Triticale (x Triticosecale Wittmack) is an artificial intergeneric hybrid between a female wheat parent (Triticum spp.) and a male rye parent (Secale spp.).
What makes the history and evolution of triticale as a species so unique compared to other cereals like wheat or rye, is that its evolution occurred during the last 140 years and is almost completely directed by humans (Mergoum et al. 2009). The origin of triticale dates back to 1873 when the Scottish botanist A. S. Wilson made the first cross between wheat and rye. However, the resulting plants were sterile and no further multiplication was possible. The first ‘true’ allopolyploid triticale according to today’s definition was bred in 1888 by the famous German plant breeder W. Rimpau who managed to create a cross between wheat and rye that was partially fertile. In 1921, G.K. Meister observed spontaneous pollinations of wheat plants with rye pollens from neighbouring plots. However, these hybrids were mostly male sterile and rarely a fertile hybrid was created by spontaneous chromosome doubling. During these early studies on this new species, researchers depended on this spontaneous chromosome doubling to obtain viable seeds. The next fundamental advance came in 1937 when botanists learned that colchicine can “double” the chromosomes in newly forming cells. The introduction of in vitro embryo rescue and chemical treatment with colchicine to double chromosomes allowed a systematic production of viable seeds. From then on making triticale fertile no longer depended on natural chance (Oettler 2005).
Triticale exhibits amphiploidy with respect to wheat (AABBDD) and rye (RR) genomes (Ammar et al. 2004). The first triticales were made using bread wheats, the offprings are called octoploids (AABBDDRR; 2n = 8x = 56), and they combine the A-, B-, D- genomes from Triticum aestivum with the R-genomes from rye. A different type of triticale was created in 1948 when J. G. O’Mara crossed a durum wheat with rye, resulting in hexaploid triticales (AABBRR; 2n = 6x = 42). These triticales are composed of the A- and B- wheat genomes and the R-genomes from rye. Driven by the belief that hexaploid triticales would be superior to its octoploid counterparts, researchers produced numerous hexaploid primaries that finally represented a much wider genetic base than that produced for octoploids. Furthermore, breeders distinguish ‘primary’ and ‘secondary’ triticales. Primary triticale refers to an allopolyploid that is newly synthesized from wheat and rye using chromosome doubling. Secondary triticales comprise all genotypes derived from crossing primary triticales with other primary triticales or with wheat or rye genotypes (Oettler 2005).
During the 1950’s and 1960’s, a few European researchers were also exploring triticales. In Spain, E. Sanchez-Monge developed a variety (Cachirulo) that was released for production in 1969. In Hungary, based on annual repeated crossing and progeny testing, A. Kiss concluded that the hexaploid type was the optimum ploidy-level for triticale. Therefore, he made crosses between octoploid and hexaploid triticale and the resulting secondary hexaploids were clearly superior to their parental stocks.
Triticale breeding programs were initiated in Mexico (CIMMYT), Poland and France in the 1960’s, and in Brazil, Portugal and Australia in the 1970’s (Oettler 2005; McGoverin et al. 2011). Those early triticales were exceptionally nutritious: their grains contained far more protein and lysine (an essential amino acid) than wheat. However, they proved to have many agronomic deficiencies, including low grain yields, poor seed set, shriveled grain, excessive height leading to lodging, premature sprouting and low baking quality. To most observers, this massive combination of difficulties seemed a barrier that could never be breached. However, much work has been done to overcome these limitations. In Mexico a handful of CIMMYT wheat scientists maintained the viable triticale research program. Before they had fairly begun to work on triticale’s manifold disadvantages, the CIMMYT breeders were the beneficiaries of a happy accident. In 1967, a triticale plant was accidentally fertilized by pollen blown in from nearby plots of dwarf bread wheats. After a few generations of selection, it resulted in a new breeding line called Armadillo. This new triticale had better fertility (seed set); also its yield was high, it was insensitive to day length, it was short and stiff-strawed, it matured early, and its grain was only slightly shriveled. Thus, in one fell swoop, Armadillo helped to resolve many of triticale’s agronomic problems. By 1970 practically every triticale at CIMMYT included Armadillo in its pedigree, and around the world the few remaining triticale breeders incorporated Armadillo materials into their own strains with renewed hope.
Additionally, in an effort to further overcome the limitations of triticale, many of the early triticales were crossbred with each other as well as with bread wheats. The low fertility and yield were improved by systematic breeding and consistent selection. Susceptibility to lodging, which is generally caused by excessive plant height, was lowered by the introduction of dominant dwarfing genes from wheat and rye. Presently, triticale has a plant height comparable to wheat and rye, as well as a grain yield that is competitive with wheat under optimal conditions or even superior under stress conditions. These promising new types began to attract favorable attention in several parts of the world. Indeed, the crop’s performance appeared so promising that it soon stimulated much press attention and enthusiastic promotion (Oettler 2005).
Today, two types of secondary hexaploid triticales are the most commercially grown triticale worldwide; complete triticales, which carry all seven pairs of unchanged chromosomes from rye, and substituted triticales, which have one or more of the rye chromosomes replaced with D-genome chromosomes from hexaploid wheat (Fox et al. 1990). There is some evidence that triticale varieties vary in their resistance depending on the number of rye chromosomes present, with varieties that have a greater number of rye chromosomes having greater resistance (Mergoum et al. 2009).
Conventional breeding programs are relatively time-consuming (10-12 years), as they are based on several rounds of crossing, inbreeding and selection of hybrids. Hence, conventional triticale breeding has evolved into a molecular breeding. Today, breeders are extremely interested in the application of modern technologies, e.g. doubled haploid breeding, embryo rescue, genomic assisted breeding, etc., for an efficient and targeted triticale breeding. The production of doubled haploids provides a solution for accelerating the breeding process. Variability at the genetic level can be fixed in a short time using the doubled haploid method to obtain completely homozygous lines from heterozygous initial material. This method is considered an indispensable tool in triticale breeding programs (Ślusarkiewicz-Jarzina et al. 2017).
Furthermore, the application of genetic DNA markers is part of modern plant breeding. Identification of markers linked or located within target genes determining valuable traits promotes selection efficiency. To facilitate the identification of markers for agronomically important traits and for genetic and genomic characteristics of triticale, Tyrka et al. (2015) constructed the first ultrahigh-density linkage map for the triticale genome.
Triticale breeding is still evolving, Wurschüm et al. (2017) evaluated the potential of genomic prediction for triticale breeding. Genomic prediction uses genomic information to obtain estimated breeding values, which are subsequently used to select candidate individuals. It was concluded that genomic selection can be utilized to increase selection gain in triticale breeding. However, the implementation of this approach in applied breeding programs is not straightforward. Furthermore, it must be noted here, that these results are only based on four families and further work is required to confirm them in a broader set of breeding material.