New hybrids with d genome wheat relatives

The cytology of nine new D genome hybrids involving Triticum syriacum, Triticum ventricosum, Triticum cyclindricum, Triticum juvenale, Triticum crassum, Triticum tauschii and Triticum aestivum is described. The calculation of numerical values of the relative affinity and the patterns of chromosome pairing indicate that the D genome in T. syriacum and T. juvenale may have been substantially modified and that of T. crassum somewhat modified from that of the diploid progenitor, T. tauschii.     

Repeated DNA sequences inTriticum (Poaceae): Chromosomal mapping and its bearing on the evolution of B and G genomes

The somatic chromosomes ofTriticum turgicum var.durum cv. Langdon andT. dicoccoides (AABB tetraploids),T. timopheevii, andT. araraticum (AAGG tetraploids) were assayed for distribution patterns of a highly repeated 120bp DNA sequence by in situ hybridization. The repeated sequence appears to be an ancient sequence shared withSecale andAegilops. The distribution patterns of the chromosomes were compared to the patterns of the A and B genome chromosomes ofT. aestivum cv. Chinese Spring (AABBDD hexaploid).T. turgidum andT. dicoccoides were observed to have identical in situ hybridization patterns. In both species, nine chromosomes with a total of 21 sites of hybridization were observed. The pattern, with few exceptions, was identical to that of Chinese Spring.T. araraticum andT. timopheevii were observed to have different patterns. InT. araraticum, six chromosomes with 21 total hybridization sites are present while inT. timopheevii nine chromosomes with 19 total sites exist. Major differences in hybridization patterns were observed between the B and G genomes. The divergence of the tetraploid wheats in this study appears to have resulted in changes in location, not in amount, of the ancient repeated sequence.     

Assessment of genomic and species relationships in Triticum and Aegilops by PAGE and by differential staining of seed albumins and globulins

Summary Endosperm protein components from common bread wheats (Triticum aestivum L.) and related species were extracted with aluminum lactate, pH 3.2, and examined by electrophoresis in the same buffer. Electrophoretic patterns of the albumins and globulins were compared to evaluate the possibility that a particular species might have contributed its genome to tetraploid or hexaploid wheat. Together with protein component mobilities, differential band staining with Coomassie Brilliant Blue R250 was employed to test the identity or non-identity of bands. Eight species and 63 accessions, representative of Triticum and Aegilops were tested. Considerable intraspecific variation was observed for patterns of diploid but not for tetraploid or hexaploid species. Patterns of some accessions of Triticum urartu agreed closely with major parts of the patterns of Triticum dicoccoides and T. aestivum. A fast-moving, green band was found in all accessions of T. urartu and of Triticum boeoticum, however, that was not found in those of T. dicoccoides or T. aestivum. This band was present in all accessions of Triticum araraticum and Triticum zhukovskyi. Patterns of Aegilops longissima, which has been suggested as the donor of the B genome, differed substantially from those of T. dicoccoides and T. aestivum. Finally, two marker proteins of intermediate mobility were also observed and may be used to discriminate between accessions of T. araraticum/T. zhukovskyi and those of T. dicoccoides/T. aestivum.     

Elimination of a tandem repeat of telomeric heterochromatin during the evolution of wheat

 An analysis of accessions of Triticum and Aegilops species (86 diploid, 91 tetraploid and 109 hexaploid) was performed using squash-dot hybridization with the tandem repeat Spelt1 sequence as a probe. The Spelt1 sequence is a highly species-specific repeat associated with the telomeric heterochromatin of Aegilops speltoides Boiss. in which its copy numbers vary from 1.5×10⁵ to 5.3×10⁵. The amounts of Spelt1 are sharply decreased in tetraploid and hexaploid species and vary widely from less than 10² to 1.2×10⁴. Two tetraploid wheats, Triticum timopheevii Zhuk. and T. carthlicum Nevski, are exceptional endemic species and within their restricted geographical distributions maintain the amounts of Spelt1 unaltered. The Spelt1 repetitive sequence was localized on the 6BL chromosome of tetraploid wheat Triticum durum Desf. cv ‘Langdon’ by dot-hybridization using D-genome disomic substitution lines. The possible causes of the loss of the telomere-associated tandem repeat Spelt1 in the process of wheat evolution and polyploidization are discussed. Received: 5 March 1998 / Accepted: 28 May 1998     

The presence of the enhanced K/Na discrimination trait in diploid Triticum species

Summary A number of accessions of the three species of diploid wheat, Triticum boeoticum, T. monococcum, and T. urartu, were grown in 50 mol m^-3 NaCl+2.5 mol m^-3 CaCl₂. Sodium accumulation in the leaves was low and potassium concentrations remained high. This was not the case in T. durum grown under the same conditions, and indicates the presence in diploid wheats of the enhanced K/Na discrimination character which has previously been found in Aegilops squarrosa and hexaploid wheat. None of the accessions of diploid wheat showed poor K/Na discrimination, which suggests that if the A genome of modern tetraploid wheats was derived from a diploid Triticum species, then the enhanced K/Na discrimination character became altered after the formation of the original allopolyploid. Another possibility is that a diploid wheat that did not have the enhanced K/Na discrimination character was involved in the hybridization event which produced tetraploid wheat, and that this diploid is now extinct or has not yet been discovered.     

A comparative study of the mitochondrial genome organization in in vitro cultures of diploid, tetraploid, and hexaploid Triticum species

Southern-blot hybridizations of total DNA to mitochondrial DNA (mtDNA) probes were used to investigate the extent of mtDNA variability in cultures derived from immature embryos of diploid (Triticum monococcum, genomic formula: AA, T. tauschii, genomic formula: DD), allotetraploid (T. durum cv Creso, genomic formula: AABB), and allohexaploid (T. aestivum, genomic formula: AABBDD) wheat species. Similar distinct changes in mtDNA organization were observed in in vitro cultures of the derived tetraploid and the hexaploid species with related genomes. The tetraploid and hexaploid species share the B genome and mtDNA variability in in vitro culture is known to be under nuclear control. These results suggest that a study of B genome diploids and other polyploid combinations would now shed light on whether or not mtDNA variability in tissue cultures is under B-genome control.     

Phylogenetic analysis of tetraploid wheat based on nuclear DMC1 gene

Tetraploid wheat (AABB or AAGG, 2n = 4x = 28) holds an important place in Triticum. It includes two allopolyploid species, Triticum turgidum and Triticum timopheevii. Many problems concerning the phylogenetic relationships among tetraploid wheat species remain unresolved. In this study, sequences data for the nuclear DMC1 gene from 61 accessions of Triticum and Aegilops species, representing diploid and tetraploid species, were used to examine the phylogenetic relationships among tetraploid wheat. Phylogenetic trees were constructed using maximum-likelihood and neighbor-joining approaches, and gene flow and genetic differentiation values were computed. The results indicated that the A genome of tetraploid wheat originated from T. urartu rather than T. monococcum, and Aegilops speltoides was the donor of the B and G genomes. Hulled tetraploid wheat accessions formed a subclade, and naked tetraploid wheat got other subclade, indicating that at least two intermediary subspecies were involved in the evolution of T. turgidum. Triticum turgidum and T. timopheevii might have simultaneously originated from a hybridization events. These results indicated that the DMC1 gene sequences are useful for resolution of the molecular phylogenetic relationships of tetraploid wheat.     

Studies on the origin and evolution of tetraploid wheats based on the internal transcribed spacer (ITS) sequences of nuclear ribosomal DNA

In this study, the internal transcribed spacer (ITS) sequences of nuclear ribosomal DNA in the tetraploid wheats, Triticum turgidum (AABB) and Triticum timopheevii (AAGG), their possible diploid donors, i.e., Triticum monococcum (AA), Triticum urartu (AA), and five species in Aegilops sect. Sitopsis (SS genome), and a related species Aegilops tauschii were cloned and sequenced. ITS1 and ITS2 regions of 24 clones from the above species were compared. Phylogenetic analysis demonstrated that Aegilops speltoides was distinct from other species in Aegilops sect. Sitopsis and was the most-likely donor of the B and G genomes to tetraploid wheats. Two types of ITS repeats were cloned from Triticum turgidum ssp. dicoccoides, one markedly similar to that from T. monococcum ssp. boeoticum (AA), and the other to that from Ae. speltoides (SS). The former might have resulted from a recent integression event. The results also indicated that T. turgidum and T. timopheevii might have simultaneously originated from a common ancestral tetraploid species or be derived from two hybridization events but within a very short interval time. ITS paralogues in tetraploid wheats have not been uniformly homogenized by concerted evolution, and high heterogeneity has been found among repeats within individuals of tetraploid wheats. In some tetraploid wheats, the observed heterogeneity originated from the same genome (B or G). Three kinds of ITS repeats from the G genome of an individual of T. timopheevii ssp. araraticum were more divergent than that from inter-specific taxa. This study also demonstrated that hybridization and polyploidization might accelerate the evolution rate of ITS repeats in tetraploid wheats.     

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.     

The origins of the genomes of Triticum biunciale,t. ovatum,t. neglectum,t. columnare,and t. rectum (poaceae) based on variation in repeated nucleotide sequences

The origins of the genomes of allotetraploid species Triticum biunciale, T. ovatum, T. neglectum, and T. columnare, and allohexaploid T. rectum were investigated by examining the presence of specific restriction fragments of repeated nucleotide sequences in DNAs of the polyploid species. The restriction fragments were detectable either in a single diploid Triticum species (unique characters) or a group of diploid species (unique shared characters). The analysis showed that Triticum biunciale and T. ovatum are closely related. In both species, one pair of genomes is closely related to the genome of T. umbellulatum and the other is a modified genome of T. comosum. The same genome formula, UUM°M°, is proposed for T. biunciale and T. ovatum. Potential reasons for the modification of the M° genome are discussed. Triticum neglectum and T. columnare are also closely related to each other and have the same genomes. They share the U genome with T. biunciale and T. ovatum, but their second pair of genomes is unrelated to the M° genome. No relationship was found of this genome to a genome of any extant diploid species of Triticum or any phylogenetic lineage leading to the extant diploid species. This unknown genome is designated X'.∗∗∗ The proposed genome formula for T. neglectum and T. columnare is UUX'X'∗∗∗. Hexaploid T. rectum originated from hybridization of one of the tetraploid species with the formula UUX'X', likely T. neglectum, with T. uniaristatum (genome N), and its genome formula is UUX'X'NN.     

Effects of Soil Flooding on Growth and Grain Yield of Populations of Tetraploid and Hexaploid Species of Wheat

Single populations of three hexaploid species of wheat, Triticumaestivum, Triticum spelta and Triticum macha, and two populationsof the tetraploid wheat, Triticum dicoccum (Pontus and Bordeaux),were grown in a greenhouse experiment at a range of soil floodingregimes: free draining, two levels of transient flooding andcontinuous flooding. Increasing severity of flooding treatment resulted in increasedsoil reduction and an increase in the concentration of reducediron and manganese in the experimental soil, and also resultedin a reduction in vegetative growth, number of inflorescences,grain number and grain weight. There were, however, large differencesbetween the wheat populations in the degree of reduction inyield caused by flooding. The population of T. macha was muchmore flooding-tolerant than the other hexaploid species andthe ‘Pontus’ population of the emmer wheat, T. dicoccum,was more tolerant than the ‘Bordeaux’ populationof this species and than T. spelta and T. aestivum. The results are discussed in relation to the origin of the populations. Soil flooding, Triticum aeslivum, Triticum macha, Triticum spelta, Triticum dicoccum     

GluDy allele variations in Aegilops tauschii and Triticum aestivum: implications for the origins of hexaploid wheats

To investigate the evolution and geographical origins of hexaploid wheat, we examined a 284 bp sequence from the promoter region of the GluDy locus, coding for the y subunit of high-molecular-weight glutenin. Fourteen different alleles were found in 100 accessions of Aegilops tauschii and 169 of Triticum aestivum. Two alleles were present in both species; the other 7 alleles from Ae. tauschii and 5 from T. aestivum were unique to their respective species. The two shared alleles differed at only one nucleotide position within the region sequenced, but their apparent association with the common haplotypes GluD1a and GluD1d, which have substantial differences within their GluDy coding regions, makes it unlikely that the alleles evolved independently in Ae. tauschii and T. aestivum. The results therefore support previous studies which suggest that there were at least two Ae. tauschii sources that contributed germplasm to the D genome of T. aestivum. The number of alleles present in T. aestivum, and the nucleotide diversity of these alleles, indicates that this region of the D genome has undergone relatively rapid change since polyploidisation. Ae. tauschii from Syria and Turkey had relatively high nucleotide diversity and possessed all the major GluDy alleles, indicating that these populations are probably ancient and not the result of adventive spread. The presence in the Turkish population of both of the shared alleles suggests that hexaploid wheat is likely to have originated in southeast Turkey or northern Syria, within the Fertile Crescent and near to the farming villages at which archaeological remains of hexaploid wheats are first found. A second, more recent, hexaploidisation probably occurred in Iran.     

A reconsideration of relationships among Japanese Trillium species based on karyology and AFLP data

To reexamine the relationships among the Japanese Trillium species that form a polyploid series, we performed principal-coordinates analysis (PCOA) based on proposed karyotypic compositions and on amplified fragment length polymorphism (AFLP) analysis. A hexaploid species, T. smallii, whose karyotypic composition had been hypothesized as K₂K₂SSUU, with hybridization between tetraploid T. apetalon (SSUU) and a presumed K₂K₂ diploid species, showed a genotype corresponding to T. yezoense (K₁SU). Accordingly, T. smallii appears to be an allopolyploid of T. yezoense, with the karyotypic composition K₁K₁SSUU. Trillium channellii, a recently described tetraploid species whose origin and genealogical position remains unclear, showed a genotype corresponding to K₁K₁K₂T. We conclude that T. channellii may be derived from hybridization of T. camschatcense (K₁K₁) as the seed parent and T. tschonoskii (K₂K₂TT) or hexaploid T. hagae (K₁K₁K₂K₂TT) as the pollen parent.     

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.     

Heterochronic development of the floret meristem determines grain number per spikelet in diploid, tetraploid and hexaploid wheats

Background and Aims

The inflorescence of grass species such as wheat, rice and maize consists of a unique reproductive structure called the spikelet, which is comprised of one, a few, or several florets (individual flowers). When reproductive growth is initiated, the inflorescence meristem differentiates a spikelet meristem as a lateral branch; the spikelet meristem then produces a floret meristem as a lateral branch. Interestingly, in wheat, the number of fertile florets per spikelet is associated with ploidy level: one or two florets in diploid, two or three in tetraploid, and more than three in hexaploid wheats. The objective of this study was to identify the mechanisms that regulate the architecture of the inflorescence in wheat and its relationship to ploidy level.

Methods

The floral anatomy of diploid (Triticum monococcum), tetraploid (T. turgidum ssp. durum) and hexaploid (T. aestivum) wheat species were investigated by light and scanning electron microscopy to describe floret development and to clarify the timing of the initiation of the floret primordia. In situ hybridization analysis using Wknox1, a wheat knotted1 orthologue, was performed to determine the patterning of meristem formation in the inflorescence.

Key Results

The recessive natural mutation of tetraploid (T. turgidum ssp. turgidum) wheat, branching head (bh), which produces branched inflorescences, was used to demonstrate the utility of Wknox1 as a molecular marker for meristematic tissue. Then an analysis of Wknox1 expression was performed in diploid, tetraploid and hexaploid wheats and heterochronic development of the floret meristems was found among these wheat species.

Conclusions

It is shown that the difference in the number of floret primordia in diploid, tetraploid and hexaploid wheats is caused by the heterochronic initiation of floret meristem development from the spikelet meristem.Key words: Triticum, wheat, inflorescence, spikelet, floret, meristem, heterochrony, heterochronic development, knotted1, polyploidy     

The origin of spelt and free-threshing hexaploid wheat

It is widely believed that hexaploid wheat originated via hybridization of hulled tetraploid emmer with Aegilops tauschii (genomes DD) and that the nascent hexaploid was spelt, from which free-threshing wheat evolved by mutations. To reassess the role of spelt in the evolution of Triticum aestivum, 4 disomic substitution lines of Ae. tauschii chromosome 2D in Chinese Spring wheat were developed and one of them was used to map the Tg locus, which controls glume tenacity in Ae. tauschii, relative to simple sequence repeat (SSR) and expressed sequence tag loci on wheat chromosome 2D. The segregation of SSR markers was used to assess the presence of Tg alleles in 11 accessions of spelt, both from Europe and from Asia. Ten of them had an inactive tg allele in the D genome and most had an active Tg allele in the B genome. This is consistent with spelt being derived from free-threshing hexaploid wheat by hybridization of free-threshing wheat with hulled emmer. It is proposed that the tetraploid parent of hexaploid wheat was not hulled emmer but a free-threshing form of tetraploid wheat.     

The origin of the B-genome of bread wheat (Triticum aestivum L.)

Understanding the origin of cultivated wheats would further their genetic improvement. The hexaploid bread wheat (Triticum aestivum L., AABBDD) is believed to have originated through one or more rare hybridization events between Aegilops tauschii (DD) and the tetraploid T. turgidum (AABB). Progenitor, of the A-genome of the tetraploid and hexaploid wheats has generally been accepted to be T. urartu. In spite of the large number of attempts and published reports about the origin of the B-genome in cultivated wheats, the donor of the B-genome is still relatively unknown and controversial and, hence, remains open. This genome has been found to be closely related to the S-genome of the Sitopsis section (Ae. speltoides, Ae. longissima, Ae. sharonensis, Ae. searsii, and Ae. bicornis) of the genus Aegilops L. Among Sitopsis species, the most positive evidence has been accumulated for Ae. speltoides as the progenitor of the B-genome. Therefore, one or more of the Sitopsis species were proposed frequently as the B-genome donor. Although several reviews have been written on the origin of the genomes of wheat over the years, this paper will attempt for the first time to review the immense literature on the subject, with a particular emphasis on the B-genome which has attracted a huge attention over some 100 years. The ambiguity and conflicting results in most of the methods employed in deducing the precise B-genome donor/s to bread wheat are also discussed.     

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     

Allopolyploidy alters gene expression in the highly stable hexaploid wheat

Hexaploid wheat (Triticum aestivum) contains triplicated genomes derived from three distinct species. To better understand how different genomes are coordinated in the same nucleus of the hexaploid wheat, we globally compared gene expression of a synthetic hexaploid wheat with its diploid (Aegilops tauschii) and tetraploid (T. turgidum) parents by cDNA-AFLP display. The results suggested that the expression of a significant fraction of genes was altered in the synthetic hexaploid; most appeared to be diminished and some were activated. We characterized nine cDNA clones in details. Cytogenetic as well as genomic sequence analyses indicated that the gene silencing was not due to chromosome/DNA loss but was caused by gene regulation. Northern and RT-PCR divided these genes into three groups: (I) four genes were down-regulated nonspecifically, likely involving both parental orthologues; (II) four genes were down-regulated in an orthologue-dependent manner; (III) one gene was activated specifically in the synthetic hexaploid wheat. These genes were often altered non-randomly in different synthetic hexaploids as well as natural hexaploid wheat, suggesting that many of the gene expression changes were intrinsically associated with polyploidy.     

Evidence of widespread hybridization among couch grasses (Elymus,Poaceae)

Hybridization, polyploidization, and crop‐to‐wild gene transfer within the agriculturally important tribe Triticeae are well explored experimentally, but the true consequences of both phenomena under natural conditions remain understudied. The present paper reports on an investigation of three species of couch grasses (Elymus hispidus, E. repens, and E. caninus) examining the ploidy levels and absolute genome sizes (1081 plants from 302 natural populations in Central Europe, verified by chromosome counts) and their morphological delimitation. In the present study, the hexaploid level prevailed in E. hispidus and E. repens whereas E. caninus was exclusively tetraploid. Introgressive hybridization between hexaploid species, unidirectionally shifted towards E. hispidus, was indicated by a continual pattern of genome size values. We did not find any evidence for heteroploid hybridization involving tetraploid E. caninus; however, we detected minority cytotypes among both E. caninus plants (hexaploid) and E. repens–E. hispidus hybrids (heptaploid and nonaploid) suggesting the formation of unreduced gametes. Morphometric results (367 plants, redundancy analysis, principal component analysis, and correlation analysis) mirrored the continual homoploid pattern of absolute genome size (including the unidirectional shift), and a significant correlation between absolute genome size and morphology was confirmed. Moreover, morphometric analyses detected additional characteristics for the delimitation of the Elymus taxa under study. Considering the crossability of E. hispidus with Triticum aestivum (bread wheat), the revealed extent of introgressive hybridization has implications for assessing the potential risk of gene flow between crops and troublesome weeds.