Chromosome and nucleotide sequence differentiation in genomes of polyploid Triticum species

Summary The nature of genome change during polyploid evolution was studied by analysing selected species within the tribe Triticeae. The levels of genome changes examined included structural alterations (translocations, inversions), heterochromatinization, and nucleotide sequence change in the rDNA regions. These analyses provided data for evaluating models of genome evolution in polyploids in the genus Triticum, postulated on the basis of chromosome pairing at metaphase I in interspecies hybrids.The significance of structural chromosome alterations with respect to reduced MI chromosome pairing in interspecific hybrids was assayed by determining the incidence of heterozygosity for translocations and paracentric inversions in the A and B genomes of T. timopheevii ssp. araraticum (referred to as T. araraticum) represented by two lines, 1760 and 2541, and T. aestivum cv. Chinese Spring. Line 1760 differed from Chinese Spring by translocations in chromosomes 1A, 3A, 4A, 6A, 7A, 3B, 4B, 7B and possibly 2B. Line 2541 differed from Chinese Spring by translocations in chromosomes 3A, 6A, 6B and possibly 2B. Line 1760 also differed from Chinese Spring by paracentric inversions in arms 1AL and 4AL whereas line 2541 differed by inversions in 1BL and 4AL (not all chromosomes arms were assayed). The incidence of structural changes in the A and B genomes did not coincide with the more extensive differentiation of the B genomes relative to the A genomes as reflected by chromosome pairing studies.To assay changing degrees of heterochromatinization among species of the genus Triticum, all the diploid and polyploid species were C-banded. No general agreement was observed between the amount of heterochromatin and the ability of the respective chromosomes to pair with chromosomes of the ancestral species. Marked changes in the amount of heterochromatin were found to have occurred during the evolution of some of the polyploids.The analysis of the rDNA region provided evidence for rapid fixation of new repeated sequences at two levels, namely, among the 130 bp repeated sequences of the spacer and at the level of the repeated arrays of the 9 kb rDNA units. These occurred both within a given rDNA region and between rDNA regions on nonhomologous chromosomes. The levels of change in the rDNA regions provided good precedent for expecting extensive nucleotide sequence changes associated with differentiation of Triticum genomes and these processes are argued to be the principal cause of genome differentiation as revealed by chromosome pairing studies.     

Albumin evolution in polyploid species of the genusXenopus

Electrophoresis of serum from 21 Xenopus species and subspecies reveals variable numbers of albumin bands. The diploid X. tropicalis has one albumin, while the tetraploid species (laevis, borealis, muelleri, clivii, fraseri, epitropicalis) have two. The octoploid species (amieti, boumbaensis, wittei, vestitus, andrei) have two to three bands, and the dodecaploid X. ruwenzoriensis has three. The molecular weight of the Xenopus albumins varies from 68 kd (in the tropicalis group) to 74 kd. The subspecies of X. laevis possess two albumins of different molecular weights (70 and 74 kd), whereas most species have only 70-kd albumins. Peptide maps have been obtained from albumin electromorphs by limited proteolysis in sodium dodecyl sulfate (SDS) gels, using S. aureus V8 protease. The peptide patterns produced by electromorphs from the same tetraploid Xenopus species generally differ from each other, suggesting that the two albumin genes contain a substantial amount of structural differences. In addition, the peptide maps are diagnostic for most tetraploid species and for some subspecies of X. laevis as well. Proteolysis of albumins from most octoploid and dodecaploid species results in patterns which are very similar to the ones produced by the electromorphs from X. fraseri. The albumins of X. vestitus differ from those of the other octoploid species. X. andrei possesses two fraseri-type and one vestitus-type albumin, which indicates that it probably originated by allopolyploidy.     

Chromosome substitutions of Triticum timopheevii in common wheat and some observations on the evolution of polyploid wheat species

Whether the two tetraploid wheat species, the well known Triticum turgidum L. (macaroni wheat, AABB genomes) and the obscure T. timopheevii Zhuk. (A^tA^tGG), have monophyletic or diphyletic origin from the same or different diploid species presents an interesting evolutionary problem. Moreover, T. timopheevii and its wild form T. araraticum are an important genetic resource for macaroni and bread-wheat improvement. To study these objectives, the substitution and genetic compensation abilities of individual T. timopheevii chromosomes for missing chromosomes of T. aestivum Chinese Spring (AABBDD) were analyzed. Chinese Spring aneuploids (nullisomic-tetrasomics) were crossed with a T. timopheevii x Aegilops tauschii amphiploid to isolate T. timopheevii chromosomes in a monosomic condition. The F₁ hybrids were backcrossed one to four times to Chinese Spring aneuploids without selection for the T. timopheevii chromosome of interest. While spontaneous substitutions involving all A^t- and G-genome chromosomes were identified, the targeted T. timopheevii chromosome was not always recovered. Lines with spontaneous substitutions from T. timopheevii were chosen for further backcrossing. Six T. timopheevii chromosome substitutions were isolated: 6A^t (6A), 2G (2B), 3G (3B), 4G (4B), 5G (5B) and 6G (6B). The substitution lines had normal morphology and fertility. The 6A^t of T. timopheevii was involved in a translocation with chromosome 1G, resulting in the transfer of the group-1 gliadin locus to 6A^t. Chromosome 2G substituted for 2B at a frequency higher than expected and may carry putative homoeoalleles of gametocidal genes present on group-2 chromosomes of several alien species. Our data indicate a common origin for tetraploid wheat species, but from separate hybridization events because of the presence of a different spectrum of intergenomic translocations.     

Reticulate evolution in Ranunculus cantonensis polyploid complex and its allied species

Hybridization is particularly likely to occur in initial and young polyploid complexes, and interspecies hybridization between diverged species usually leads to a complicated reticulate evolution. The Ranunculus cantoniensis complex and its allied species include R. chinensis (2x), R. silerifolius var. silerifolius (2x), R. cantoniensis (4x), R. trigonus (2x), R. shuichengensis (2x), R. diffusus (4x), R. repens (4x), R. vaginatus (5x) and R. sieboldii (6x, 8x). Many morphological intermediates can be found between the members of this complex, and the relationship among members is complicated. By analyzing internal transcribed spacers and nrDNA FISH (fluorescence in situ hybridization) signals, we unraveled the phylogenetic and genetic constitution of the various taxonomic units of this complex. Haplotypes were highly separated by median-joining network analysis and at least four haplogroups emerged in which there were 11 primary haplotypes; six out of ten taxa shared haplotype 1, suggesting that haplotype 1, a variation of the primary haplotype R. chinensis, served as the pivotal genome in the complex. The pollen characteristics and electrophoretic patterns of R. vaginatus (5x) showed it to be an intermediate between R. diffusus (4x) and R. sieboldii (6x). The distribution of R. vaginatus (5x) was located at the junction of the distributions of R. diffusus (4x) and R. sieboldii (6x). Ranunculus vaginatus (5x) shared haplotypes 7 and 8 with R. diffusus (4x), and haplotypes 8 and 9 with R. sieboldii (6x). This proved that R. vaginatus (5x) emerged from hybridization between R. diffusus (4x) and R. sieboldii (6x). The results of FISH also support a hybrid origin of R. vaginatus (5x). The findings of this study clearly show that there are only eight taxa in this polyploid complex including R. chinensis (2x), R. silerifolius var. silerifolius (2x), R. trigonus (2x), R. silerifolius var. dolicanthus(2x), R. cantoniensis (4x), R. diffusus (2x), R. vaginatus (5x) and R. sieboldii (6x, 8x). These taxa associated with each other by hybridizing with the pivotal genome. Ranunculus cantoniensis (4x) and R. vaginatus (5x) arose from hybridization events between diverged species in the polyploid complex, leading to a complicated reticulate evolution.     

Chromosome diminution in Ascaris suum: Chromatin structure

Ascaris suum loses 56% of its nuclear DNA during chromosome diminution. Measured values of histones per nucleus are relatively constant, resulting in an approximate doubling of histone: DNA ratios during this process. Experiments were performed in an effort directed towards ascertaining the location of the increased histones. Repeat lengths of micrococcal nuclease protected pre- and post-diminution DNA were determined. Nuclear sap proteins from post-diminution nuclei were also examined in order to test the possibility of nuclear pools of free histones.     

Chromosome organisation in polyploid mouse trophoblast nuclei

At least one-third of mouse trophoblast cells undergo endoreduplication during the first half of gestation. It has been suggested that the endoreduplicated chromosomes may be polytenised. Here it is shown, using in situ hybridisation to the -1 antitrypsin genes, which map at a unique site, that while there is a tendency for duplicated chromosomes to cluster, this does not involve the complete fusion of replicated chromatids found in fully polytene chromosomes, and in a substantial proportion of homologues the sites on the chromosome arms corresponding to these genes are widely separated. The centromeres do not fuse into a single chromocentre but the possibility is not ruled out that individual chromosomes may be polytenised in the centromeric region. Evidence is also presented showing that endoreduplication in trophoblast nuclei is not accompanied by the formation of new prekinetochore structures, in contrast to the situation in polyploid mouse liver and C127 cells.     

Early adjustment of patterns of metaphase association in the evolution of a polyploid species

Evidence is presented for the adjustment of chromosomal association at metaphase in the very early stages of establishment of a new level of polyploidy. The species concerned is S. cambrensis Rossner, the hexaploid hybrid between S. squalidus L. (2n=20) and S. vulgaris (2n=40). It is suggested that the perfect bivalent pairing of S. vulgaris is the basis for the success of S. cambrensis.     

Studies on a species of Monosporascus isolated from triticum

Hyphal parasitic behaviour of Fusarium oxysporum on Rhizoctonia solani and consecutive changes during this phenomenon have been investigated and studied. The hyphal parasitism was very commonly recorded between the test fungi. During the course of parasitism coiling, penetration, lysis and formation of chlamydospores by F. oxysporum on R. solani were observed. R. solani is a new host record for the mycoparasite F. oxysporum.     

Biochemistry of photosynthesis in species of triticum of differing ploidy

Illuminated flag leaves of Triticum monococcum(2X), T. urartu(2X), T. dicoccum(4X), T. dicoccoides(4X), and T. aestivum(6X) were exposed to ¹⁴CO₂ for 10 seconds and subsequently allowed to continue photosynthesis in the ambient air for periods of up to 2 minutes. The relative distribution of ¹⁴C among water-soluble products in the leaves was similar for each species at each sampling time. After the 10-second pulse of ¹⁴CO₂, radioactivity was mainly in phosphate esters with less than 5% in C₄ acids. Subsequently, radioactivity increased in sucrose, glycine, and serine at the expense of that in phosphate esters. By 2 minutes, between 18% and 29% of the ¹⁴C was in glycine plus serine. The results suggest rapid photorespiration in all species and an absence of C₄ photosynthesis.     

The ups and downs of genome size evolution in polyploid species of Nicotiana (Solanaceae)

BACKGROUND: In studies looking at individual polyploid species, the most common patterns of genomic change are that either genome size in the polyploid is additive (i.e. the sum of parental genome donors) or there is evidence of genome downsizing. Reports showing an increase in genome size are rare. In a large-scale analysis of 3008 species, genome downsizing was shown to be a widespread biological response to polyploidy. Polyploidy in the genus Nicotiana (Solanaceae) is common with approx. 40 % of the approx. 75 species being allotetraploid. Recent advances in understanding phylogenetic relationships of Nicotiana species and dating polyploid formation enable a temporal dimension to be added to the analysis of genome size evolution in these polyploids. METHODS: Genome sizes were measured in 18 species of Nicotiana (nine diploids and nine polyploids) ranging in age from <200,000 years to approx. 4.5 Myr old, to determine the direction and extent of genome size change following polyploidy. These data were combined with data from genomic in situ hybridization and increasing amounts of information on sequence composition in Nicotiana to provide insights into the molecular basis of genome size changes. KEY RESULTS AND CONCLUSIONS: By comparing the expected genome size of the polyploid (based on summing the genome size of species identified as either a parent or most closely related to the diploid progenitors) with the observed genome size, four polyploids showed genome downsizing and five showed increases. There was no discernable pattern in the direction of genome size change with age of polyploids, although with increasing age the amount of genome size change increased. In older polyploids (approx. 4.5 million years old) the increase in genome size was associated with loss of detectable genomic in situ hybridization signal, whereas some hybridization signal was still detected in species exhibiting genome downsizing. The possible significance of these results is discussed.     

Recombinant genome of cereals: the pattern of formation and the role in evolution of polyploid species

The pivotal-differential model of evolution of polyploid species of cereals has been experimentally reproduced, and the pattern of the formation of a recombinant genome has been analyzed. It has been found that mutual substitution of chromosomes of the original genomes is subjected to selection pressure and, hence, is nonrandom. The selection occurs at the level of homeologs, whose selective advantages are determined by interactions between the genotype and the environment. If a homeolog has distinct selective advantages, the chromosomal composition of the corresponding homeologous group is completed rapidly, which leads to the formation of intergenomic recombination at the level of whole chromosomes. If homeologs have the same competitiveness, the composition of the group is stabilized more slowly. Domination of the genetic systems of the basic genome ensures a high rate of pairing of homeologous chromosomes of the recombinant genome during meiosis, which leads to recombinations at the level of chromosomal segments. It has been demonstrated that different combinations of chromosomes from original genomes are selected at different conditions of plant growth.     

Phylogeny of Alisma (Alismataceae) revisited: implications for polyploid evolution and species delimitation

Journal of Plant Research - Alisma L. is a genus of aquatic and wetland plants belonging to family Alismataceae. At present, it is thought to contain ten species. Variation in ploidy level is known...     

The evolution of polyploid wheats: identification of the A genome donor species.

Cytogenetic work has shown that the tetraploid wheats, Triticum turgidum and T. timopheevii, and the hexaploid wheat T. aestivum have one pair of A genomes, whereas hexaploid T. zhukovskyi has two. Variation in 16 repeated nucleotide sequences was used to identify sources of the A genomes. The A genomes of T. turgidum, T. timopheevii, and T. aestivum were shown to be contributed by T. urartu. Little divergence in the repeated nucleotide sequences was detected in the A genomes of these species from the genome of T. urartu. In T. zhukovskyi one A genome was contributed by T. urartu and the other was contributed by T. monococcum. It is concluded that T. zhukovskyi originated from hybridization of T. timopheevii with T. monococcum. The repeated nucleotide sequence profiles in the A genomes of T. zhukovskyi showed reduced correspondence with those in the genomes of both ancestral species, T. urartu and T. monococcum. This differentiation is attributed to heterogenetic chromosome pairing and segregation among chromosomes of the two A genomes in T. zhukovskyi.     

Recombinant genome of cereals: The pattern of formation and the role in evolution of polyploid species

The pivotal-differential model of evolution of polyploid species of cereals has been experimentally reproduced, and the pattern of the formation of a recombinant genome has been analyzed. It has been found that mutual substitution of chromosomes of the original genomes is subjected to selection pressure and, hence, is nonrandom. The selection occurs at the level of homeologs, whose selective advantages are determined by interactions between the genotype and the environment. If a homeolog has distinct selective advantages, the chromosomal composition of the corresponding homeologous group is completed rapidly, which leads to the formation of intergenomic recombination at the level of whole chromosomes. If homeologs have the same competitiveness, the composition of the group is stabilized more slowly. Domination of the genetic systems of the basic genome ensures a high rate of pairing of homeologous chromosomes of the recombinant genome during meiosis, which leads to recombinations at the level of chromosomal segments. It has been demonstrated that different combinations of chromosomes from original genomes are selected at different conditions of plant growth.     

Oilseed rape: learning about ancient and recent polyploid evolution from a recent crop species

Oilseed rape (Brassica napus) is one of our youngest crop species, arising several times under cultivation in the last few thousand years and completely unknown in the wild. Oilseed rape originated from hybridisation events between progenitor diploid species B. rapa and B. oleracea, both important vegetable species. The diploid progenitors are also ancient polyploids, with remnants of two previous polyploidisation events evident in the triplicated genome structure. This history of polyploid evolution and human agricultural selection makes B. napus an excellent model with which to investigate processes of genomic evolution and selection in polyploid crops. The ease of de novo interspecific hybridisation, responsiveness to tissue culture, and the close relationship of oilseed rape to the model plant Arabidopsis thaliana, coupled with the recent availability of reference genome sequences and suites of molecular cytogenetic and high‐throughput genotyping tools, allow detailed dissection of genetic, genomic and phenotypic interactions in this crop. In this review we discuss the past and present uses of B. napus as a model for polyploid speciation and evolution in crop species, along with current and developing analysis tools and resources. We further outline unanswered questions that may now be tractable to investigation.     

植物多倍体基因组的形成与进化

多倍化是植物进化变异的自然现象,也是促进植物发生进化改变的重要力量。在被子植物中,约 70％的种类在进化史中曾发生过一次或多次多倍化的过程。目前的研究结果表明,自然界绝大多数多倍体是通过未减数配子的融合而形成的,并且很多多倍体种是通过多次独立的多倍化过程而重复发生的。由多倍化所导致的重复基因在多倍体基因组中可能有三种不同的命运,即：保持原有的功能、基因沉默或分化并执行新的功能。多倍化以后,重复基因组的进化动态则主要表现在染色体重排和“染色体二倍化”、不同基因组之间的相互渗透、以及核-质之间的相互作用等方面。     

Leaf photosynthesis and conductance of selected triticum species at different water potentials

Leaf gas exchange characteristics of a desert annual (Triticum kotschyi [Boiss.] Bowden) and the wheat cultivar TAM W-101 (Triticum aestivum L. em Thell) were compared over a range of leaf water potentials from −0.50 to −2.9 megapascals. At an ambient [CO₂] of 330 microliters per liter, T. kotschyi had higher conductance and CO₂ assimilation (A) at a given water potential than T. aestivum. Under well watered conditions, A versus internal CO₂ concentration (C_i) response curves for both species were similar in shape and magnitude, and the higher A of T. kotschyi at an ambient [CO₂] of 330 microliters per liter was mostly related to the higher stomatal conductance of T. kotschyi. The higher conductance of T. kotschyi than T. aestivum under well watered conditions was associated with higher C_i and lower water use efficiency. Under water deficits, however, C_i at 330 microliters per liter ambient [CO₂] did not differ significantly between species. T. kotschyi had higher A under water deficits than T. aestivum primarily because its A versus C_i response curves had higher A at C_i values above about 150 microliters per liter. The results show that conductance played an important role in the high A of T. kotschyi under well watered conditions, but under water deficits the high A of T. kotschyi was related more to the maintenance of a higher capacity for mesophyll photosynthesis.