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 Germany, France, Poland, Australia, 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 milling techniques used for wheat don't suit 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 crops biomass in bioethanol production.
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, 925) 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 (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.
The aim of a triticale breeding programme mainly focuses 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, 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.
Abundant information exists concerning disease resistance (R) genes for wheat and a continuously updated on-line catalogue (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 and are thus 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 (i.e. the reconstitution of triticale), 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.
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)
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)
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 greatly been 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.
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