Wheat Breeding:  From Nature's Way to the Synthetic Way

March, 2000

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The roots of the family tree of wheat run deep, and in a far-away land.  Archeological samples dating to 8,000 BC indicate that the first domestication of wheat probably occurred in the Fertile Crescent, an area around the Tigres and Euphrates Rivers in Iraq, Syria, and southeastern Turkey. 

These very primitive forms were called einkorn wheat (scientific name, Triticum monococcum) and emmer wheat (T. dicoccoides).  Einkorn is a diploid wheat, meaning it has two sets of chromosomes, with seven chromosomes per set.  Geneticists label those sets as AA.  Emmer wheat is a tetraploid, with four sets of seven chromosomes. Its genetic designation is AABB.

Emmer wheat was produced from a natural cross between einkorn and a yet unverified donor of the B chromosomes, probably one of the native grasses of the Middle East region.  The cross occurred naturally, probably due to proximity of the two grasses in natural stands.

Deriving emmer wheat in this way was a “minor miracle.”  A cross of an AA plant with a BB plant produces an AB hybrid plant.  This F₁ plant does not have matching sets of chromosomes, so it is sterile.  By some quirk of nature, the chromosomes in the F₁ plant were doubled to produce a fertile AABB plant.  Emmer wheat, the progenitor of modern durum wheat, evolved from this ancestor plant through natural selection.

In a similar manner, hexaploid wheat, or bread wheat of the type grown here in Oklahoma, most likely originated from a cross of a cultivated emmer wheat plant with a goat grass plant (Triticum tauschii).  Goat grass is common throughout the Middle East and the Fertile Crescent region.

Goat grass has two sets of seven chromosomes, designated DD.  When emmer wheat (AABB) crossed with goat grass (DD), the resultant hybrid was sterile (ABD).  Again, something in nature caused the chromosomes in the F₁ hybrid to double—yet, another miracle.

The resulting hexaploid plant (AABBDD) eventually evolved through natural selection to provide the human race with bread wheat grown throughout the world today.  The hard red winter, hard red spring, hard white, soft white, and soft red winter classes of wheat are descendants from this original hexaploid.

Bread wheat probably evolved from only a few natural crosses as described above.  Therefore, only a limited amount of the genetic variation that exists within the goat grass and emmer wheat species was incorporated into the genetic pool breeders use today.   We already know that many useful genes are present within the different goat grasses present in the Middle East, as well as in many other regions of the world, and of course, durum wheat--or the modern form of emmer wheat--has much to offer in the same way.

But, so much for history!  How is history helping us breed wheat varieties for Oklahoma wheat producers?

Wheat breeders have begun utilizing the natural variation that exists within goat grass by making crosses between varieties of durum and different selections of goat grass.  These artificial crosses might be considered a 21^st-century version of the crosses that naturally occurred several thousand years ago.

By forcing the chromosomes to double in the laboratory, a fertile “synthetic hexaploid” is formed with the genetic composition of AABBDD.  These synthetics are similar to the original hexaploid produced in nature except, thanks to the contribution of the modern durum varieties, they have a much improved genetic makeup.

Further breeding is necessary, however, to “weed out” undesirable genes contributed by the goat grass parent.  This is possible because synthetics cross readily to modern wheat varieties, including those grown in Oklahoma.

Hundreds of the new synthetics have been produced in the last 10-15 years. The CIMMYT wheat research program based in Mexico has used synthetics for many years in their varietal development program.  Results to date indicate that they contain useful genes for disease and insect resistance, drought tolerance, heat tolerance, increased yield potential, and other useful traits.

The Wheat Improvement Team recently obtained more than 100 synthetics from CIMMYT, as well as many breeding lines derived from crosses of spring bread wheat x synthetic wheat (referred to as synthetic derivatives).  These synthetics and synthetic derivatives are now being crossed to locally adapted wheat varieties by WIT researchers.

Without a doubt, these crosses will introduce new genetic variation into OSU’s wheat improvement program.  The ultimate objective will be to introduce useful genes from the synthetics, which will lead to varieties with better disease and insect resistance as well as increased yield potential for the producers of Oklahoma.

Submitted by Art Klatt, on behalf of OSU’s Wheat Improvement Team.