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Triticale
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Triticale (× Triticosecale) is a hybrid of wheat (Triticum) and rye (Secale) first bred in laboratories during the late 19th century. The grain was originally bred in Scotland and Sweden. Commercially available triticale is almost always a 2nd generation hybrid, i.e. a cross between two kinds of triticale (primary triticales). As a rule, triticale combines the high yield potential and good grain quality of wheat with the disease and environmental tolerance (including soil conditions) of rye.
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Triticale (× Triticosecale) is a hybrid of wheat (Triticum) and rye (Secale) first bred in laboratories during the late 19th century. The grain was originally bred in Scotland and Sweden. Commercially available triticale is almost always a 2nd generation hybrid, i.e. a cross between two kinds of triticale (primary triticales). As a rule, triticale combines the high yield potential and good grain quality of wheat with the disease and environmental tolerance (including soil conditions) of rye. Only recently has it been developed into a commercially viable crop. Depending on the cultivar, triticale can more or less resemble either of its parents. It is grown mostly for forage or animal feed although some triticale-based foods can be purchased at health food stores or are to be found in some breakfast cereals.
The word 'triticale' is a fusion of the latin words triticum (or wheat) and secale (rye). When crossing wheat and rye, wheat is used as the female parent and rye as the male parent (pollen donor). The resulting hybrid is sterile and thus has to be treated with the alkaloid chemical colchicine to make it fertile and thus able to reproduce itself.
The primary producers of triticale are Poland, Australia, Germany, France, China and Belarus. In 2005, according to the Food and Agriculture Organization (FAO), 13.5 million tons were harvested in 28 countries across the world.
The triticale hybrids are all amphidiploid, which means the plant is diploid for two genomes derived from different species, in other words triticale is an allotetraploid. In earlier years most work was done on octoploid triticale. Different ploidy levels have been created and evaluated over time. The tetraploids showed little promise, but hexaploid triticale was successful enough to find commercial application. (Oetler 2005)
The CIMMYT triticale improvement program wanted to improve food production and nutrition in developing countries. According to Villegas (1973) triticale has potential in the production of bread and other food products such as pasta and breakfast cereals. The protein content is higher than that of wheat although the glutenin fraction is less. The grain has also been stated to have higher levels of lysine than wheat. Assuming increased acceptance, the milling industry will have to adapt to triticale, as the milling techniques employed for wheat are unsuited to triticale. Sell et al. (1962) delivered reports of triticale suitability as a grain feed and it is a better ruminant feed than other cereals due to its high starch digestibility. (Bird et al. 1999) As a feed grain, triticale is already well established and of high economic importance. Triticale has received attention as a potential energy crop and research is currently being conducted on the use of the crop's biomass in bioethanol production.
Biology and genetics
Earlier work with wheat-rye crosses was difficult due to low survival of the resulting hybrid embryo and spontaneous chromosome doubling. (Oetler, 2005). These two factors were difficult to predict and control. To improve the viability of the embryo and thus avoid its abortion, in vitro culture techniques were developed. (Laibach, 1925) Colchicine was used as a chemical agent to double the chromosomes. (Blakeslee & Avery 1937) After these developments a new era of triticale breeding was introduced. Earlier triticale hybrids had four reproductive disorders--namely, meiotic instability, high aneuploid frequency, low fertility and shriveled seed. (Muntzing 1939; Krolow 1966). Cytogenetical studies were encouraged and well funded to overcome these problems.
It is especially difficult to see the expression of rye genes in the background of wheat cytoplasm and the predominant wheat nuclear genome. This makes it difficult to realise the potential of rye in disease resistance and ecological adaptation. One of the ways to relieve this problem was to produce secalotricum, in which rye cytoplasm was used instead of that from wheat.
Triticale is essentially a self-fertilizing, or naturally inbred, crop. This mode of reproduction results in a more homozygous genome. The crop is, however, adapted to this form of reproduction from an evolutionary point of view. Cross-fertilization is also possible, but it is not the primary form of reproduction.
Conventional breeding approaches
The aim of a triticale breeding programme is mainly focused on the improvement of quantitative traits such as grain yield, nutritional quality, plant height, as well as traits which are more difficult to improve, such as earlier maturity and improved test weight (a measure of yield). These traits are controlled by more than one gene. (Triticale Production and Utilization Manual 2005) However, problems arise because such polygenic traits involve the integration of several physiological processes in their expression. Thus the lack of single-gene control (or simple inheritance) results in low trait heritability. (Zumelzú et al. 1998)
Since the induction of the CIMMYT triticale breeding programme in 1964, the improvement in realized grain yield has been remarkable. In 1968, at Ciudad Obregon, Sonora State in Northwest Mexico, the highest yielding triticale line produced 2.4 t/ha. Today, CIMMYT has released high yielding spring triticale lines (e.g. Pollmer-2) which have surpassed the 10 t/ha yield barrier under optimum production conditions. (Hede 2000)
Based on the commercial success of other hybrid crops, the use of hybrid triticales as a strategy for enhancing yield in favourable as well as marginal environments has proven successful over time. Earlier research conducted by CIMMYT made use of a chemical hybridising agent in order to evaluate heterosis in hexaploid triticale hybrids. To select the most promising parents for hybrid production, testcrosses conducted in various environments are required. This is because the variance of their specific combining ability (sca) under differing environmental conditions is the most important component in evaluating their potential as parents to produce promising hybrids. The prediction of general combining ability (gca) of any triticale plant from the performance of its parents is only moderate with respect to grain yield. Commercially exploitable yield advantages of hybrid triticale cultivars is dependent on improving parent heterosis and on advances in inbred-line development.
Triticale is useful as an animal feed grain. However, it is necessary to improve its milling and bread-making quality aspects in order to increase its potential for human consumption. It was initially noted that the relationship between the constituent wheat and rye genomes produced meiotic irregularities and that genome instability and incompatibility presented numerous problems when attempts were made to improve triticale. This led to two alternative methods to study and improve the crops reproductive performance, namely the improvement of the number of grains per floral spikelet and its meiotic behaviour. The number of grains per spikelet has an associated low heritability value. [de Zumelzú et al. 1998] In improving yield, indirect selection (the selection of correlated/related traits other than that to be improved) is not necessarily as effective as direct selection. (Gallais 1984)
Lodging (the toppling over of the plant stem especially under windy conditions) resistance is a complexly inherited (expression is controlled by many genes) trait and has thus been an important breeding aim in the past. (Tikhnenko et al. 2002) The use of dwarfing genes known as Rht genes, which have been incorporated from both Triticum and Secale, has resulted in a decrease of up to 20cm in plant height without causing any adverse side effects.
Application of newer techniques
Abundant information exists concerning disease resistance (R) genes for wheat and a continuously updated on-line catalogue, the Catalogue of Gene Symbols, of these genes can be found at http://wheat.pw.usda.gov/ggpages/wgc/98/. Another on-line database of cereal rust resistance genes is available at http://www.cdl.umn.edu/res_gene/res_gene.html. Unfortunately, less is known about rye and particularly triticale R-genes. Many R-genes have been transferred to wheat from its wild relatives and appear in the catalogue, thus making them available to triticale breeding. The two mentioned databases are significant contributors to improving the genetic variability of the triticale gene pool through gene (or more specifically, allele) provision. Genetic variability is essential for progress in breeding. In addition, genetic variability can also be achieved by producing new primary triticales, which essentially means the reconstitution of triticale, and the development of various hybrids involving triticale such as triticale-rye hybrids. In this way some chromosomes from the R genome have been replaced by some from the D genome. The resulting so-called substitution and translocation triticale facilitates the transfer of R-genes.
Introgression
Introgression involves the crossing of closely related plant relatives and results in the transfer of ‘blocks’ of genes, i.e. larger segments of chromosomes compared to single genes. R-genes are generally introduced within such blocks, which are usually incorporated/translocated/introgressed into the distal (extreme) regions of chromosomes of the crop being introgressed. Genes located in the proximal areas of chromosomes may be completely linked (very closely spaced) thus preventing or severely hampering genetic recombination which is necessary to incorporate such blocks. (Chelkowski & Tyrka 2004) Molecular markers (small lengths of DNA of a characterized/known sequence) are used to ‘tag’ and thus track such translocations. A weak colchicine chemical solution has been employed to increase the probability of recombination in the proximal chromosome regions and thus the introduction of the translocation to that region. The resultant translocation of smaller blocks that indeed carry the R-gene/s of interest has decreased the probability of introducing unwanted genes. (Lukaszewski 1995)
Production of doubled haploids
Doubled haploid (DH) plants have the potential to save much time in the development of inbred lines. This is achieved in a single generation as opposed to many which would otherwise occupy much physical space/facilities. DHs also express deleterious recessive alleles that are otherwise masked by dominance effects in a genome containing more than one copy of each chromosome. (And thus more than one copy of each gene) Various techniques exist to create DHs. The in-vitro culture of anthers and microspores is most often used in cereals including triticale. (Bernard & Charmet 1984; González and Jouve 2000; González et al. 1997) These two techniques are referred to as androgenesis, which refers to the development of pollen. Many plant species and cultivars within species including triticale are recalcitrant in that the success rate of achieving whole newly generated (diploid) plants is very low. GenotypeXculture medium interaction is responsible for varying success rates, as is a high degree of microspore abortion during culturing. (Gonzalez & Jouve 2005; Johansson et al. 2000) It is known that the response of parental triticale lines to anther culture is correlated (related) to the response of their progeny. (Anderson et al. 1989; Gonzalez et al. 1997; Konzak & Zhou 1992)
Chromosome elimination is another method of producing DHs and involves hybridisation of wheat with maize (Zea mays L.) followed by auxin treatment and the artificial rescue of the resultant haploid embryos before they naturally abort. This technique is applied rather extensively to wheat. (Bennet et al. 1990) Its success is in large part due to the insensitivity of maize pollen to the crossability inhibitor genes known as Kr1 and Kr2 that are expressed in the floral style of many wheat cultivars. (Bennett & Laurie 1987) The technique is unfortunately less successful in triticale. (Marcinska et al. 1998) However, Imperata cylindrica (a grass) was found to be just as effective as maize with respect to the production of DHs in both wheat and triticale. (Chaudhary et al. 2005)
Application of molecular markers
An important advantage of biotechnology applied to plant breeding is the speeding up of cultivar release that would otherwise take 8-12 years. It is the process of selection that is actually enhanced, i.e. retaining that which is desirable or promising and ridding that which is not. This carries with it the aim of changing the genetic structure of the plant population. The website http://maswheat.ucdavis.edu/protocols/protocols.htm is a valuable resource for Marker assisted selection (MAS) protocols relating to R-genes in wheat. MAS is a form of indirect selection. The Catalogue of Gene Symbols mentioned earlier is an additional source of molecular and morphological markers. Again, triticale has not been well characterized with respect to molecular markers although an abundance of rye molecular markers makes it possible to track rye chromosomes and segments thereof within a triticale background.
Yield improvements of up to 20% have been achieved in hybrid triticale cultivars due to a phenomenon described as heterosis. (Becker et al. 2001; Burger et al. 2003; Góral 2002; Góral et al. 1999) This raises the question of what inbred lines should be crossed (to produce hybrids) with each other as parents in order to maximize yield in their hybrid progeny. This is termed the ‘combining ability’ of the parental lines. The identification of good combining ability at an early stage in the breeding program can reduce the costs associated with ‘carrying’ a large number of plants (literally thousands) through the program and thus forms part of efficient selection. Combining ability is assessed by taking into consideration all available information on descent (genetic relatedness), morphology, qualitative (simply inherited) traits and biochemical and molecular markers. There exists exceptionally little information on the use of molecular markers to predict heterosis in triticale. (Góral et al. 2005) It is generally accepted that molecular markers are better predictors than morphological markers (agronomic traits) due to their insensitivity to variation in environmental conditions.
A useful molecular marker known as a Simple Sequence Repeat (or SSR) is used in breeding with respect to selection. SSRs are segments of a genome composed of tandem repeats of a short sequence of nucleotides, usually 2–6 bp. They are popular tools in genetics and breeding because of their relative abundance compared to other molecular marker types, high degree of polymorphism (number of variants), and easy assaying by PCR. However, they are expensive to identify and develop. Comparative genome mapping has revealed a high degree of similarity in terms of sequence co-linearity between closely related crop species. This allows the exchange of such markers within a group of related species such as wheat, rye and triticale. One study established a 58% and 39% transferability rate to triticale from wheat and rye respectively. (Baenziger et al. 2004) ‘Transferability’ refers to the phenomenon where the sequence of DNA nucleotides flanking the SSR loci (position on the chromosome) is sufficiently homologous (similar) between genomes of closely related species. Thus DNA primers (a generally short sequence of nucleotides are used to direct a copying reaction during PCR) designed for one species can be used to detect SSRs in related species. SSR markers are available in wheat and rye but very few if any are available for triticale. (Baenziger et al. 2004)
Genetic transformation
The genetic transformation of crops involves the incorporation of ‘foreign’ genes or rather, very small DNA fragments compared to introgression discussed earlier. Amongst other uses transformation is a useful tool to introduce new traits/characteristics into the transformed crop. Two methods are commonly employed, i.e infectious bacteria (Agrobacterium) -mediated and biolistics with the last-mentioned being most commonly applied to allopolyploid cereals such as triticale. Agrobacterium-mediated transformation however holds several advantages such as a low level of transgenic DNA rearrangement, low number of introduced copies of the transforming DNA, stable integration of a priory characterized T-DNA fragment (containing the DNA expressing the trait of interest) and an expected higher level of transgene expression. Triticale has until recently only been transformed via biolistics with a 3.3% success rate. (Zimny et al. 1995) Little has been documented on Agrobacterium-mediated transformation of wheat while nothing exists with respect to triticale until a recent study by Binka et al. (2005) in which the success rate was nevertheless low.
Conclusion
Triticale holds much promise as a commercial crop as it goes a long way toward addressing specific problems within the cereal industry. Research of a high standard is currently being conducted worldwide such as that at Stellenbosch University in South Africa.
Conventional breeding has helped establish triticale as a valuable crop and more particularly where conditions are less favourable for wheat cultivation. Notwithstanding the fact that triticale is a man-synthesized grain, many initial limitations such as an inability to reproduce due to infertility and seed shrivelling, low yield and poor nutritional value have been largely eliminated.
Tissue culture techniques with respect to wheat and triticale have seen continuous improvements, but the isolation and culturing of individual microspores seems to hold the most promise. Many molecular markers can be applied to marker-assisted gene transfer, but the expression of R-genes in the new genetic background of triticale remains to be investigated. (Baenziger et al. 2004) More than 750 wheat microsatellite primer pairs are available in public wheat breeding programs and could be exploited in the development of SSRs in triticale. (Baenziger et al. 2004) Another type of molecular marker known as an SNP (Single Nucleotide Polymorphism) is likely to have a significant impact on the future of triticale breeding.
Triticale in fiction
An episode of the popular TV series Star Trek, "The Trouble with Tribbles," revolved around the protection of a grain developed from triticale, which writer David Gerrold called "quadro-triticale" at producer Gene Coon's suggestion, and to which he ascribed four distinct lobes per kernel. A later episode titled "More Tribbles, More Troubles," in the animated series that was also written by Gerrold, dealt with "quinto-triticale," an improvement on the original that apparently had five lobes per kernel. These two grains exist only in the realm of Star Trek. In addition, the video game Metroid Prime makes a reference to "deca-triticale;" the reference apparently ascribes TEN distinct lobes per kernel to this grain. (There is an inexplicit link within these names to the crops ploidy level, i.e. a specific characteristic of the genome.)
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