To produce wheat-barley cross, embryo is excised from immature barley endosperm (A) and is replaced by frail hybrid embryo (B) produced by hand pollination. The endosperm serves as nurse tissue for the hybrid (C). About 70 percent of the embryo cultures may give rise to plants (D), but of 270 plants obtained by this process at Davis, only 20 were true wheat x barley hybrids (E, center; spike at left is wheat; right, barley).

A major factor limiting the rate of progress criterion in tomatoes. in plant breeding has been low heritability. One strength of cellular and molecular ge- This occurs frequently in complex traits such netic approaches is the possibility of perform- as yield and quality, which are controlled by a ing selection with chemicals or physical con- large number of genes and are environmen- ditions on extremely large populations of tally unstable. Plant breeders and physiolo- haploid individuals in a small space and over gists are cooperating in an effort to overcome a short time. Numerous schemes have been this problem by breaking a complex character devised to select desirable genetic variation down to components, and ultimately to the with respect to nutrition, stress tolerance, actual molecules that mediate the expression herbicide resistance, and disease resistance. of the character. By selecting for the compo- Until now, variant cells selected in vitro have nents of a character or for its underlying only rarely been translated into functionally molecular constitution, breeders hope to equivalent whole plants. Recent studies show reduce complexity and environmental im- that it may be possible to circumvent some of pact, allowing for faster progress. the problems associated with selection of cul- Areas currently being explored for applica- tured cells by substituting whole plants. For tion of this approach are yield, quality, salt example, chemicals have been used to screen and drought tolerance, water-logging toler- barley seedlings exhibiting increased nutri- ance, chilling tolerance, disease and insect re- tional value. Other experiments have shown sistance, and self-incompatibility phenotypes that pollen can be responsive to selection in (which are used in FIhybrid production). For vivo, opening up the possibility for screening example, in processing tomatoes the major large populations of haploid individuals. component of yield is volume of finished Plant breeding has brought about drastic product (sauce, paste, catsup) per unit of cul- changes in plant architecture or genetic tivated area. In breeding for harvest yield makeup, revolutionizing production and wt eat-ba Iey hybrids alone, much of the progress observed results marketing methods. Recent examples include from increased water content, which must, in determinate growth habit for mechanical har- Chao-Chien Jan turn, be eliminated by the processor. The so- vesting, disease-resistant cultivars, and hy- lution has been to maintain or increase the brid varieties. New ideas are constantly Calvin 0. Qualset economic yield of processing tomatoes (sol- emerging, such as techniques to permit the se- Jan Dvorak ids) while decreasing water content. The lection of plants with multiple tolerance genes alternative character used by breeders as a (“horizontal” resistance) as opposed to often selective criterion is sugar (soluble solids) unstable single gene (“vertical”) resistance. some crop plants have simple inherited concentration, which is negatively correlated Dwarf fruit trees are being developed that characters that would be desirable if trans- with water content. By combining this char- will produce more with lower management ferred to another crop. Our current work on acter with additional selection for gross yield, and harvesting costs. transfer of resistance to the barley yellow uniform maturity, thick skin, and fiiflesh, Although genetic engineering for crop im- dwarf virus (BYDV) from barley to wheat is it has been possible to make more rapid pro- provement holds great promise, plant breed- an example of a simple modification to the gress in improving this crop than in the past. ing and other aspects of whole plant genetics embryo culture method that produced hybrid Another approach to solving the problem are also becoming increasingly sophisticated. plants from two difficult-to-hybridizespecies. of low heritability, recently developed at the Advances in breeding technology guarantee BYDV is an aphid-transmitted virus that University of California, is the use of electro- that plant breeders will continue to make occurs on many grasses, including forage phoretic variation as an alternative selection valuable contributions to agriculture. grasses and cereal grains. Discovered in Cali-

CALIFORNIA AGRICULTURE, AUGUST 1982 23 fornia in 1951, BYDV causes serious yield ly into a wheat chromosome. Techniques are chromosomes can occur. One approach em- losses in barley, wheat, and oats. Resistance already available for this step, but the key to ploys hybrids between such species and to BYDV was found only in barley from success in making the difficult hybrid was the wheat. When an alien set of chromosomes is Ethiopia and was subsequently transferred to rescue of the hybrid embryos with the endo- present in the hybrid along with the donor several California barley varieties. A single sperm nurse tissue. chromosome carrying the desired trait, the gene, Yd2, in barley confers resistance. An Chao- Chien Jan, formerly Postgraduate Re- chances of pairing between the donor and extensive search among wheats from around search Agronomist (now Research Geneticist, wheat chromosomes are enhanced. The sec- the world failed to find a resistant variety, so USDA-ARS, Davis, Cat$), Calvin 0. Qualset, ond approach manipulates dosages of genes Professor, and Jan Dvorak, Associate Professor, it appeared necessary to transfer Yd2 from Agronomy and Range Science, Davis. This work known to regulate chromosome pairing in barley to wheat. was supported by USDA grants 801-15-42 and wheat. The wheat x barley hybrid is very difficult 50-2063- 01402. A recent successful example of the first ap- to make because the two species are not close- proach involves the wild diploid species ly related. Barley has 7 pairs of chromo- Aegilops speltoides, which is native to the somes, and bread wheat has 21 pairs. When Middle East. This species has a genotype cap- pollen from barley is applied to wheat, fertili- able of inducing pairing between homoeo- zation can take place, but at a very low fre- Induced chromosome logues in hybrids with bread wheat. It also quency. The resulting embryos abort unless possesses resistance to the leaf rust fungus excised and cultured on a sterile growth pairing Puccinia recondita. Following initial hybridi- medium; even then few embryos survive (less zation between the susceptible wheat cultivar Jonathan lrvine than 1 percent). Pitic 62 and resistant Ae. speltoides plants, In the early 1970s Anthon Kruse, working Patrick McGuire the hybrids were backcrossed to Pitic 62 for in Denmark, discovered a simple embryo res- several generations. The F, and first back- cue technique. He placed the hybrid embryo cross plants were male sterile, but by the third on immature endosperm excised from the de- Success in hybridizing wild with cultivated backcross, pollen fertility was substantially veloping seed of one of the parent species. species ultimately depends on the ability of restored in the derivative lines. When pro- The endosperm serves as a nurse tissue for the the alien chromosomes to pair and recombine geny of the third and fourth backcrosses were hybrid. Australian workers found that barley with the chromosomes of the crop species at screened for resistance to the fungus, resis- endosperm was the best nurse tissue and re- meiosis. In bread wheat, pairing and thus re- tant plants were found. Observations of ported that about 70 percent of the cultured combination normally occur only between chromosomes at meiosis in resistant plants of hybrid embryos produced plants. identical or homologous chromosomes and the fourth backcross revealed some 42-chro- We adopted these techniques at Davis in a not, unfortunately, between a wheat chromo- mosome plants with normal wheat-like meio- program that will transfer to wheat the Yd2 some and an alien chromosome. However, sis. Thus, the required portion of an Ae. spel- gene, which is on barley chromosome 3. Even increasing knowledge of the components of toides chromosome conferring resistance had with the method described, it is difficult to the genetic system controlling pairing has been translocated to a wheat chromosome. produce the hybrid plant. In two years, more made it possible to manipulate them to The exchange occurred in plants of the F, than 50,000 individual wheat ovaries were achieve pairing between nonhomologous generation following initial hybridization pollinated by barley, and of those, only about chromosomes. A major advantage of such through homoeologous chromosome pairing 440 showed embryo development. These induced pairing is that it usually occurs be- permitted by the interaction of the Ae. spel- were excised and transferred to nurse tissue tween closely related but not homologous toides genotype with the wheat genotype. on sterile culture medium. About 270 plants chromosomes -termed homoeologues. Re- The desired exchange was only one of many were obtained, but only 20 were true wheat combination of genetic material between ho- that may have occurred. Backcrossing and x barley hybrids. moeologues is likely to result in genetically screening were necessary to exclude un- The hybrid plants were completely male- more balanced chromosomes than if recom- wanted exchanges and retain plants with sterile but did produce seeds when pollinated bination were between unrelated chromo- wheat characteristics and resistance. After with wheat. These seeds, called backcross F,, somes. the first backcross, some of the Ae. speltoides produced plants that had the 7 chromosomes Two approaches can be used to transfer chromosomes were lost, since they had no of barley and 21 wheat chromosome pairs. genes from related species into wheat. There pairing partner at meiosis. Normal meiosis We have backcrossed and self-pollinated are diploid species (with 14 chromosomes) re- returned to the derivatives as backcrossing several more generations and now have lated to wheat (42 chromosomes) that modify continued. wheat-like plants with 44 chromosomes-all the chromosome pairing control system of This technique is efficient, because in one of the wheat chromosomes (42) and one wheat so that pairing between homoeologous step it incorporates resistance and provides chromosome pair from barley. an opportunity for the recombination of Seven types of wheat plants have indivi- homoeologous chromosomes. From these re- dual barley chromosomes, one type for each combinants, backcrossing and screening se- of the 7 barley chromosomes. Our task now is lect any plants that possess the desired por- to identify the wheat plant with barley chro- tion of the alien chromosome translocated to mosome 3. We can then expose those plants wheat without interfering with normal chro- to BYDV to see if the Yd2 gene functions in mosome pairing and fertility once the wheat wheat. The next step will be to induce recom- background is restored. bination of a wheat chromosome with the Jonathan Zrvine, Research Associate, Znterna- barley chromosome, so that we eventually tional Plant Research Institute, Son Carlos, Cali- Reactions to leaf rust on leaves of (from fornia, and Patrick McGuire, Postgraduate have a wheat plant with 21 pairs of chromo- left): susceptible Pitic 62 wheat cultivar; Research Agronomist, Agronomy and Range somes with the Yd2 gene incorporated direct- susceptible derivative; resistant derivative. Science, U.C., Davis.

24 CALIFORNIA AGRICULTURE, AUGUST 1982