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Euphytica 45 : 257-266,1990. 1990KluwerAcademicPublishers.PrintedintheNetherlands . Hybridreconstructioninmaize M .Koutsika-Sotiriou',I .Bos2 andA .Fasoulas' t DepartmentofGeneticsandPlantBreeding,AristotelianUniversity,Thessaloniki,Greece ; 2 DepartmentofPlantBreeding-IvP,AgriculturalUniversity,Wageningen,TheNetherlands Received 29 March 1988 ; acceptedinrevisedform 24 March 1989 Keywords :Zeamays, maizebreeding,honeycombselection,inbredlinedevelopment Summary TheproductionofseedofthemaizesinglecrosshybridF68*NE2isuneconomicbecauseofthe lowgrain yieldofthematernalline .Thereforetheaimwastoproduceitfromnewlydevelopedinbredlinesobtained by reshufflingthegenesinthehybrid,accompaniedbyselection .Thusinopenpollinatinggenerations derived fromthishybrid,i .e .inCO,C1,C2,C3andC4,honeycombselectionforgrainyield improvementwas applied .SelfingofoneearandopenpollinationofanotherearofselectedprolificC4plantsyielded 20pairs ofSl/halfsibprogenies .PlantsgrownfromremnantS1seedcorrespondingtosuperiorprogeny pairswere selfed .IneachS2-lineasingleplantwasselectedandselfed .TheS3-lineswereevaluated foryield.Two S3-lines,i .e .6Dand2B,attractedattentionbecausetheyyieldedtwoandahalftimesasmuch asthebest commercialinbredlineB73 . TheS1-andS2-linesweretestedforcombiningabilitywiththerelatedinbredlinesNE2andF68bymeans ofhoneycombdesignexperimentsandforcombiningabilitywithunrelated,freelyavailable inbredlinesby meansofrandomizedcompleteblockdesigns .TwoS2-lines,i .e .5Cand6E,wereselected fortheirgood combiningability .ThesixsinglecrosshybridsproducedbycrossingthefourS3-lines6D,2B,SCand6E were comparedwiththeoriginalhybridF68*NE2inahoneycombdesignattwosites .Thegrainyieldsofthe single crosshybrids6D*6Eand5C*6EweresimilartothatofF68*NE2 .However,thesetworeconstructedhybrids canbeproducedinacheaperwaybecausethenewmaternalinbredlinesyieldasgoodasB73 (line5C)or muchbetter(line6D) . Introduction Theincreasinggrowingofsinglecrosshybridvarie- tiesinmaize (ZeamaysL .) hascausedagreater emphasisonobtaininghighyieldingmaternallines . Consequentlythedevelopmentofgoodperform- inginbredlineswithsuperiorhybridperformance isnowaprimarygoalofmostmaizebreedingpro- grams .Suchinbredlinesmaybedevelopedfrom improvedpopulations .Recurrentselectionbefore highyieldinglinesareextractedcanbeeffective, butsuchsuperiorlinesarenotfrequentlyobtained (Zuber,1982) .Anotherproblemistheshortperiod ofextensiveuseofinbredlines(Zuber&Darrah, 1980) .Mostofthesuccessfulinbredlinesappearto haveabout10yearsofpeakusage . InGreecetheinbredlineF68,originatingfrom Australia,yieldsontheaverageonlyabout500kg perha .TheinbredlineNE2yieldsontheaverage 3000kgperha .ThislinewasdevelopedinAustra- liafromtheF2ofthesinglecrossrFE701*07 .The singlecrosshybridF68*NE2yieldsabout12000kg perha .Becauseofitslowkernelyieldtheuseof F68fortheproductionofsinglecrosshybridseedis

Hybrid reconstruction in maize

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Page 1: Hybrid reconstruction in maize

Euphytica 45 : 257-266, 1990.1990 Kluwer Academic Publishers. Printed in the Netherlands .

Hybrid reconstruction in maize

M. Koutsika-Sotiriou', I . Bos2 and A. Fasoulas't Department of Genetics and Plant Breeding, Aristotelian University, Thessaloniki, Greece ;2 Department of Plant Breeding-IvP, Agricultural University, Wageningen, The Netherlands

Received 29 March 1988 ; accepted in revised form 24 March 1989

Key words: Zea mays, maize breeding, honeycomb selection, inbred line development

Summary

The production of seed of the maize single cross hybrid F68*NE2 is uneconomic because of the low grainyield of the maternal line. Therefore the aim was to produce it from newly developed inbred lines obtained byreshuffling the genes in the hybrid, accompanied by selection . Thus in open pollinating generations derivedfrom this hybrid, i .e . in CO, C1, C2, C3 and C4, honeycomb selection for grain yield improvement wasapplied. Selfing of one ear and open pollination of another ear of selected prolific C4 plants yielded 20 pairsof Sl/half sib progenies. Plants grown from remnant S1 seed corresponding to superior progeny pairs wereselfed. In each S2-line a single plant was selected and selfed . The S3-lines were evaluated for yield. TwoS3-lines, i .e. 6D and 2B, attracted attention because they yielded two and a half times as much as the bestcommercial inbred line B73 .

The S1- and S2-lines were tested for combining ability with the related inbred lines NE2 and F68 by meansof honeycomb design experiments and for combining ability with unrelated, freely available inbred lines bymeans of randomized complete block designs . Two S2-lines, i .e. 5C and 6E, were selected for their goodcombining ability. The six single cross hybrids produced by crossing the four S3-lines 6D, 2B, SC and 6E werecompared with the original hybrid F68*NE2 in a honeycomb design at two sites . The grain yields of the singlecross hybrids 6D*6E and 5C*6E were similar to that of F68*NE2 . However, these two reconstructed hybridscan be produced in a cheaper way because the new maternal inbred lines yield as good as B73 (line 5C) ormuch better (line 6D) .

Introduction

The increasing growing of single cross hybrid varie-ties in maize (Zea mays L .) has caused a greateremphasis on obtaining high yielding maternal lines .Consequently the development of good perform-ing inbred lines with superior hybrid performanceis now a primary goal of most maize breeding pro-grams. Such inbred lines may be developed fromimproved populations . Recurrent selection beforehigh yielding lines are extracted can be effective,but such superior lines are not frequently obtained

(Zuber,1982) . Another problem is the short periodof extensive use of inbred lines (Zuber & Darrah,1980). Most of the successful inbred lines appear tohave about 10 years of peak usage .

In Greece the inbred line F68, originating fromAustralia, yields on the average only about 500 kgper ha. The inbred line NE2 yields on the average3000 kg per ha. This line was developed in Austra-lia from the F2 of the single cross rFE701*07. Thesingle cross hybrid F68*NE2 yields about 12000 kgper ha. Because of its low kernel yield the use ofF68 for the production of single cross hybrid seed is

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unprofitable in Greece . Thus our objective was toreconstruct the single cross hybrid by reshuffling itsgenes such that new inbred lines to be developedallow a more economic production of an equivalentsingle cross hybrid . To reach this goal a special`second cycle' breeding program (Allard, 1960 ;p. 276) was performed .

Materials and methods

In short, the following procedure was applied forthe reconstruction of the heterotic maize hybridF68*NE2 (see Fig . 1). In 1977 this single crosshybrid produced seed by open pollination . Theseed thus obtained was planted in 1978 as startingmaterial and is, accordingly, indicated as gener-ation CO. In each of the next five years, i .e. in 1978(CO), 1979 (Cl), 1980 (C2), 1981 (C3) and in 1982(C4), honeycomb selection (Fasoulas, 1981) forhigh grain yield per plant was applied. The in-tensity of selection was 1 .6%, i.e. a plant was se-lected if it outyielded each neighbour in four sur-rounding hexagons . The progress appears from theyield of CO and C4 as related to the yield of thesingle cross hybrid . For CO the relative yieldamounted to 41% and for C4 to 83% (Gogas,1986). In 1982 and 1984 selected plants were selfed .In 1983 good yielding Sl-lines were selected . Yield-ing capacity of S3 lines and combining ability ofselected S1- and S2-lines were evaluated in 1985 .Finally, in 1987 reconstructed single cross hybridswere compared with the original one at two loca-tions .

All trials were regularly irrigated to avoid severedrought stress . Ears harvested from individualplants or from plots were thoroughly dried beforeweighing .

The development of the Sl-, S2- and S3-lines

In 1982, 20 SO-plants belonging to the C4 pop-ulation were selected by eye because of their vigourand prolificacy . The upper ear of each plant wasselfed and the lower ear was open pollinated . InApril 1983 the Sl-line from the selfed and the half

sib (= HS-) family from the open pollinated ear ofeach of these 20 plants were evaluated in compari-son to the original single cross hybrid as well as theCO population . The evaluation was by means of anR-49 honeycomb design for single-plant evaluation(Fasoulas, 1981) . This design requires 49 entries,which may be coded iA to iG ; i = 1, . . ., 7. In thepresent case forty codes were for the 20 pairs con-sisting of an Si-line and a HS-family . The singlecross hybrid and its offspring after open pollina-tion, i .e. CO, were coded 1A and 3A and sevencodes (i .e . 7A, . . ., 7G) were assigned to additionalCO material . The interplant distance was 1.25m.Because several plants were missing, 39 to 52 plantswere observed per entry .

The inbreeding depression of each S1-line wascalculated according to Goulas & Lonnquist(1976) . Thus the relative difference in yield withregard to the corresponding HS-family was calcu-lated. For CO the inbreeding depression was calcu-lated as the relative difference with regard to theoriginal hybrid (Meghji et al ., 1984) .

Seven Sl-lines, showing inbreeding superiorityor a small inbreeding depression were selected(these lines are the top seven in Table 1). For eachof these 7 S1-lines a few plants were grown early in1984 in the greenhouse from remnant seed . Fromthe Sl-line coded 6B six selected plants were self-ed. The obtained S2-lines were named 1B, . . ., 6B .From the Sl-lines coded 6D, 5C and 6E only oneplant was selfed and the obtained S2-lines werecoded 6D, 5C and 6E respectively . Selfing of plantsfrom lines 3, 4 and 6 was not successful. Later in1984 the 9 S2-lines were grown in the field in anear-to-row design at a low plant density . From eachS2-line a single plant was selected and selfed .

The yielding capacity of the S3-lines and thecombining ability of the S1- and S2-lines

In April 1985 three experiments were sown . Thefirst was intended to determine the yielding capac-ity of eight S3-lines per se in comparison to thefreely available inbred lines B73, B84 and Va22 .(Because of lack of seed S3-line 4B was not in-cluded.) B73 was developed from BSSS(HT)C5

Page 3: Hybrid reconstruction in maize

3. Reconstruction ofsingle crossF68 x NE2 andits evaluation(2 years)

1984(Summv)

1985

1987

F68xNE2

00000000000000000000ea00000 p( 0,1000000CO000000000000000000000

0 00000 •00 •00000000 •000000000 •0 0000000•0 00000000 •00000000 •00

Fig. 1 . Pathway for the reconstruction of the single cross maize hybrid F68 X NE2.

Open pollination in F68*N52followed by honeycomb selec-tion in generations Co (=F2 ),C1, C2 and C3.

Selling one ear and openpollination of another earof selected C4- plants .

Evaluation of HS-S1 proge-nias in the same trial .•

HS-families•

S1 -lines•

CHECK Co

0 0 -® Selfing of selected S1 -plants

'® 0 0 0

in selected S1 -lines .

•0 -® 0

-0 0 0-0 '0 -0 -0

Selling of selected S2-plants

• -0 0 -0-0 0 0 0•

0 -0 0

0 -0 -0 0

Evaluation of S3-lines.

0 0 00

0

1986

Crosses between S3-lines.

000000000

Evaluations of reconstructed00000000

hybrids000000000

0 Reconstructed hybrids00000000000090000 • CHECK F68xNE200000 •00000000 •00

259

1. Recombination andselection(5 years) 1977-81

2. Inbreeding and 1982selection (4 years)

1983

1984(Wintw)

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260

(Russell, 1972) . According to Dudley (1984) ityielded 95g per plant in Urbana, Illinois, USA,when grown at a density of 5 plants per m2 (More-no-Gonzalez & Dudley, 1981) . B84 was derivedfrom BSSS(HT)C7 (Zuber, 1982) and Va22 fromthe synthetic population [(WF9*T8)*T8]*[(CI .21*C103)*C103] (Josephson et al ., 1976) . The plotsconsisted of single rows of 28 plants each . Theintrarow distance was 1 .25 m and the interrow dis-tance 1.08m .

Two other experiments aimed at determining thecombining ability of the S1- and S2-lines . The firstof these involved testcrosses with related inbredlines, i .e. families obtained by crossing some of theS1- or S2-lines with maternal line F68 or paternalline NE2. Thirteen of such testcrosses were eval-uated together with the single cross hybrid F68*NE2 as the check . An R-49 honeycomb design wasused because there were not enough seeds avail-

Table 1 . Grain yield (in g per plant) and inbreeding depression (in %) for 20 pairs of HS-families and Sl-lines . Each pair of HS-familyplus Sl-line originates from a selected SO-plant . The ranking is according to the inbreeding depression . Data from 1983

* 1A : the hybrid, serving as check .** 3A + (7A + . . . + 7G) : its offspring from open pollination, i .e . CO .

able for yield trials in row plots . From the 49 codesused in the honeycomb design three were randomlyassigned to each entry and the remaining ten codes(i .e . 7A, . . ., 7G, 3A, 4F, 5A) to the check . Theinterplant distance was 1 .5m. (Because testcross6D(S2)*NE2 was not included in this experiment inApril 1987 six testcrosses involving the parentalline NE2 (including 6D(S2)*NE2) and the singlecross hybrid F68*NE2 for check were sown in anR-7 honeycomb design) . To test the hypothesisthat pairs of entries grown in the R-49 and the R-7honeycomb design were equivalent for mean yieldthe t-test for independent samples from popula-tions with different standard deviations was ap-plied with Cochran's approximation (Snedecor &Cochran, 1967) .

The other experiment concerned testcrosseswith unrelated inbred lines . Thus all nine S2-lineswere crossed with two or more of the twelve un-

Code number (number of plants) Grain yield Inbreeding depression

HS-family S1-line HS-family Sl-line

2B (44) 6B (44) 1025 1182 - 152D (48) 6D (48) 572 650 -141C (46) 3C (48) 996 792 204F (48) 5F (44) 875 650 264C (49) 5C (44) 1075 738 311E (43) 3E (42) 927 625 332E (49) 6E (44) 1253 790 371F (49) 3F (49) 1046 603 422G (49) 6G (47) 902 514 431G (48) 3G (44) 869 473 462F (45) 6F (43) 760 397 484D (45) 5D (41) 873 441 491D (45) 3D (47) 836 398 524G (47) 5G (47) 930 445 522C (47) 6C (44) 949 445 534E (45) 5E (39) 957 442 544B (48) 513 (45) 976 404 591A (47)* (369)** 1226 463 624A (42) 5A (47) 959 335 65113 (52) 3B (41) 747 251 662A (46) 6A (43) 989 330 67Mean (excl . check) : 925 .8 545 .3 41 .0

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related inbred lines B73, B84, Va22, Va92, H60,H98, Mo17, ZPT72, 37431, FR177, FRVa26 andFRB73 . Altogether 52 testcrosses were obtained .These were divided in 4 groups and each group wasevaluated in a different randomized completeblock design with four replications . Two groupscontained 15 testcrosses, the other two 11 testcross-es. The commercial single cross hybrids Pioneer3183, Peruviano and Nickerson 702 were added toeach group . The experimental plot consisted of two5 m long rows spaced 80 cm apart . All plots con-tained 50 plants, i .e. 25 plants per row . Thus, thedensity was 6 .25 plants per m 2 . Analyses of varia-nce were used to test the hypothesis that testcrossand hybrid grain yield were identical .

The evaluation of the reconstructed hybrids

In April 1987 the six single cross hybrids betweenthe four newly developed S3-lines were evaluatedin comparison with the original hybrid. The eval-uation was done by means of an R-7 honeycombdesign. The reconstructed hybrids obtained codenumbers 1, . . ., 6 and the original hybrid codenumber 7. The evaluation occurred at locations A(= Farm School of Thessaloniki) and B (= Val-tos) . Site A is a well-drained sandy-clay soil, site Baffords heavy clay soil . The interplant distance was1.5 m .

Across all seven hybrids as well as across the six

* Entries with the same letter were equivalent .

reconstructed hybrids simple correlation coeffi-cients between the two sites were estimated foryield .

Results

The performance of the S1 -lines

For yield per plant the inbreeding depression ofeach S1-line in comparison to the correspondingHS-family ranged from 20 to 67% . Two Sl-linesshowed inbreeding superiority (Fasoulas, 1981 ;p. 29) of 15 and 14% respectively (Table 1) . Themean inbreeding depression across the 20 Sl-linesamounted to 41% . The inbreeding depression ofCO in comparison to the single cross hybrid was62% . Apparently CO contained more plants with ahomozygous genotype for a deleterious gene onone or more loci (Sprague, 1983) than the Sl-linesobtained from C4 plants . Thus honeycomb selec-tion for four generations has been quite successfulin improving the performance after selling. The sixhighest yielding Sl-lines were ranked 1, 2, 3, 4, 5and 7 for inbreeding depression . The inbreedingdepression of the Sl-lines corresponding with thesix highest yielding HS-families were ranked 1, 3, 5,7, 8 and 20. Thus the progenies with an inbreedingdepression ranked 1, 2, 3, 4, 5, or 7 made a goodoverall impression, both as HS-family and as S1-line .

261

Table 2 . The grain yield (in g per plant) of inbred lines B73, B84 and Va22, serving as checks, and of eight S3-lines derived from Sl-linescoded 6B, 6D, 5C and 6E in Table 1 . The number of plants and the statistical equivalence are also indicated . Data from 1985

S3-line (Sl-parent) or check Yield Number of plants Equivalence of entries*

6D(6D) 1257 .7 15 a2B(6B) 1018 .8 20 a6B(6B) 480 .4 14 b5C(5C) 447 .2 16 bB73 399 .7 28 b3B(6B) 367 .1 19 b5B(6B) 336 .4 22 bB84 326 .2 21 b1B(6B) 318 .1 26 b6E(6E) 312 .7 15 bVa22 127 .3 20 c

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The yield and combining ability evaluation

The yield evaluation of eight S3-lines and the in-bred lines B73, B84 and Va22 (Table 2) did notshow many significant differences . It was remark-able that S3-lines 6D and 2B yielded about threetimes as much as check B73 .

These results directed our attention to lines 6Dand 2B. Their yields correspond to 9294 kg/ha and7529 kg/ha respectively, whereas B73 arrives at2954 kg/ha .

Table 3 presents the 1985 yield of families ob-tained by crossing Sl- or S2-lines with the relatedtester lines F68 or NE2 . It also shows their yieldrelative to that of the original hybrid F68*NE2 .These range from 36 .1 to 106.1% . Only the test-crosses 6E*NE2 and 5C*NE2 yielded similar to theoriginal hybrid, while the remaining eleven test-crosses were significantly inferior . It is noteworthythat testcross 2B*NE2 yielded lowest . Table 4 pre-sents the 1987 yield data of families obtained bycrossing S2-lines with the related inbred line NE2 .The yield relative to that of the original hybridF68*NE2 is also given . The relative yields rangedfrom 47 .7 to 86.9% . Equally to the 1985 experi-ment only the testcrosses of S2-lines 5C and 6Eyielded similar to the original hybrid . The other 4

* Entries with the same letter were equivalent .

.testcrosses were significantly inferior, with 6D*NE2 yielding lowest .

The good combining lines 6E and 5C have adifferent origin compared to the high yielding lines6D and 2B. This makes them potential partners forsuperior hybrids .

To concentrate attention to the four S2-lines 5C,6E, 6D and 2B (Table 5) presents only their test-cross yields when using the unrelated tester linesB73, B84 and Va22. (In contrast to the 9 otherunrelated tester lines, these three tester lines wereshared by the 4 S2-lines .) The table also includesthe yields relative to those of the commercial singlecross hybrids Pioneer 3183 and Nickerson 702 inthe same randomized complete block experiment .Because their hybrids were more productive S2-lines 5C and 6E combined also here better thanS2-lines 6D and 2B (wich were selected for highyield per se) .

The performance of the reconstructed hybrids

The yield of the six reconstructed hybrids and thatof the original hybrid are shown in Table 6 . Themean yield across the two sites ranged from 687 .7to 1033 .4g per plant. The hybrids 6D*6E and 5C*

Table 3. The grain yield (in g per plant, as well as relative to the yield of the original single cross hybrid F68*NE2) of 13 testcrosses, i .e .progenies resulting from crosses between an S1- or S2-line and the related lines F68 or NE2 . The number of plants being observed andthe equivalence are also presented . Data from 1985

Testcross Yield (in %) Number of plants Equivalence of entries*

6E(S2)*NE2 1082 .4 (106) 57 aCheck(= F68*NE2) 1020 .5 (100) 116 a5C(S2)*NE2 1010 .1 (98) 50 a6A(S1)*NE2 890 .7 (87) 57 b3B(S2)*NE2 879 .8 (86) 52 b5B(S2)*NE2 852 .7 (84) 57 b3A(Sl)*NE2 796 .5 (78) 60 c6B(S1)*NE2 769 .6 (75) 51 c3B(S1)*F68 681 .9 (67) 46 d3B(S1)*NE2 673 .2 (66) 47 d5A(S1)*NE2 670 .7 (66) 48 d6B(S1)*F68 466 .9 (46) 43 e5C(Sl)*F68 422 .5 (41) 52 e2B(S2)*NE2 368 .1 (36) 52 f

Page 7: Hybrid reconstruction in maize

6E exceeded the original hybrid but the differencewas not significant. These hybrids were at leastequivalent in yield to the original hybrid at site A .For site B there were four reconstructed hybridswhich exceeded F68*NE2 .

For yield per plant the correlation coefficientbetween the two sites across all seven hybrids wasr = 0.86* ; across the six reconstructed hybrids itwas r= 0.93** .

Discussion

In the present programme hybrid reconstructionwas accomplished in three stages (Fig . 1) . The firststage lasted five years . It started with open pollina-tion of the original hybrid. In this stage honeycombselection was applied among open pollinatingplants. The selection aimed at increasing grainyield and prolificacy . The second stage lasted fouryears. In each generation one ear from each visual-ly selected plant was selfed to develop homozygos-ity and to eliminate deleterious recessive genes . Inthe third stage single cross hybrids were successful-ly produced .

Population CO, obtained by intermating withinthe single cross hybrid, is a population in linkageequilibrium in as far as the loci segregate independ-ently. The performance of CO, in 1983 amountingto 463 g per plant in a relatively large evaluation, isthus representative for the open pollinating varietythat can be related to the single cross hybrid beingstudied. Simultaneously CO can be considered as anS1-line directly obtained from the single cross . The

Table4. The grain yield (in g per plant, as well as relative to the yield of the check) of testcrosses between S2-lines and the related lineNE2. The number of observed plants and the equivalence of the entries are also indicated . Data from 1987

Testcross

Yield (in %)

* Entries with the same letter were equivalent .

20 HS-families grown in 1983 were obtained fromselected plants from a population developed bycontinued honeycomb mass selection starting withpopulation CO . Thus the difference between themean performance across the 20-HS-families andthat of CO, i .e. 925 .8 - 463 = 462.8 g, reflects pro-gress due to selection . Important progress appearsalso from the mean yield across the 20 Sl-linesobtained from the same 20 plants, i .e. 545 .3 g perplant, when compared to the yield of CO, i.e. the`Sl-line' from the original hybrid, which amountedto 463 g per plant .

The inbreeding depression of CO in comparisonto the single cross hybrid (62%) can also be consid-ered as the heterosis of the single cross hybrid incomparison to a related open pollinating popula-tion .

It has long been known (Jenkins, 1935 ; Sprague,1946) that the success of breeding programs uti-lizing inbreeding followed by hybridization of thedeveloped inbred lines depends on the testing pro-cedures involving lines per se as well as progeniesobtained by crossing these lines with inbred testerlines .

In absence of selection S3-lines are expected tobe heterozygous for only one eighth of the numberof heterozygous loci in the original SO-plants . Thusif the selection did not hamper the reduction ofheterozygosity, the heterozygosity remaining inthe S3-lines is relatively restricted. Therefore fur-ther selling in combination with selection will notnecessarily produce completely homozygous lineswith lower yields than those of the tested S3-lines .The present evaluation of the S3-lines per se

Number of plants

Equivalence of entries*

263

Check(= F68*NE2) 827 .8 (100) 28 a5C*NE2 741 .5 (87) 25 a6E*NE2 671 .0 (81) 25 a6A*NE2 510 .6 (62) 23 b5A*NE2 427 (52) 26 c3B*NE2 417 .8 (51) 28 c6D*NE2 395 (48) 23 c

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showed that 6D and 2B were .high yielding lines,and that the other six lines, including 5C and 6E,had a grain yield equivalent to that of the freelyavailable lines B73 and B84 (see Table 2) . TheseS3-lines were obtained after three successive gen-erations of selfing in combination with selection .

When determining the combining ability of theeight S3-lines the following was considered. Raw-lings & Thompson (1962) presented evidence that aline which is homozygous recessive at all loci is theideal tester. However, for loci controlling quantita-tive traits the general rule is that recessive allelesinduce in homozygous state lower levels of pheno-typic expression than corresponding dominant al-leles. Thus the ideal tester is expected to suffer toomuch from lack of vigour .

As a way-out of this problem two procedureshave been suggested :(i) to use the broad-base source population, i .e. theparental variety as tester (Hallauer, 1975) .(ii) to use unrelated elite lines as tester (Hallauer &Lopez-Perez (1979) found that such tester was aseffective as a related low-performance tester) .

In our experiments in stead of a broad-base pa-rental variety the parental inbred line NE2 wasused as a tester and alternatively the unrelated lines

Table 5. The grain yield (in g per plot) of testcrosses obtained by crossing S2-lines 5C, 6E, 6D and 2B with the freely available tester linesB73, B84 and Va22. The yield (in %) relative to that of Pioneer 3183 and Nickerson 702 in the same experiment is also given. Data from1985

B73, B84 and Va22 were used . Under Greek con-ditions lines B73 and B84 are elite lines .

Lines 5C and 6E showed good combining abilitywith both types of testers : the related tester line(Tables 3 and 4) and the three unrelated elite lines(Table 5). These results are in agreement withHaullauer & Lopez-Perez (1979) . In their study anunrelated elite line tester appeared to be the appro-priate tester for both S1- and S8-lines .

The high yielding capacity of lines 6D and 2B andthe good combining ability of lines 5C and 6E led tothe reconstruction of the original hybrid by meansof these four inbred lines . The similar performanceof the two reconstructed hybrids 6D*6E or 5C*6Eand the original single cross hybrid F68*NE2 in-dicates that selection for yield in the CO and latergenerations and for combining ability when devel-oping lines resulted in the recovery of some lineswhose hybrids yielded as good as the original hy-brid .

The advantage of the new derived inbred lines isthat they have either the same grain yield as theinbred lines B73 or B84, viz . lines 5C and 6E, orthat they have a grain yield two to three times thatof B73, viz . line 6D and 2B (see Table 2) . This isimportant because it shows how the problem of the

Testcross Yield Yield relative to Pioneer 3183 Yield relative to Nickerson 702

5C*B73 16079 117 1575C*B84 14060 105 1535C*Va22 13128 98 143

6E*B73 13080 97 1196E*B84 15935 116 1566E*Va22 13635 101 124

6D*B73 11781 88 1286D*B84 10243 76 1126D*Va22 9409 70 102

2B*B73 12706 95 1132B*B84 12126 95 1132B*Va22 8878 66 97

Pioneer 3183 100Nickerson 702 100

Page 9: Hybrid reconstruction in maize

low seed yield of F68, which hampers cheap pro-duction of seed of the original hybrid F68*NE2,can be overcome .The new derived lines allow reduced prices for

hybrid seed, which is advantageous for maize grow-ers because the hybrid performance is not affected .Farmers will certainly respond to this because inthe U.S.A. the cost of good seed was about $ 10 .00per bushel in 1957 but in 1982 it amounted about$ 60 .00 per bushel (Stone, 1983) .

The correlation coefficient for grain yield be-tween the two sites across all seven hybrids was0.82* ; across the six reconstructed hybrids it waseven 0 .93** . These high correlations indicate thathybrid*site interaction was of minor importance .

Heterosis is defined as the superiority of themean phenotypic value across the plants belongingto a hybrid in comparison to the mean phenotypicvalue across the two parental lines (Rieger et al .,1976). Then, because of the high grain yield of thenew derived lines the midparental value is in-creased and the heterosis is reduced . It is, however,the hybrid's yield itself which is decisive for itsattractiveness and not the amount of heterosis .

S5-lines were obtained from the four S3-linesthat were used for making the six reconstructedhybrids. Together with the two parental lines thesewere assayed for their allozyme pattern for nineenzyms. After that the diversity index, i .e . thenumber of non-identical patterns of allozyms, wasdetermined for each possible pair of inbred lines .

* Entries with the same letter were equivalent .

265

This diversity index measures the . genetic distancesbetween the lines . The diversity index for the linesconstituting each of the six reconstructed hybrids aswell as the original single cross hybrid was correlat-ed with the grain yield of the corresponding hybrid(Hunter & Kannenberg, 1971) . The estimation forthe correlation coefficient was positive (r = 0 .26),but not significantly different from zero . Frei et al .(1986) found that allozyme dissimilarity of parentallines was only associated with hybrid yield whenthe lines had a common genetic background .

Hybrid reconstruction seems a valuable new ap-proach in maize improvement . The quantitativegenetic justification of hybrid reconstruction isbased on the fact that the predominant type of geneaction concerning yield and other quantitativelyinherited traits in maize is additive with partial tocomplete dominance of favourable alleles (Hal-lauer & Miranda, 1981, p. 129). Since dominantgenes hide deleterious recessive genes in hetero-zygous genotypes, selection against such recessivegenes is indispensable for improving the inbred lineperformance . Genter (1982) showed how success-ful such selection can be .

Acknowledgements

The authors thank Mr . L.C.J.M. Suurs, Depart-ment of Plant Breeding, Agricultural University,

Table 6 . Grain yield (in g per plant) of reconstructed single cross hybrids and the original hybrid F68*NE2 at each two sites, as well asacross these two sites . Data from 1987

Hybrid Site A Site B Mean yieldacross sites

Number ofplants

Yield Equivalence Number ofplants

Yield Equivalence

6D*6E 31 1119 .4 a 40 947 .4 a 1033 .45C*6E 21 997 .4 ab 41 950 .6 a 974 .0F68*NE2 34 1058 .5 ab 42 871 .5 ab 965 .02B*5C 32 900 .3 bcd 39 894 .5 ab 897 .46D*5C 26 866 .8 bcd 41 897 .9 ab 869 .22B*6D 29 766 .8 cd 37 763 .5 be 765 .22B*6E 34 643 .2 d 41 732.2 c 687 .7

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Wageningen, The Netherlands, for the allozymeanalyses of the inbred lines .

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