CHAPTER 23

THE USE OF CYTOPLASMIC MALE STERILITYFOR HYBRID SEED PRODUCTION

MICHAEL J. HAVEYUSDA -ARS, Department of Horticulture, University of Wisconsin, 1575 Linden

Drive, Madison, WI USA

Abstract Cytoplasmically inherited male sterility (CMS) results from an interaction between theorganellar and nuclear genomes that conditions the failure to produce functional pollen . CMS providesan expedient mechanism to produce large populations of male-sterile plants for commercial Fl hybridseed production. In cases where the Fl hybrid crop must produce pollen and set seed, male sterility can bereversed by nuclear-encoded restorer-of-fertility alleles. Although the unfortunate epidemic of SouthernCorn Leaf Blight on T-cytoplasmic maize revealed the dangers of hybrid-seed production using a singlesource of CMS, no other genetic vulnerability to disease or stress has been attributed directly to a CMSgene, in spite of the worldwide use of CMS. In some cases, mi tochondrial -encoded CMS genes havebeen linked to undesirable, plastid-encoded traits, and nuclear restorer-of-fertility alleles have been linkedto undesirable nuclear-encoded trails, but these linkages were successfully broken by somatic cellgenetics and conventional plant breeding, respectively. The use of CMS to produce hybrid seed is verycost effective and has been widely exploited in a plethora of agronomic and horticultural crops. Thepurpose of this chapter is to list and review the major sources of CMS used commercially to producehybrid seed.

1. INTRODUCTION

Male-sterility-inducing cytoplasms were recognized early in the 1900s. Bateson andGairdner (1921) reported that male sterility in flax was inherited from the femaleparent, although genes passed from both the male and female parents affected itsexpression. Chittenden and PelIow (1927) recognized that male sterility in flax wasdue to an interaction between the cytoplasm and nucleus, with the male sterilityconditioned by the homozygous recessive nuclear genotype in combination with thesterility-inducing cytoplasm. Rhoades (1931, 1933) observed that male sterility inmaize was inherited only through the female parent, although occasional male fertileprogeny were observed. In a seminal paper, Jones and Clarke (1943) established in

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624 MICHAEL J. HAVEY

onion that male sterility is conditioned by the interaction of the male-sterile (S)cytoplasm with the homozygous recessive genotype at a single male-fertilityrestoration locus (Ms) in the nucleus, written in onion as S msms. A dominant alleleat Ms conditions male fertility in plants possessing S cytoplasm (S Ms--). Plantspossessing the normal (N) male-fertile cytoplasm are male fertile regardless of thegenotype at Ms. In the same manuscript. Jones and Clarke (1943) described thetechnique used today to exploit cytoplasmic-genic male sterility (CMS) for theproduction of hybrid seed. A male-sterile (S msms) inbred line [termed the A lineby Jones and Clarke (1943)] is seed propagated by growing it in isolation with amale-fertile maintainer (B) line that is N cytoplasmic and homozygous recessive atthe nuclear male-fertility restoration locus (N msms). All seed harvested off of theA line is male sterile (S msms); all seed harvested off of the maintainer line is malefertile (N msrns). The A line then becomes the female parent for hybrid-seedproduction. This system to seed propagate CMS inbred lines has been widely usedfor hybrid production of many crops.

The use of CMS for hybrid seed production received a "black eye" after theepidemic of Bipo/aris maydis on T-cytoplasmic maize (Pring and Lonsdale, 1989).This epidemic is often cited as a classic example of genetic vulnerability of ourmajor crop plants. However the B. mavdis epidemic on T cytoplasmic maizeappears to be the exception rather than the rule, and such a serious epidemic hasnever been observed in any other CMS system, even though single sources of CMSare widely used around the world to produce hybrid seed. This does not mean thatbreeders should ignore cytoplasmic uniformity, only that the epidemic of SouthernCorn Leaf Blight on T-cytoplasmic maize appears to date to be an anomalyassociated with this specific source of CMS. Onion is an example of continuedcytoplasmic uniformity in a major crop plant. The majority of hybrid-onion seed isproduced using S cytoplasm (Havey, 1995; 2000), which traces back to a singleonion plant identified in Davis, CA, in 1925 (Jones and Emsweller, 1936). RFLPsin the organellar genomes have demonstrated that, in addition to almost all hybrids,many significant open-pollinated populations of onion also possess S cytoplasm(Courcel et al., 1989; Havey, 1993; Satoh et al., 1993; Havey and Bark, 1994). Bulband seed production can occur simultaneously, such as in the Treasure Valley ofIdaho or the Central Valley of California, resulting in year round cultivation ofcytoplasmically uniform onion.

Numerous hybrid crops representing significant (>50%) components ofworld production are generated without CMS, such as tomato and maize by handemasculation, spinach and the cucurbits by monoecy, and the Brassicas by selfincompatibility (Janick, 1998). Hybrids of other crop plants may be produced usingsystems of genic (nuclear) male sterility. An example is leek (A/hum ampeloprasumL.), a vegetable crop for which growers demand high uniformity. Because leek is anautotetraploid, it shows extreme inbreeding depression and relatively pooruniformity. A source of genic male sterility was identified in leek (Smith andCrowther, 1995). Leek hybrids are produced by propagating in vitro male-sterileplants conditioned by this single source of genic male sterility. The asexuallypropagated leek plants are used as the female to produce hybrids that showsignificantly greater uniformity than open pollinated (OP) cultivars (Smith and

THE USE OF CMS FOR HYBRID SEED PRODUCTION 625

Crowther, 1995). Engineered sources of nuclear male sterility have been developedin model systems (Mariani et al., 1990; Hemould et al., 1993; Perez-Prat and vanLookeren Campagne, 2002). A problem with these nuclear transformants is thatthey segregate for male fertility or sterility and must be over planted and rogued byhand or sprayed with herbicides to remove male-fertile plants. Nevertheless

Brassica hybrid-seed is produced using the Bayer SeedLink tm system, in which a

transgene conditioning male sterility is linked to herbicide resistance. HybridBrassicas produced using the SeedLinktm system have reached a significant marketshare by combining herbicide tolerance with consistent high performance.

An important consideration for hybrid development using CMS systems isthe requirement, or not, for male-fertility restoration. For vegetable, fruit, or foragecrops, nuclear restoration of male fertility in the hybrid is not necessary. Thissimplifies the production of hybrids because effort can concentrate on maintainerline development, without concern whether the pollinator restores male fertility inthe hybrid. For crops with seeds as the economically important product, such ascanola, sunflower, or maize, one or both of the hybrid's parents must bring in male-fertility restoration factors or the male-sterile hybrid seed must be blended withmale-fertile hybrid seed (see maize section below).

In this chapter, my intention is to document and review the use of CMS toproduce hybrids for a wide range of economically important plants. Plants werechosen to illustrate the various uses of CMS to produce hybrid seed, and theomission of any specific crop is unintentional. I contacted leading plant researchersto estimate the predominance of hybrids in these crops and types of CMS used toproduce the hybrids. Some researchers in the private sector were willing to sharetheir opinions, but asked that I not report their names. Molecular-genetic aspects ofCMS and fertility restoration, including many of the systems discussed below, arereviewed in chapter 22 of this volume.

2. CMS IN HYBRID CROP PRODUCTION

2.1. Alfalfa

Alfalfa or lucerne (Medicago sativa L.) is an outcrossing, autotetraploid legumewidely grown as forage for ruminant animals. Historically, alfalfa cultivars orpopulations were developed by selecting and interpollinating among superior plants.Sources of CMS in alfalfa have been described (Pedersen and Stucker, 1970; Brownand Bingham, 1984) and significant specific combining abilities among plantsdemonstrated (Pedersen and Hill, 1972). Although I could find no example of truesingle-cross hybrids in alfalfa, there exist on the North American market "hybridswarms". These populations are created by planting together in seed fields mixturesof CMS plants with male-fertile pollinators. My colleague, Dr. Edwin Bingham atthe University of Wisconsin, estimates that these seed-production fields possess 10to 20% CMS plants. The CMS plants increase outcrosslng, contributing to superiorforage yields by reduced inbreeding.

626 MICHAEL J. HAVEY

2.2. A/hums

Hybrid-onion cultivars dominate the North American, European, and Japanesemarkets, although specific regions still grow OP populations for specific markets(such as the Walla Walla Sweets in Washington, USA). Numerous areas of theworld with significant onion production also grow OP cultivars (e.g., Argentina,Turkey, Australia, India, and New Zealand). Reasons for the prevalence of OPcultivars in these regions include lower seed prices, or that adapted indigenouspopulations possess exclusively or predominantly the male-sterile cytoplasm,making the extraction of maintainer lines difficult to impossible (Havey, 1993).Two sources of CMS are commercially used to produce hybrid onion. The mostwidely used source of CMS is S cytoplasm, as described by Jones and Clarke(1943). T cytoplasm is a second source of CMS (Berninger, 1965) used to producehybrids in Europe and Japan (Havey, 2000). The vast majority of onion hybrids areproduced using S cytoplasm, likely because it was the first source of onion CMSreleased in 1951 and was made available both in long- and short-day germplasms(Goldman et al., 2000). CMS has also been described in chive (A. schoenoprasuinL.) and Japanese bunching onion (A. fistulosum L.) and exploited for hybridproduction in both crops. CMS in chive shows unique sensitivity to tetracycline,which restores male fertility (Tatlioglu, 1986) and tetracycline susceptibility isconditioned by recessive alleles at a single locus (aa) (Tatlioglu and Wricke, 1988).CMS in Japanese bunching onion is conditioned by the male-sterile cytoplasm andrecessive alleles at two nuclear restorer loci (Moue and Uehara, 1985). Althoughthese sources of CMS are used to produce hybrid and Japanese bunching onioncultivars, I was not able to acquire information on the prevalence of hybrid versusOP cultivars for theSe Alliums. Alloplasmic sources of CMS conditioned by thecytoplasm of A. galanthum have been transferred to onion, shallot, and the Japanesebunching onion (A. JIstulosum L.) (Havey, 1999; Yamashita and Tashiro, 1999;Yamashita et al., 1999). The main advantage of the galanthum CMS system is thatnuclear male-fertility restoration loci appear to be rare or non-existent (Havey,1999), indicating that many populations may be used to maintain this source ofCMS.

2.3. Brassicas

Almost 100% of broccoli and cabbage cultivars are hybrid, with fewer(approximately 50%) cauliflower and collard cultivars being hybrid. For theBrassicas, hybrids were historically produced using self-incompatibility (SI).Although the SI system is very effective for hybrid production, some self pollinationcan occur, allowing competitors to screen for and isolate one, or in some cases both,of the hybrid's parental inbred lines. This has been an impetus for the developmentof an effective CMS system to produce hybrid Brassicas. Ogura (1968) identifiedCMS in radish, and this cytoplasm was transferred by backcrossing to B. oleracea.Unfortunately the Ogura CMS was associated with cold susceptibility, conditionedby the chloroplast genome. This defect was overcome by several differentlaboratories after protoplast fusion and organellar sorting to combine the CMS

LWTHE USE OF CMS FOR HYBRID SEED PRODUCTION 627

conditioned by the radish mitochondrial genome with cold tolerance conditioned bythe Brassica chloroplast genome. A cold-tolerant form of Ogura CMS was patentedby Syngenta and is used to produce hybrid Brassicas. Other companies and publiclabs (Walters and Earle, 1993) have developed similar cold-tolerant CMS linesindependent of the Syngenta source. One problem with the Brassica CMS is that theflower structure of some male-sterile lines is not as attractive to insects, which cannegatively affect seed production (Mark Farnham. USDA, personalcommunication).

The proportion of hybrid canola or rapeseed is approximately 80% inChina, 50% in Germany and the USA, 35% in Canada and Austria, 20% in theCzech Republic, and France, 10% to 15% in Australia, Denmark, Poland, and theUK. In Canada and Europe, approximately 60% of the hybrid seed is producedusing the nuclear SeedLink tm system. The other 40% is produced using CMS,largely based on the [NRA Ogura system the was discovered in Raphanus sativus,transferred to B. napus after protoplast fusion, and later improved upon to makeogu-[NRA CMS. A small part of the hybrid market is based on the Polimacytoplasm and MSL (male-sterile Lembke) systems. The Polima CMS wasidentified by Dr. Fu in China in the Polima cultivar of Polish B. napus. In the USA,approximately 50% of the hybrid market is produced using the Polima cytoplasm,while the SeedLink tm and Ogura systems are increasing in market share registered(Dale Bums, personal communication). In China, approximately 95% of hybrids arebased on Polima type CMS with the remaining 5% produced using genic malesterility (Dr. Fu, personal communication). In Australia, all hybrid seed productionis based on Ogura-INRA CMS (Dr. Radisa Gjuric, DSV Canada Inc., Winnipeg,Manitoba).

Canola and rapeseed hybrids produced using the SeedLink tm systemcontinue to increase in market share. Both the Ogura and Polima sources of CMShave been successfully commercialised. Polima hybrids are in decline due toproblems with sterility breakdown under both high and low temperatures. Flowersare also not attractive to pollinators due to flower morphology and absence ofnectaries. Ogura CMS is more stable than Polima, but possesses similar problemswith flower morphology. Under lower temperatures (<12 C), Ogura CMS plantsdevelop severe chlorosis. A major improvement made by [NRA in France producedstable CMS and monogenic restoration. Initially, the restoration factor introducedfrom radish (Raphanus sativus) was linked with high glucosinolate contents in theseed. Because canola is low in erucic acid and glucosinolates, it was not possible touse Ogura CMS to develop commercial hybrids for registration in Canada, butpossible for the US and Australia. This explains the relative higher percentage ofCMS hybrids in these two markets. The first low glucosinolates restorers weredeveloped first by Advanta Seeds in 1996 (first hybrid in 2001), followed by otherbreeders in Canada and EU. At the moment the majority of breeding organizationsare focused on ogu-INRA and this system is expected to become the dominanthybrid system worldwide (Dr. Radisa Gjuric and Dr. Dale Bums, personalcommunications).

628 MICHAEL J. HAVEY

2.4. Carrot

For carrot, two sources of CMS are used to produce hybrid seed. The predominantCMS is the petaloid male-sterile cytoplasm, in which the anthers are replaced by awhorl of petals (Eisa and Wallace, 1969). The second source of CMS is brownanther, in which complete flowers produce shrivelled anthers with no pollen (Welchand Grimball 1947). Male-fertility restoration for these sources of CMS iscomplexly inherited with up to five loci affecting this trait (Peterson and Simon,1986). Hybrid carrot represents approximately 50% of the world market, withpetaloidy as the predominant source of CMS used for hybrid-seed production in theUS (over 90%) and world (70%) markets. The rest of hybrid-carrot seed isproduced using the brown anther source of CMS (Philipp Simon, USDA, personalcommunication). The pentaloid CMS is generally preferred because of less frequentreversion to male fertility; however seed yields on the brown-anther CMS aregenerally higher (Peterson and Simon, 1986).

2.5. Lolium

Two sources of CMS have been developed in ryegrass (Lolium perenne L.), onefrom an interspecific cross between L. multflorum and L. perenne (Wit, 1974) andone from an intergeneric cross between Festuca pratensis and L. perenne (Connollyand Wright-Turner, 1984). However no ryegrass hybrids are commercially availableat this time (Michael Casler, USDA, personal communication).

2.6. Maize

Maize was the first hybrid crop to be produced on a large scale. In Europe, Japan,Australia, New Zealand, and the US, the field and sweet-corn production is basedexclusively on hybrid cultivars. Open-pollinated cultivars are still widely cultivatedin areas where the cost of hybrid seed is prohibitive. The prevalence of CMS in theproduction of hybrids is not clear. In the case of sweet corns, the relatively short life(three to five years) of a specific hybrid means that there may not be enough time toconvert new inbreds to CMS by backcrossing and the hybrid will be produced byemasculation. When CMS is used to produce hybrid seed, C and S cytoplasms arecommon; T cytoplasm is not used due to its susceptibility to B. maydis. Thepreference of C versus S cytoplasm is dependent on the specific population andstability of the male sterility over environments. Many maize hybrids do not carrynuclear male-fertility restoration alleles and, as a result, are male sterile. The hybridseed is then blended with N-cytoplasmic, hand emasculated hybrid seed that isobviously male fertile and provides enough pollen in the production field. Mostmaize breeders reported that the blended hybrids are 50% CMS with 50% N-cytoplasmic plants. The recent incorporation of transgenes in maize hybrids hasdecreased the use of CMS to produce hybrid seed, because of the time requirementto backcross sources of CMS into transgenic lines.

THE USE OF CMS FOR HYBRID SEED PRODUCTION 629

2.7. Millets

The prevalence of pearl-millet hybrids ranges from nearly 100% in the US, toapproximately 50% in India, to very low in some regions of Africa (Dr. WayneHanna, University of Georgia, personal communication). CMS in pearl millet(Pennisectum glaucum L. R. Br.) was discovered in the 1950s (Burton, 1958;Menon, 1959) and first released as the male-sterile inbred Tifton 23A, termed theAl or milo cytoplasm. Most, if not all, of the world's hybrid pearl millet isproduced using the Al cytoplasm (Smith and Chowdhury, 1989). Other sources ofCMS were identified (Burton and Athwal, 1967), but were not stable enough forcommercial use (Smith and Chowdhury, 1989) or were difficult to find nuclearrestorers (A4). The A4 cytoplasm is useful for production of forage hybrids that donot require male-fertility restoration.

2.8. Pepper

Hybrid cultivars dominate the production of some market classes of peppers. Forexample, approximately 100% of bell peppers are hybrid, as compared to 50-75% ofJalapenos. The other major types of pepper are almost exclusively open-pollinated(Paul Bosland, New Mexico State University, personal communication). The onlysource of CMS used to produce hybrid-pepper cultivars was described by Peterson,(1958).

2.9. Rye

Rye is an important grain crop in Germany, Poland, and parts of Russia. InGermany, hybrid cultivars represent approximately 60% of rye production; hybridrye is starting to be cultivated in Poland at about 5 to 10% of production (Dr.Thomas Miedaner, University of Hohenheim, Germany, personal communication).Rye is naturally outcrossing due to gametophytic self-incompatibility; however self-fertile plants have been identified allowing for inbred line development. Hybrid ryeis produced using the Pampa (P) cytoplasmic source of CMS, which , wasintrogressed at the University of Hohenheim, Germany, from a primitive Argentinapopulation (Geiger and Schnell, 1970). The P cytoplasm is widely used because itsmale sterility is stable across environments and research has focused onidentification effective restorer genes (Geiger and Miedaner, 1996; Miedaner et al.,2004). A second CMS (G cytoplasm) was developed in the former GermanDemocratic Republic and is used to produce one registered hybrid in Germany(Novus). Other sources of CMS have been described and their relationships to the Pand G cytoplasms are unclear.

2.10. Sorghums

Hybrid sorghum predominates the US and world markets. Hybrid sudangrasscultivars also exist, however OP populations still represent a significant componentof production. CMS is routinely used to produce hybrid seed of sorghum and

630 MICHAEL J. HAVEY

sundangrass. Although several sources of CMS have been described (Al throughA4), the predominant male-sterile cytoplasm is the Al (milo) cytoplasm (Hollandand Stephans, 1954; Pring et al., 1995 Schertz et al., 1997). Fertility restoration ofthe Al cytoplasm requires the action of two complementary nuclear male-fertilityrestorers (RfI and RJ2), similar to T-cytoplasmic maize. There was some effort inproducing male sterile sorghum x sudangrass hybrids in systems with fewer fertilityrestorers. However the emergence of sorghum ergot (Claviceps africana) in theWestern Hemisphere in the 1990s caused abandonment of this effort. The infectionsite for sorghum ergot is the unfertilized stigma and it was feared that largepopulations of CMS sudangrass hybrids could serve as an inoculum source for othersorghums (Dr. Jeff Peterson, University of Nebraska, personal communciation).

2.11 Sugar and table beets

CMS is used to produce hybrids of both table and sugar beets. Sugar-beet cultivarsare almost exclusively hybrids in the US and Europe, with some open-pollinatedcultivars grown in regions of the world with lower inputs such as Morocco andEgypt (Mitch McGrath, USDA, personal communication). Both diploid and triploidsugar-beet hybrids are widely grown. Approximately 50% of table-beet cultivars arehybrid; OP cultivars are still produced with the advantage of cheaper seed (I.Goldman, personal communication). The sole source of CMS used to producehybrid beets was described by Owen in 1945. In beets, male sterility is conditionedby the interaction of the S cytoplasm with recessive alleles at two nuclear loci (xxzz). Owen (1945) recognized that the approach of Jones and Clarke (1943) could beused to produce beet hybrids. A second alloplasmic source of CMS from Betamaritima has been described (Boutin et al., 1987), but has not been used to date toproduce commercial beet hybrids.

2.12. Sunflower

Hybrid sunflower dominates many of the word's production areas, including 100%in Argentina, Australia, Europe, India, South Africa, Turkey, and USA. Othercountries, such as China, have significant (>35%) hybrid production. Hybridsunflower is produced using the PET1 cytoplasm, which was developed bytransferring the cytoplasm of Helianthus petiolaris to H. annuus (LeClerq, 1969).Although other sources of CMS in sunflower can be distinguished by organellarpolymorphisms (Crouzillat et al., 1991), PET is the only cytoplasm used to producesunflower hybrids. The predominance of the PET 1 source of CMS is due primarilyto its stable expression and complete male-fertility restoration by one dominantlocus (Rfl) (Jerry Miller, USDA, personal communication).

3. FUTURE ASPECTS

This chapter documents the worldwide use of CMS to produce competitivehybrid cultivars. Major investments of time and resources are required to backcross

THE USE OF CMS FOR HYBRID SEED PRODUCTION 631

a male-sterility-inducing cytoplasm into elite lines. These generations ofbackcrossing could be avoided by transformation of an organellar genome of theelite male-fertile inbred to produce female inbred lines for hybrid seed production.Because the male-fertile parental and male-sterile transformed lines would bedeveloped from the same inbred, they should be highly uniform and possess thesame nuclear genotype (excluding mutations and residual heterozygosity).Therefore, the male-fertile parental line becomes the maintainer line to seed-propagate the newly transformed male-sterile line (Havey, in press). A fewgenerations of seed increases would produce a CMS-maintainer pair for hybrid seedproduction. An additional advantage of organellar transformation would be thediversification of CMS sources used in commercial hybrid-seed production.Transformation of the mitochondrial genome would allow breeders to introducedifferent male-sterility-inducing factors into superior inbred lines. These male-sterility-inducing factors could be from the same species, such as T-cytoplasmiconion (Berninger, 1965), or possibly from another species, such as the mitochondrialfactors conditioning CMS from Petunia (Nivison et al., 1994), sorghum (Tang et al.,1996), or sunflower (Moneger et al., 1994). Although the nuclear genome has beensuccessfully transformed with male-sterility-inducing factors (Mariani et al., 1990;Hemould et al., 1993), these nuclear transformants segregate for male fertility orsterility and must be rogued by hand or with herbicides to remove male-fertile plants(Perez-Prat and van Lookeren Campagne, 2002).

Transformation of the mitochondrial genome to produce CMS lines is morepractical for hybrid-seed production because removal of male-fertile plants iseliminated or greatly reduced. The chioroplast genome of Chiamydomonas(Boynton et al.. 1988; Kindle et al., 1991) was the first organellar genome to besuccessfully transformed, followed by tobacco (Svab and Maliga, 1993; O'Neill etal., 1993; Koop et al., 1996; Daniell et al., 2004; Devine and Daniell, 2004, see alsochapter 16 of this book), Arabidopsis (Sikdar et al., 1998), carrot (Kumar et al.,2004a), Cotton (Kumar et al., 2004b), soybean (Dufourmantel et al., 2004) andtomato (Rufet al., 2001). Although transformation of the mitochondrial genome hasbeen reported for Chiamydomonas (Randolph-Anderson et al., 1993) and yeast(Butow et al., 1996), there is no report of mitochondrial transformation of a higherplant. Introduction of a male-sterility inducing transgene into one of the organellargenomes of a higher plant would be a major breakthrough in the production of male-sterile inbred lines. This has been indeed accomplished recently by expressing thephaA gene coding for 13-ketothiolase via the tobacco chloroplast genome (seechapter 16). The transgenic lines were normal except for the male sterilityphenotype, lacking pollen. Scanning electron microscopy revealed a collapsedmorphology of the pollen grains. Transgenic lines followed an accelerated antherdevelopmental pattern, affecting their development and maturation, resulting inaberrant tissue patterns. Abnormal thickening of the outer wall, enlargedendothecium and vacuolation, which decreased the inner space of the locules,affecting pollen grain and resulting in the irregular shape and collapsed phenotype.This is the first report of engineered cytoplasmic male sterility and offers a new toolfor transgene containment for both nuclear and organelle genomes. This technique

Mir

632 MICHAEL J. HAVEY

would be of great potential importance in the production of hybrid crops by avoidinggenerations of backcrossing, an approach especially advantageous for crop plantswith longer generation times (Havey, 2004).

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