy (natural hybridization) must have played a vital role because this rare natural phenomenon exposes new genes and chromosomes for functional interaction through division cycles and meiotic screening. The incumbent individual segregate(s) struggle for survival by producing numerous morphological and chromosomal combinations as revealed by population cytogenetic studies carried out on two pteridophytic genera, a heterosporous lycophyte Isoetes and another a primitive homosporous plant also referred as a fern, Ophioglossum . In both these unrelated genera, hybrid segregates have exhibited uniform story of struggling variants and have supported the concept “that polyploidization, more effectively allopolyploidy can trigger abrupt changes at the genomic DNA levels, thereby causing variations in cytosine methylation and also influence heterochromatinization on the chromatin. Appearance of new chromosomes within the genome are being understood to have been evolved primarily on account of allopolyploidy and new individuals with variables are obviously facing diversified selection pressures. This is also conceived that the giant heterosporous lycopods of Carboniferous must have been polyploids that had dominated the homosporous pteridophytes during Carboniferous (300 mya) until they had to face biological extinction on account of drastic climatic turmoil during the Mesozoic. After that as estimated by palaeobotanists, giant occupancy on marshy land gradually culminated to greatest reduction series of evolution finally to herbaceous lycopods such as quillworts. Also, the homosporous ferns emerged as dominating vegetation (polyploidization might have been helpful) at the same time when angiosperms arose as contemporary flora. The reduction series imposed on heterosporous plants probably might have also involved allopolyploidy and other mechanisms resulting in downsizing of genomes. In many extant genera, as exemplified here on homosporous palaeopolyploid Ophioglossum L and heterosporous lycopod, Isoetes L allopolyploidy offers both superior as well as inferior genotypes with many variable at morphological and chromosomal levels so that reproductive fitness is decisively handled by natural selection. Autopolyploidy in nature rarely follows allopolyploidy so natural segregates often have irregular meiosis. In Summary, it is emphasized that polyploidy has been a fundamental mechanism “prima facie” responsible to trigger speciation processes.

Molecular Cytological Characterization of Somatic Variation in Rice Aneuploids

Aneuploids cannot be stably preserved by sexual propagation, though they can maintain their genetic identity by asexual propagation. However, it is possible for somaclonal variation to occur during asexual propagation. Surveys taken from 2005 to 2007 showed that there were 26 lines with morphological variations from a total of 114 asexually propagated rice lines. Among these, 12 lines were detected that had either increased or decreased chromosome numbers, while the other 14 lines did not show any changes in chromosome number. Lines with increased chromosome numbers include the following four classes: (1) an extra chromosome was retained, and another normal individual chromosome was increased; (2) an extra chromosome was increased, and the normal chromosome numbers did not change; (3) an extra chromosome was lost, and another chromosome was increased; and (4) the genome was doubled. We studied 543 somatic cells from the 12 lines during mitosis and observed abnormal mitotic behaviors such as chromosome lagging, uneven distribution, and chromosome breakage at anaphase. These results show that abnormal mitotic behavior led to the somaclonal variation in chromosome number. However, cytological variation can only explain a minority of the asexual separated lines.

Analysis of karyotype diversity of 40 Chinese chrysanthemum cultivars

AbstractTo investigate the genetic differences in Chinese large‐flowered chrysanthemum (Chrysanthemum×morifolium Ramat.) cultivars, we selected 40 typical and stable cultivars on which to carry out cytological studies using karyotype analysis. The results showed that 67.5% of these cultivars were hexaploid‐based aneuploid and that the proportion of hexaploid decreased with passing time. Moreover, 35% of the cultivars had 1–4 satellite chromosome(s). The probability of satellite chromosomes rose with increasing chromosome number. Most of the karyotypes were 2A and 2B. The probability of types 2A and 2C also increased with increasing ploidy of the cultivars. The mean of long‐/short‐arm ratio and the variation of long‐/short‐arm ratio were positively correlated (r2 = 0.72). There was no obvious difference in the asymmetry coefficient of karyotypes, but the discrepancy in the variance of karyotype asymmetry index and relative length of chromosomes was quite distinct. In terms of karyotype parameters, the petal types of chrysanthemums were classified to five groups as flat, tubular, spoon, abnormal, and anemone. We did not observe any obvious orderliness among flower head types. Considering the relationship between karyotype parameters and phenotypic characters, variation of long‐/short‐arm ratio and asymmetry coefficient of karyotypes had the greatest relevance toward most phenotypic characters. The above results indicate that karyotype parameters possess great values for cultivar identification, classification, and genetic analysis in chrysanthemums.

Nuclear DNA content in the polyploid complex Turnera ulmifolia (Turnera L., Passifloraceae)

Turnera ulmifolia constitutes a well-studied polyploid complex with allo- and autopolyploid species ranging from 2 to 8x. Flow cytometry was used to determine nuclear DNA content, and to estimate 2C- and 1Cx-values with the aim of analysing the genome size in Turnera in terms of polyploid speciation. The 2C-value and 1Cx-value were evaluated in 12 species of the T. ulmifolia complex. Nuclear DNA content was estimated by flow cytometry of nuclei stained with propidium iodide. The 2C DNA content ranged from 1.38 to 1.83 pg in diploids, from 2.67 to 3.96 pg in tetraploids, from 2.73 to 4.31 pg in hexaploids, and from 3.53 to 5.90 pg in octoploids. The 1Cx-value ranged from 0.44 to 0.99 pg. The Turnera ulmifolia complex showed an increase in total DNA content in the ploidy level, but not in the expected proportion. The general tendency indicated a decrease in the 1Cx-value with increasing chromosome number, with T. grandidentata 4x being an outstanding exception. The 1Cx-values in the allooctoploids T. aurelii and T. cuneiformis differed by 1.6-fold from each other, probably as a result of different evolutionary histories following divergence from the last common ancestor.

Chromosomal fission accounts for small-scale radiations in Zamia (Zamiaceae; Cycadales)

Zamia is unique among Cycadales in its diversity of morphology, ecology and chromosome numbers. The chromosome numbers in Zamia range from 16 to 28, excluding 20, manifest as both interspecific and intraspecific series. It has long been recognized that Robertsonian transformations (chromosomal fission or fusion) probably dominate karyotype evolution in Zamiaceae, although it has been debated whether chromosome numbers are increasing or decreasing. We re‐analyse published karyotypes of Zamia spp., relating both chromosome forms and sizes to recent phylogenetic data. We show that karyotype evolution is most probably moving towards increased asymmetry, with higher numbers of smaller chromosomes, thus supporting chromosomal fission. We also address additional hypotheses for increasing chromosome numbers, namely pericentric inversions and unequal translocations. Finally, we discuss the role of these chromosomal changes in evolutionary radiations.

Apomixis and sexuality in the Potentilla argentea complex III. Euploid and aneuploid derivatives (including trisomics) of some apomictic biotypes

Three apomictic biotypes belonging to the Potentilla argentea complex formed the basis of this investigation, one of them diploid, one tetraploid and one hexaploid. The effects of chromosome doubling and other changes in chromosome number were studied. Hybrids between the same biotypes were discussed in a previous paper. The diploid was a facultative apomict, but autotetraploids produced by colchicine treatment were totally sexual. This was also true of autotriploids, probably also of trisomics and other aneuploids. Five out of seven possible primary trisomics were obtained; they were morphologically well separated from each other and from the diploid. As shown by the result of test-crosses, facultative apomixis was restored in “recombined diploids” in progenies from triploids and trisomics. This indicates that apomixis is partly dependent on homozygosity for recessive genes, but that also the degree of ploidy as well as chromosomal and genic balance directly influence the mode of reproduction. — In the more strictly apomictic tetraploid (2n= 28) biotype, unreduced egg cells were fertilized in exceptional cases, whereas the chromosome-doubled plants formed some aberrants by fertilization of reduced egg cells. However, an “autotriploid” (2n= 42) formed half of its offspring by fertilization of unreduced and reduced egg cells. Aberrants with 2n= 35 and 2n= 63 were obligate apomicts. — The hexaploid was obligately apomictic, whereas the poorly fertile plants resulting from chromosome doubling gave rise to one plant with further increased chromosome number; thus, even here apomixis was interfered with to some extent. — The concluding discussion also deals with problems of mitotic and meiotic instability, crossing barriers in pseudogamous taxa, and viewpoints on the mechanisms and regulation of apomixis.