Euphytica 92 : 135-138, 1996.

135© 1996 Kluwer Academic Publishers. Printed in the Netherlands .

Breeding components for mixture performance

J. HillDepartment of Agricultural Sciences, Royal Veterinary and Agricultural University, Copenhagen, Denmark ;Present address : Broneirian, Llanbadarn Road, Aberystwyth, Dyfed SY23, 1HB, Wales, U .K.

Key words : breeding mixture components, coevolution, ecological combining ability

Summary

The problems posed by breeding components for use in binary mixtures are discussed . Breeding and selectionstrategies designed to overcome some of these problems are outlined .

The problems

Before a breeder embarks upon a breeding and selec-tion programme he would, as a matter of routine, definehis objectives . In particular what would be the likelyend-use of the potential cultivar ; what, in other words,would be its agricultural niche . Defining his objectivesallows the breeder to choose a base population andbreeding and selection strategies appropriate for theirfullfilment. Thus, for example, breeding for diseaseor drought resistance would require that potential vari-eties are selected in environments prone to these bioticand abiotic stresses . Yet, when it comes to the breed-ing of crop varieties destined for use in mixtures, rarelyif ever is one component of the mixture evaluated inthe presence of the other component(s) . Rather is thebreeding process aimed at identifying superior geno-types on the basis of their performance in pure stands .It is only afterwards that the putative components arebrought together in the hope, often misplaced, that asuperior mixture will result. One reason for the fail-ure of this haphazard process is that yielding ability,which is required for high monoculture performance, isnot necessarily the same as competitive ability (Sakai,1961), which is the attribute determining performancein mixtures. Competitive ability may be defined as theability of one component to obtain limited resourceswhen grown in mixtures with another component, com-pared to its ability to secure those same resourceswhen grown in pure stands (modified from Snaydon,1991). Monoculture performance is not necessarily

therefore a reliable guide to behaviour in mixtures,and indeed, yielding and competitive ability may evenbe inversely related. Moreover, in monocultures onlyinfra-component competition is present, whereas inmixtures both infra- and inter-component competitionexists. Stresses induced by infra-component compe-tition are expected to be the greater because similarspatial and temporal demands are being made uponthe available environmental resources (Mather, 1961 ;Caligari, 1980) .

So what are the requirements of those breeding andselection strategies designed to produce varieties des-tined for use in mixtures, such as the grass/clover mix-tures encountered in temperate pastures, or the mixedcropping systems employed in many developing coun-tries? Clearly the main pre-requisite is to conduct suchprogrammes in the presence of the other component, ifwe consider just binary mixtures, since only then is itlikely that genotypes will `possess biological charac-ters conducive to synergism' (Allard & Adams, 1969) .Allard & Adams were able to substantiate this viewfrom their own work with barley mixtures: selection insuch mixed populations they observed `favours the sur-vival of genotypes that are good competitors and at thesame time good neighbours,' thereby improving whatis often referred to as the ecological combining ability(Harper, 1967 ; Allard & Adams, 1969) of the com-ponents concerned . By analogy with the well-knowngenetic concept of combining ability, ecological com-bining ability may be divided into general and specific .And, again by analogy with Sprague & Tatum's (1942)

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original definitions of the genetic concept, the generalecological combining ability (g .e .c .a .) of a competi-tor may be defined as the average performance of allmixtures based on that competitor, while specific com-bining ability (s .e.c .a.) relates to those mixtures whichperform significantly better or worse than expectedon the basis of the average of its constituent compo-nents (Hill, 1990) . Thus the yield of a binary mixturebetween the ith competitor and the jth associate (Yii)can be written as

Yj = p + ci + aj + cia, + eai

where µ is the overall mean, c i is the g .e.c .a. of the ithcompetitor, a; is the g .e .c.a. of the jth associate, cia; isthe s.e .c .a. of the ith competitor and the jth associate,and e2 is an error term . Note that this model mea-sures neither the competitive ability of the individualcomponents nor the relative magnitudes of intra- andinter-component competition as such. This latter aspectof competition in particular requires an entirely differ-ent experimental approach, which has been reported indetail elsewhere in the literature .

Breeding and selection strategies

Two broad breeding strategies can be envisaged . Inthe first, attention is focussed on one of the compo-nents only. For the sake of convenience this will bereferred to as the passive approach . In the second bothcomponents may be improved using alternating cyclesof selection . This will be termed the active approach .The passive approach lends itself to a North Caroli-na 2 mating design, in which a number of genotypesfrom one component could be assessed in all possiblebinary combinations against a set of testers or asso-ciates drawn from the other component (Figure 1) .Ideally testers should not be strong competitors, oth-erwise differences between the genotypes under testmay go undetected . Equally the testers should not beso weak as to offer no competitive stress to the geno-types. Material selected on the basis of its generalecological combining ability may then be polycrossedand re-tested against the associates in further cycles ofselection. Specific ecological combining ability wouldalso be revealed by this mating design .

In the active approach, the roles of tester and test-ed are reversed in alternate cycles of selection (Figure1), but apart from this the basic strategy is the sameas for the passive approach . Both a North Carolina 2or a diallel mating design could be used. The former

design would be appropriate for binary mixtures basedon different species, such as grass/clover, while thelatter is suited to the evaluation of different genotypesor cultivars of the same species . If a diallel designis employed it should preferably be based on Griff-ing's experimental method 4 (Griffing, 1956), as thiswould avoid any complications arising from estimatesobtained from both monocultures and mixtures .

Furthermore, a diallel should be reserved for thelater phases of any breeding programme, by which timethe number of components has been whittled down tomanageable proportions and when specific ecologicalcombining ability is being sought .

Coevolution and ecological combining ability

So far no mention has been made of coevolution . Itis axiomatic that species or genotypes must coexistbefore they can coevolve . According to Janzen (1980)coevolution occurs when a trait of one species evolvesin response to a trait of another species, which traithas itself evolved in response to the trait in the firstspecies . The process is both reciprocal and specific .If only one species diverges, coevolution sense strictohas not occurred (Futuyma & Slatkin, 1983). Con-vincing experimental evidence of coevolution is hardto find (Snaydon, 1985), as opportunities for indepen-dent plant species to coevolve will depend upon theirexploitation of the available niches . Similarly, Rhodes(1981) suggested that improved ecological combiningability may be achieved by increasing both the tempo-ral and spatial compatibility of the components . Bothcoevolution and ecological combining ability dependtherefore on the availability and exploitation of suit-able niches, such that the competitive stresses betweenthe components are somehow avoided, minimized ortolerated (Hill et al ., 1986). The distinction betweenthem is that the former is a process which occurs natu-rally and often passes unnoticed, while the latter is anattribute which can be bred for, given the appropriatestrategy .

In determining whether coevolution has occurredthere is a danger of applying agronomic rather thangenetic criteria. Coevolution is primarily concernedwith improving the genetic fitness of the participants,not with maximizing their agricultural productivity . AsHarper (1978) states `many of the processes that favourmembers of one species over another in respect ofleaving more descendants than their neighbours, maylead in a direction quite opposite to that of agronom-

Select and evaluate A

with testers of B.

Base population(s)

Repeat as necessary.

is optimisation.' Although the evolutionary and plantbreeding concepts of coevolution may not be synony-mous, it could be argued that artificial selection mayenforce coevolution for agronomic traits, since thesetraits could be related to fitness under these condi-tions . Greater synonymity between these two conceptsmay also result if the character concerned is itselfrelated to fitness, such as seed yield, as opposed toa vegetative character such as dry matter production .Thus, although Aarssen & Turkington (1985), Evanset al. (1989) and others observed increased yields fromgrass/clover mixtures based on components which hadpreviously coexisted, it comes as no surprise to discov-er that not all such mixtures behave in this way (Collins& Rhodes, 1989). Nevertheless, mixtures based oncomponents having a history of coexistence offer astopgap until material developed by the strategies sug-gested earlier becomes available .

The art - or science - of plant breeding could per-haps be construed as the ability to re-assort in decadeswhat it has taken nature centuries to build, in order toaccumulate as many agronomically desirable traits aspossible into a single variety . It is no easy task ; devel-

Multiplication, etc .

Select and evaluate A

in mixtures with B.

Polycross survivors of A .

Use resultant material as

testers for B .

4Select and evaluate B in

mixtures ofA .

yPolycross survivors of B .

Use resultant material as

testers for A.

Figure 1 . Outline of the active and passive breeding and selection strategies for improving ecological combining ability (after Hill, 1990) .

oping varieties specifically for use in mixtures will beno easy task, but then whoever said plant breeding waseasy?

Acknowledgement

I am indebted to Dr. E.L. Breese for reading and com-menting upon this script .

References

Aarssen, L.W. & R. Turkington, 1985 . Biotic specialization betweenneighbouring genotypes in Lolium perenne and Trifolium repensfrom a permanent pasture . J Ecol 73 : 605-614 .

Allard, R.W. & J. Adams, 1969 . Population studies in predominant-ly self-pollinating species . XIII . Intergenotypic competition andniche differentiation in barley and wheat. Am Nat 103 : 621-645 .

Caligari, P.D.S ., 1980. Competitive interactions in Drosophilamelanogaster . Heredity 45: 219-231 .

Collins, R. & I . Rhodes, 1989 . Yields of white clover populations inmixture with contrasting perennial ryegrasses. Grass Forage Sci44: 111-115 .

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Evans, D.R ., J . Hill, T.A . Williams & I . Rhodes, 1989 . Coexistenceand the productivity of white clover-perennial ryegrass mixtures .Theor Appl Genet 77: 65-70.

Futuyma, D .J . & M . Slatkin, 1983 . Coevolution. Sinauer Associates,Sunderland.

Griffing, B ., 1956. Concept of general and specific combining abilityin relation to diallel crossing systems . Aust J Biol Sci 9 : 463-493 .

Harper, J.L., 1967. A Darwinian approach to plant ecology . J Ecol55:247-270 .

Harper, J .L ., 1978 . Plant relations in pastures. In : J.R . Wilson (Ed .),Plant Relations in Pastures, pp 3-14. CSIRO, East Melbourne .

Hill, J ., 1990 . The three C's - competition, coexistence and coevo-lution - and their impact on the breeding of forage crop mixtures .Theor App! Genet 79: 168-176 .

Hill, J., K . Mather, R.M. Chirwa &P.D.S.Caligari, 1986. The analy-sis of competitive ability in forage crops . In : M.J. Kearsey & C .P.Werner (Eds), Proc . Sixth Meeting Eucarpia Section Biometricsin Plant Breeding, University of Birmingham, UK, 28 July-IAugust, 1986, pp 193-204.

Janzen, D.H ., 1980 . When is it coevolution? Evolution 34 : 611-612.

Mather, K ., 1961 . Competition and co-operation. Symp Soc ExplBiol 15 : 264-281 .

Rhodes, I., 1981 . The physiological basis of variation in the yield ofgrass/clover mixtures . Br Grassl Soc Occas Symp 13 : 149-161 .

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Snaydon, R .W., 1985 . Aspects of the ecological genetics of pas-ture species . In: J . Haeck & J.W. Woldendorp (Eds), Structureand Functioning of Plant Populations . 2 . Phenotypic and geno-typic variation in plant populations, pp 127-152. North Holland,Amsterdam .

Snaydon, R.W., 1991 . Replacement or additive designs for compe-tition studies? J Appl Ecol 28 : 930-946 .

Sprague, G.F. & L.A . Tatum, 1942 . General vs specific combiningability in single crosses of corn . J Am Soc Agron 34: 923-932.