5 Breeding Triticale

Plant breeding is a long, repetitious undertaking demanding infinite patience and promising no certain success. Success, if attained, is rarely dramatic and often illusory.
B. S. DODGE
It Started in Eden

Plants in different genera are normally separated by strong barriers of infertility: they will not crossbreed because the pollen of one is incompatible with the ovum of the other. This natural barrier makes triticale difficult to create. In nature, the necessary cross-pollination occurs infrequently and is successful only when rye is the male parent.

WHEAT

Wheat itself evolved through natural hybridization. It, too, is a composite of different species. The various cultivated wheats are of three main types, each genetically distinct and characterized by different numbers of chromosomes.

Diploid wheat. The most primitive cultivated wheat is a grass called "einkorn" (Triticum monococcum). This is a diploid species and is still cultivated in parts of Turkey and the southern Soviet Union.

Tetraploid wheat. Thousands of years ago, before the dawn of civilisation, einkorn hybridized with another diploid wild grass. (The actual species is not known for certain, but it was probably similar to the wild goat grass Aegilops speltoides, also called Triticum speltoides.) Chance chromosomal doubling resulted in tetraploid wheats. At first, these held their grains tightly, but after they were taken into cultivation, mutants appeared that had "naked" grains that could be easily threshed out. For at least 7,000 years, farmers have selected, planted, nurtured, and replanted the seeds of these scrawny grasses, gradually transforming them into plump, productive, modern wheats. Among their creations is durum wheat (Triticum turgidum var.durum), one of the two wheats that today feed much of the world.

Hexaploid wheat. Subsequently' the tetraploid wheats hybridized with yet another wild, weedy goat grass (called either Aegilops squarrosa or Triticum tauschii). Again chromosome doubling occurred, resulting in hexaploid wheat. After thousands of years of nurturing, it, too, was transformed into a valuable crop-bread wheat, or common wheat (Triticum aestivam). It has become the world's most widely grown breadmaking crop. Sears, 1987.

Chromosome Designations

As a shorthand designation, geneticists use letters to indicate the chromosome sets contributed by each wheat ancestor. The chromosome set of einkorn is designated A; the chromosome set from the species that resembles Aegilops speltoides is designated B; and the chromosome set from Aegilops squarrosa is designated D.

With this nomenclature, the series of steps leading to the genomes in today's wheats becomes clearer. When einkorn (AA) hybridized with the species resembling Aegilops speltoides (BB), it gave rise to the tetraploid durum wheats, whose chromosome composition is designated AABB. When one of these tetraploids, in turn, hybridized with

Aegilops squarrosa (DD), it gave rise to the hexaploid wheats AABBDD. The D set markedly contributes to the strong gluten that makes these "bread wheats."

Creating Triticales

Triticale is merely a recent extension of the prehistoric hybridization process that led to modern wheat: the rye set is simply added to wheat.

To create triticale, the chosen wheat plant is emasculated-in other words, its immature male reproductive organs (anthers) are removed so that self-fertilization cannot occur. At flowering time, fresh pollen from the chosen rye plant is transferred to the chosen wheat's female reproductive organs (stigmas).

The fertilized cells (embryos) produced from this cross-fertilization carry a single set of chromosomes from both parents. Rye, a diploid, has two sets of chromosomes, designated R. If breadwheat (AABBDD) is pollinated with rye (RR) it produces a hybrid with a chromosome formula designated ABDR. This is the sterile type of haploid hybrid that A.S. Wilson produced in 1876.

FIGURE 5.1 First-generation triticale seeds are usually sterile even though they may germinate. The sequence at top shows the system of crossing bread wheats with rye. In this case, the sterility is overcome by treating seedlings with the alkaloid colchicine. Applied to the tip of a growing sprout, this substance interferes with mitotic division in the fast- reproducing cells in such a manner that the number of chromosomes is doubled. The doubled chromosome complement gives rise to fertile flowers that later grow on the stem. The seeds produced by these flowers are also usually fertile. This pathway yields octoploid triticales. The sequence at bottom shows the system used in crossing durum wheats with rye. This produces seeds that will not germinate, and embryo culture is used to produce first- generation seedlings. The technique consists in excising the embryo from seeds that are immature (14-20 days after fertilization) and culturing the embryo in an agar medium. The culture is kept in the dark until the embryo sprouts and then is kept in constant illumination. Later the sprout is transplanted to a peat pot and treated with colchicine as above. This pathway yields hexaploid triticales. (Copyright 1974 by Scientific American, Inc. All rights reserved.)

To make a seed-producing triticale, the haploid seedling is treated with a weak solution of colchicine. This is usually fed to the plant through the roots, and it migrates to the crown area of the growing shoot. In the apical growing regions where cell division is occurring, colchicine suppresses the orderly separation of chromosomes during cell division. Instead of pulling away from each other, the products of cell division (the sister chromatics) usually remain side by side. The cell wall that normally would grow between them does not form. The resulting nucleus therefore retains both sets of chromosomes. Instead of being ABDR, it doubles into an octoploid triticale, AABBDDRR. Every chromosome now has a partner and normal reduction division can proceed when the time comes for germ-cell formation.

If durum wheat (AABB) is simply pollinated with rye (RR), mature hybrid seeds (haploid, ABR) are almost never produced. However, the pollen does fertilize some of the ova, and the resulting embryos begin growing-only to abort approximately 15 days later because of endosperm incompatibility. However, these embryos can be excised and maintained artificially in a sterile culture medium. The culture medium provides them with nutrients, and they grow into plantlets with roots and leaves. They are still sterile, but when treated with colchicine, as described above, these plantlets produce partially fertile hexaploid triticales (AABBRR).

Hexaploid Triticales

Almost all work worldwide centers on hexaploid triticales. The main reason is that they have better vigor and reproductive stability than the octoploids currently available. Also, in general they show better disease resistance and selection response. However, as they are made by crossing durum wheat (AABB) with rye (RR), they lack the bread wheat D chromosomes that should contribute the best breadmaking qualities.

Octoploid Triticales

Octoploid triticales, as noted, result when bread wheat is employed in the cross. Because they contain the D-genome chromosomes of bread wheat, they normally have good breadmaking qualities. However, at present, they tend to be unstable and unpredictable in the field, and currently are used only in the People's Republic of China. Nonetheless, some of the most valuable genetic traits are found in octoploids, and the creation of practical octoploid triticales is an important promise for the future.

So far, octoploids have been most valuable for improving hexaploid triticales. At CIMMYT in the 1970s, the crop's agronomic breakthroughs came from crosses between Armadillo (a hexaploid) and some newly created octoploid triticales, of which the Maya group were among the first. The variety Mapache was selected from the cross between Armadillo and Maya 2. It was released for commercial production in Mexico under the name "Cananea 79." Its performance in international trials was outstanding. In 38 locations it outyielded wheat. Today, almost half of the spring hexaploid varieties released worldwide are derived from it either directly or indirectly.

Primary Triticales

The products that arise directly from crossing wheat with rye (followed by chromosome doubling) are collectively called primary triticales. They are pure triticales, unmodified by further hybridization. To differing degrees, they share problems of partial steriliti, shriveled seed, low yield, and poor agronomic type. But they are pure lines- homozygous as a result of chromosome doubling for all gene pairs- and so they are stable. Like a standard crop variety, they remain much the same year after year. (They are, however, a little more unstable than normal varieties of wheat.)

Secondary Triticales

Plant breeders, on discovering that primary triticales were not agronomically useful, began to hybridize these triticales with other primary types and with wheat, or even some other species. From such crosses arose an array of triticale types that are generally called "secondary triticales" because they have new combinations of wheat and/or rye chromosomes. Except for some successful octoploid primary triticales developed and used in China, secondary triticales form the basis of the world's triticale industry at this time.

Complete and Substituted Triticales

Among CIMMYT's spring triticales there are two major types: "completes" and "substitutes." Complete triticales retain unchanged all the chromosomes of the rye parent. Many of the most resilient triticale varieties are of this type. Completes tend to be more productive under stressful conditions. They retain much of rye's inherent robustness, and they tend to thrive under various difficult conditions, including sandy soils, high elevations, and high rainfall. Thus, they appear to be the triticales of choice for marginal environments.

TABLE 5.1 Important CIMMYT Advanced Lines of Hexaploid Triticale Available in 1987.

Name	Type.	Qualities
Armadillo	S	Highly fertile, daylength-insensitive, one-gene dwarf.
Yoreme 75	S	First line released. Good plant type, good productivity.
Cananea 79	S	First triticale in international nurseries with wide adaptability and high yield potential
Panda	S	First triticale with good test weight (over 75 kg/hl).
Beagle	C	First complete with high adaptability and yield. Released in Portugal, Australia, USA, and Canada.
Juanillo	C	Most productive spring triticale in international trials since1980. Most widely adapted spring triticale. Released in many countries under different names.
Juan	C	Derived from Juanillo. Developed in California.
Currency	C	One of CIMMYT's first dwarf completes. Main cultivar used in Australia
Gnu' Stier	C	Dwarfs with high yield and adaptation. Have 6D/6A substitution.
Rhino	C	High-yielding line with stable test weight.
Tatu	C	Best complete triticale for breadmaking.
Civet	C	One of the most drought-tolerant varieties.
Ardilla	C	First early-maturing complete triticale Comparable to earliest substituted triticales and bread wheats. Has 6D/6A substitution.

C = complete R genome present
S = wheat chromosome 2D substituted for chromosome 2R
All CIMMYT triticales are of the spring type.

SOURCE. C. Varughese and T. Barker.

Two of CIMMYT's complete triticales, Beagle and Drira, are progenitors of many of the present-day commercial spring triticales used in Australia, Spain, and various Third World countries. In addition to CIMMYT lines, other completes, such as the winter triticale Lasko, are being used in Europe (see chapter 7).

Substituted triticales, in CIMMYT's terminology, are those in which rye chromosome 2R has been replaced by chromosome 2D of bread wheat. The discovery of this chromosome repatterning was an important event in the history of triticale. It began with Armadillo, which has this trait.

Under nonstressed conditions, the 2D for 2R (2D/2R) substituted types perform better than the completes. They tend to mature earlier and may be better for breadmaking. Like the completes, several of the substituted types appear to maintain a high Iysine content. And improvements in dough strength and breadmaking quality are considered less difficult to achieve in the substituted than in the complete type. (Whether this is due to the presence of chromosome 2D or to the loss of chromosome 2R is uncertain.) Of 42 commercial triticale cultivars for which the chromosome constitution is known, 16 are completes and 26 are substitutes carrying the 2D/2R substitution.

TABLE 5.2 Promising Non-CIMMYT Advanced Lines of Hexaploid Triticale Available in 1987.

Origin	Name	Type(a)	Qualities
Poland	Lasko	C	Very productive winter type, resistant to cold and disease. Requires plant-growth regulator
	Salvo	C
Hungary	Bokolo	C	Promising, dwarf winter type, bred for wheat soils. Widely grown in Eastern Europe and France
France	Clercal
	Raboliot
Spain	Triticor
Canada	Carman	C
	Wintri	C	One of the most winter hardy.
	Decade
	T44		Spring type, high yielding
United States	6TABI
	Marvel
	Siskiyou	C

C = complete set of chromosomes present

In addition, about 20 percent of CIMMYT lines and some European winter lines have a 6D/6A substitution. In these triticales, the rye genome is intact, and chromosomes from the D- genome of wheat have replaced those from either the A or B-genomes, also of wheat. This allows for both D-genome and R-genome chromosomes to be present in hexaploid triticale, thus combining the best qualities of both parents- the breadmaking qualities of wheat and the robustness of the rye plant.

Chromosome substitution raises the possibility of a far-reaching reorganization of the chromosomal and genomic composition. Researchers can make many different wheat-like triticales with various combinations of two or more substituted wheat chromosomes, or even triticale-like wheats with one or a few rye chromosomes. 'S'-contain part of a single rye chromosome (JR) to which much of the yield advantage has been attributed. These, like triticale, suffer from the "sticky dough" syndrome.

This can be of benefit to breeders of all three crops: wheat, rye, and triticale (see chapter 8).