196 VA Fasou D.A.Fasoul/Field Crops Research 75(02)191-209 inbreds in maize and other nay be raised at inbred line perfon ticed effectively higher levels by converting ulsion into coupling deleterious alleles will be replaced by additive alleles vigor and coupling into inbred vigor through intralo- increasing the productivity of inbred lines(Fasoulas, cus recombination and fixation(Fasoula and Fasoula, 1997:Fasoula and Fasoula,1997b).Fixation of het. 1997b).In fact,Genter (1967)identified two inbred sis and improvement of the productivity of inbrec lines in maize that showed little inbreeding depres- ines have already been accomplished in maiz sion (Tokatlidis t al. 98 999),nce (Ntar aing the genetic basis of 20 ,1998 by the and Eas and mato(Christ 20011R pla nt by fo inbred lines at the other,arrived at the following wo advantages:(D the amelioration of the additive conclusion "True overdominance has been shown allelic action responsible for inbred vigor and genetic to have no demonstrable role in hybrid vigor.although progress through selection.and(2)the steady transmis spurious overdominance arising from a linkage dis sion of the accomplished amelioration from generation equilibrium with or without superimposed nonallelic to generation through sexual reproduction. 3.Conditions optimizing response to selection ge Genetic imn vement is closely linked with the Response to selection.or genetic gain.is the elimination of the undesirable action of deleterious improvement of the population mean as a result of genes,first at the allelic and,by extension,at the selection.The simplest form of selection is to choose epistatic level.Elimination of the undesirable action is individuals on the basis of their own phenotypic accomplished by two means:either through the con favorab 品 Ihe parameter nse (R)in one terious eOnt9breame.culivan.Thedevelor ment of modern hybrids involves the difficult task of discover- R=aphi ing inbred lines that cover mutually their defects,a fact that renders the whole process a numbers game. uth n r henpuy the coefficient of heritability,h=/ Conversely,the development of inbred line cultivars and i the standardized selection differential, should aim at a constant cement of less favorable ap,with eing the nean yield of tr by more favorable addit e allele proces of the beaaiaceentO the ie the of the selected point is reached where hybrid developr ent is no tion to be used as ts Therefore the sm ner the more cost effective (Fasoulas,1997).When this has proportion of the selected parental plants.the large been accomplished,cultivar production should be the standardized selection differential and,by exten targeted not toward developing hybrid cultivars,but ion,the higher the expected response through selec- toward boosting the yield of inbred lines beyond that ton is. of hybrids. The conditions that optimize response toselectio of vigorous inbred li hat approa hybri ar心，in gencof E bilit for inbred line anc ection (2)he e pe of high
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VA.Fasoula.D.A.Fasoula/Field Crops Research 75 (2002)191-209 197 Phenotypic variance is partitioned into three main that attributed large and heritable components: phenotypic varation 3.1.Maximizing phenotypic variance whereis the phenotypic variance.the genetic the viro The mean ()the phenotypic standard deviation and ype- (p).and the CV of single-plant yields are affected differently in the absence vs.the presence of competi- gful if the la tion(Kyriakou and Fasoulas,1985).When the same the ed to the environme rye (Secale cereale L.)population was compared at and the genotype-by-environment interaction var- two dif ent pl 1.4 plant iances.When and are significantly minimized, heritability approaches the maximal value of 1Eq.(3). e vas reduced by 70 and the Cy wa ased by 03 61%(Fig.2).In other words,the competition envir 2= ++0x 3) onment reduced the mean and standard deviation 7.5 Evidently.the conditions that allow to maximize CV application of high Similar results were obtained by Hamblin et al ction pressures are toog as compared cm.The mear NIL-COMPETITION COMPETITION (1.4 plants/m2) (51.3 plants/m2) 112.4g 14.9g 37.5g 0, （7.9g 33%4 CV →53% oes the range of yield ex n.In the of etition the range of yield expression is maximized and
, - / & + + 0' - / + + + H ( * $ . 0' - / - / 0 & $* & ! '' )= # ) -/ 4 + + -J % .6:5/ H - I/ & & .9 < -+ / 5. < -/ & :"E & "6E 4 & 8.E - / 3 & + "5 9" & 4 + .8 1 & ) -.6":/ & & ++ 9 9 # @ ) -/ 4 + & & ! .9 < -+/ 5. < -/ # $ 3 $ $* 4 + - J % .6:5/ 2' ' 3 ' ' 3 4 3 % $* 5" "6 78" 7 .6"