Hybrid Seed Production by Cms

8/6/2019 Hybrid Seed Production by Cms

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Hybrid seed production: an applied usage ofmaternal inheritance

Plant breeders have recognized for many years that the progeny from a specificcross can out yield either of the two parents used in the cross. This is called

heterosis, the phenomenon were the phenotypic value of the heterozygote is greaterthan either of the two parents. For plant breeding, the observation of heterosis foryield has lead to the development of inbred lines that exhibit a heterotic yieldadvantage.

Corn was the first crop species in which heterosis was exploited. The originalapproach to develop hybrid corn seed required the manual detasseling of the femaleparent to prevent self-pollination. The field would be planted with the two lines andhigh school students would walk the field and detassel the female parent. As you canimagine this was a labor intensive proposition.

Because it was realized that manual detasseling of corn plants would not be required

if male sterile system could be developed, attemp ts were made to apply the twotypes of male sterility systems, genic and cyotplasmic male sterility. Genic malesterility is controlled by nuclear genes. The weakness of each of the geneticsystems, though, was that a portion of the F2 were male fertile, a nd thus a portion ofthe seed that was developed was not hybrid.

The ultimate solution to this problem was the use of cytoplasmic male sterility ( cms).As the name suggests, this type ofcytoplasmic male sterility is controlled by acytoplasmic factor and is maternally inherited. (Current molecular researchsuggests, but has not conclusively proven, that the sterility is a mitochondrialencoded function.) Thus all the males that contain the appropriate cytoplasm would

be sterile.

But this is only part of the solution for hybrid seed production. The seed companysells hybrid seed to the farmer, and the farmer expects this seed to be fertile. If thehybrid seed sold to the farmer was sterile, the seed company would have to providea pollinator source to be p lanted along with with the hybrid seed to obtain the seed.The need for this step though is alleviated by the use of restorer of fertility (Rf)genes. These dominant nuclear genes can override the cytoplasmic male sterilityfactors. Thus plants that have the cms cytoplasm contain a dominant Rfallelewill be male fertile.

Taking the above discussion into consideration, the following is a procedure to

produce hybrid corn seed without manual detasseling. A line that contains thatcontains a male sterile cytoplasm and is recessive for the restorer of fertility alleles(rfrf) is the female parent in a cross with a male that has male sterile cytoplasm andis heterozygous for the restorer of fertility alleles ( Rfrf). The F1 progeny from thiscross will exhibit the heterotic effects for yield. Furthermore, all the plants will becytoplasmically male sterile because they contain the cms cytoplasm.

The plants will also segregate 1 Rfrf :1 rfrf. Those that are recessive for the restorerfactor will still produce seed because the other heterozygous Rfrfplants will produce


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ample pollen to pollinate those plant Thus, the farmer will realize the advantages ofhybrid seed, and the seed company will not have to use resoruces for manualdetasseling. The following diagram de monstrates the process.

Although this system is functional, some difficulties have arisen with it practicalapplication. In corn breeding, the cytoplasm that was used initially to provide themale sterility was the Texas or T cytoplasm. Nearly all of th e hybrid corn grown until1970 contained this cytoplasm. During that year a fungal disease (Southern corn leafblight) appeared that preferentially attacked plants with the T cytoplasm.

This is an example of genetic vulnerability. This term refers to the genetic conditionwere all of the individuals in a region have the same genotype that makes them allvulnerable to a single disease or pathogen that could destroy the whole population.Thus, all of the hybrid corn was at great risk the following year.

Fortunately, USDA scientists recognized the upcoming problem, and seed stocks

were developed that contained other cytoplasms. These seed stocks had to bemanually detasseled, and its hybrid performance was not as good as the previousmaterial, but it did provide a source of seed that was resistant to the disease andsaved the United States from losing the entire hybrid corn harvest that year. Sincethat time though sources of the T cytoplasm have been developed that are resistantto the disease, and hybrid seed production utilizing cytoplasmic male sterility hasresumed.

Copyright 1997. Phillip McClean


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The use of cytoplasmic male-sterility in maize seed production --Has, V, Has, I, Grecu, C

The use of cytoplasmic male sterility (cms) in maize hybrid seed production is of

economic importance and is also advantageous for genetic purity of seeds. Threetypes of male sterile cytoplasms in maize are used as cms maternal parents to

produce hybrids: cms-C, cms-S, cms-T. The concern that in a few years all maize

might again be in C or S cytoplasms gave rise to ideas as to how to prevent thisnarrowing of the cytoplasmic gene base. Thus, a new technique of producing

hybrids was proposed, using multiplasm, respectively a blend of several kinds ofmale sterile cytoplasms. The aim of this investigation was: 1) to detect the presence

of dominant Rf genes in more than 600 inbred lines by crossing with different types

of cms: C, ES, M, T; 2) to compare some registered "TURDA" hybrids developed withnormal and cms and/or Rf parental forms, in different environmental conditions, for

three agronomic traits. Restoration reactions of 600 inbreds lines on the cms: C, ES,

M and T were scored using Josephsons scale (Josephson et al., 1978). The

observations were performed at the Agricultural Research Station -Turda, between

1995-2001. Nine registered "Turda" hybrids carrying both fertile and sterile

cytoplasms were grown in two years at five locations.

When using cms in maize breeding programs it is as necess ary as it is difficult toidentify the inbred lines by their composition of Rf genes. Identifying restorers of

cms-C and cms-ES becomes much more complicated, due both to the involvement ofat least two-three complementary Rf4, Rf5, Rf6 genes, and to certain modifyingfactors, probably quantitative ones which, in some specific environmental conditions,

act in the absence of the Rf gene, influencing the reactions of lines by the "late -break" phenomenon. The percentage of non -restorer genotypes was 40% both to

cms-C and to cms-ES (Table 1).

Table 1. The distribution of inbred lines according to their reaction in crosses to fourcms types.

% inbred lines

cms

types

No.

cms

tester

lines

No.

studied

lines

Nonrestorers

Restorers Differentreactions

partially fully

cms-C

5 198 34 4 55 7

cms-ES

3 94 35 5 51 9


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cms-M

2 121 20 33 20 27

cms-

T

7 223 74 1 16 9

Total-studied lines 636

121 inbred lines have been tested with cms -M, only 20% of them being identified as

Rf3/Rf3. The inbred lines which partially restore fertility or have a variable reactionaccording to the environmental conditions represent 27% of the inbred lines tested

with cms-M. Because cms-T is only used in areas less favorable to the diseasecaused by Helminthosporium maydis T-race, research on the use of this cms type islimited. Table 2 presents the synthetic results of the comparison between the

cytoplasmic (N or cms) effects on certain agronomic traits of registered hybridsdeveloped at the Agricultural Research Station, Turda, Romania. Trial conditions

(years, locations) have emphasized a series of significant differences between thetwo cytoplasms as far as grain yield is concerned. These differences are greatlydetermined by nuclear-cytoplasmic interaction or by hybrid x local conditions

interaction.

Table 2. Cytoplasmic male sterility effect for some traits in 9 registered "TURDA"

hybrids.

Hybrid Cytoplasms Grain

yield

q/ha

Dry

matter

ofgrain

%

Erect

plants

atharvest

%

Synthetic

relative

index

%

1 2 3 4 5 6

Turda-SU182

N 98.6 76.6 85.4 100

cmsC 95.0 77.1 79.3 90

(%)cms/N 96 101 93 -

Turda-

Mold 188

N 101.1 77.7 79.0 100

cmsC 97.3 77.9 81.8 100

(%)cms/N 96 100 103 -

Turda N 85.1 75.3 81.6 100


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Super

cmsC 86.7 76.0 78.2 98

(%)cms/N 102 101 96 -

Saturn N 78.2 73.2 78.0 100

cmsC 92.1 75.1 83.2 129

(%)cms/N 118** 102** 107 -

Turda215

N 97.5 76.0 73.5 100

cmsT 86.2 74.4 70.0 82

(%)cms/N 880 9800 95 -

Turda-SU210

N 84.9 76.0 73.6 100

cmsC 89.2 77.0 75.8 110

(%)cms/N 105 101 103 -

TurdaFavorite

N 93.3 75.8 80.7 100

cmsC 104.5 74.7 74.3 102

(%)cms/N 112* 99 92 -

Turda

198

N 102.1 76.8 76.1 100

cmsES 101.2 76.5 75.5 98

(%)cms/N 99 100 99 -

Turda

160

N 90.0 77.9 78.7 100

cmsC 87.8 77.1 81.2 97

(%)cms/N 97 99 100 -

Trial N 92.3 76.1 78.5 100

mean cms 93.4 76.1 77.7 100

(%)cms/N 101 100 99 -


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*** Significant at 5% and 1%, respectively*Si%= {col.3x4x5(cms)/col.3x4x5(N)}. 100

The nine hybrids carrying cms did not differ generally from their counterparts withfertile cytoplasm (N) for yield and for two other traits.

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may

be cited only with consent of the authors.

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Hybrid Seed Production by Cms