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VI. Alteration of Populations through Selection and Hybridization

VI. Alteration of Populations through Selection and Hybridization

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RED CLOVER BREEDING AND GENETICS



139



Selected plants may be intercrossed among themselves to develop a new

population for testing and/or selection. Continued repetition of this process is

referred to as phenotypic recurrent selection.

Progeny tests may be conducted by using open-pollinated seed from the

selected individuals in the original source nursery. Further selection can then be

applied on a maternal line basis (best maternal line continued into next generation

or cycle, or incorporated into a synthetic) or on a single-plant basis within a

family.

A third scheme is to bulk open-pollinated seed from selected plants in the

source nursery and repeat the selection process in successive generations.

Fergus and Hollowell (1960) reviewed the use of these breeding schemes in

red clover prior to the late 1950s. Plants resulting from three cycles of phenotypic

recurrent selection were superior for persistence, growth habit, and vigor (Malm

and Hittle, 1963). Each cycle consisted of three growing seasons, and selection

against root-rotting organisms was effective. The phenotypic recurrent selection

scheme also has been effective in breeding for resistance to northern anthracnose

(Maxwell and Smith, 1971; Smith and Maxwell, 1973; Smith et al., 1973),

powdery mildew (Owen, 1977), clover rot (Ludin and Jonsson, 1974), yellow

clover aphid [Theroaphistrifolii (Monell)], and pea aphid [Acyrthosiphurnpisum

(Harris)] (Gorz er al., 1979), and for reduced formononetin content (Francis and

Quinlivan, 1974).

Mokhtarzadek et al. (1967) reported that maternal line selection and

phenotypic selection were effective in improving persistence in red clover. Improved persistence was accompanied by decreased forage yield and flowering in

first-year stands but increased forage yield and delayed flowering in the second

harvest year. Selection for resistance to the stem nematode was effective when

maternal line selection was used (Bingefors, 1956; Dijkstra, 1956). Dijkstra

(1969) used this same method to increase the percentage of two-seeded pods in

diploid red clover. It was not effective for tetraploid germplasm. However,

Anderson (1973) effectively used maternal line selection to develop a lateflowering tetraploid red clover, GRASSLANDS PAWERA.

Mass selection has been employed effectively to increase height of nectar in

the corolla (Hawkins, 1971), dry matter yield (Novoselova and Matskiv, 1974;

Julen, 1971), and protein content (Novoselova and Matskiv, 1974). Mass selection was effective in developing germplasm adapted to the Altai and Kalinin

Regions of the USSR (Burdina, 1971; Sorokin, 1972). Mass selection followed

by progeny testing was used to identify the superior clones of the cultivars

NORLAC (Folkins et al., 1976) and KENSTAR (Taylor and Anderson, 1973a).

Combinations of mass and individual selection with and without progeny

testing have improved dry matter yield, protein percentage, digestibility, disease

and insect resistance, and seed yield in red clover (Julen and Lager, 1966;

Schieblich, 1966; Novoselova and Piskovatskaya, 1972; Pamfil et al., 1972;



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N . L. TAYLOR AND R . R. 'SMITH



Cemy and Vasak, 1973; Rogash et al., 1973; Litvinenko, 1974; Vestad, 1974;

Julen, 1974, 1975; Novoselova and Cheprasova, 1975; Goral, 1976).

C. PROGENY TESTING, COMBINING ABILITY, AND

HERITABILITY



Knowledge of the combining ability of clones or lines becomes important

when the breeder considers the basic material to be used in developing a hybrid

or synthetic cultivar. Testing of progenies to evaluate breeding value of parents

has not been conducted extensively in red clover because of the difficulty of

maintaining parents, Red clover propagules are short-lived and susceptible to

virus diseases. Taylor et al. (1962) found that clones tolerant of viruses and other

diseases could be maintained up to 5 years. Of 1500 third-year plants dug from

fields of KENLAND red clover, all except 20 were eventually eliminated

through clonal evaluation for response to viruses and other diseases (Taylor et

al., 1968).

Progeny testing by use of selfs has not been conducted extensively, because

most plants are self-incompatible. Limited amounts of seed may be produced via

PSC after heat treatment (Kendall and Taylor, 1969; Kendall, 1973). Taylor et

al. (1970) found that one clone of four tested was nonpersistent both as an I, and

an I, clone. Other I, clones showed segregational effects, however.

Top crosses and open-pollinated crosses, although useful in maternal line

selection, have not been used extensively for progeny testing. Top-cross progenies were evaluated by Dijkstra (1970) to assess the potential of using reciprocal

recurrent selection for the improvement of red clover. He concluded that the

performance of the progenies did not warrant a program of reciprocal recurrent

selection. Dry matter yield, growth habit, and corolla tube length as measured on

open-pollinated progeny of nine clonal genotypes had high heritability values

(Lawson, 1971). The cultivar KENSTAR was developed on the basis of the

polycross progeny test to evaluate parental clones (Taylor and Anderson, 1973a).

The polycross nursery, consisting of 10 plants of 20 clones in 20 replications of a

randomized block design, was maintained for 3 years, during which seed was

harvested for subsequent polycross progeny testing. Subsequent progeny tests

showed significant parent-progeny correlations for persistence, vigor or yield,

blooming or seed yield, and mildew resistance. Ten clones were selected for the

synthetic cultivar KENSTAR (Taylor et al., 1968). Competitive effects relative

to the polycross progeny test have been evaluated by Taylor and Kendall(l965).

They reported that seed of outstanding progenies do not always produce similar

performance in polycross tests as when composited in synthetics. The polycross

progeny test also has been used successfully to evaluate sib lines used to maintain

parental genotypes (Torrie et al., 1952).



RED CLOVER BREEDING AND GENETICS



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Diallel crosses of parental clones probably have been used more extensively in

red clover than has any other type of progeny testing. Anderson ( 1 960) made

diallel crosses of seven noninbred, selected parents and tested the progeny in

field spacings of a 7 x 7 balanced lattice design with six replications. He

observed significant general combining ability (GCA) and specific combining

ability (SCA) between the seven parents for yield, growth habit, persistence, and

flowering. GCA variances were greater than SCA variances, and both GCA and

SCA interacted with the season. Relatively high heritabilities were obtained for

yield, habit of growth, persistence, and flowering time. Anderson et ul. (1974b)

progeny-tested 10 noninbred clones of red clover by diallel crosses. Seeds of the

45 single crosses were produced by hand-crossing in a greenhouse. GCA was the

most important source of genetic variation in this study, and additive genetic

variance constituted over 8 1% of genetic variance. Heritability estimates for

persistence, yield, vigor, and date of first bloom ranged from 17 to 42%. In

another study, additive genetic variance was a significant portion of the genetic

variance among diallel progeny from plants selected from wild and cultivated

populations (Ceccarelli, 1971). Genetic variance was greater among populations

than within, and there were reciprocal differences for forage yield. Significant

GCA effects were observed among male-sterile types of red clover from the

variety MOSCOW 1 (Zvyagina, 1973).

Smith and Puskulcu (1976) evaluated the combining ability of six I,sib, lines

of red clover by testing diallel progeny in both space-planted and broadcastplanted experiments. Both GCA and SCA were significant for dry matter yield,

maturity, visual yield, and regrowth after harvest. Specific effects were large for

some combinations, suggesting that nonadditive genetic variance was of importance in this material. However, the SCA x location interaction was significant.

Little association was observed between space-planted and broadcast-planted

material. Reciprocal effects were of little importance.

Evaluating diallel progeny of 10 I, red clover clones, Cornelius et ul. (1977)

also stressed the importance of nonadditive genetic variance. Reciprocal effects

were important in this case. Heritabilities were estimated on individual plants and

on hybrid means, and performances of all possible double crosses were predicted. They concluded that progeny testing of I, clones was not as useful as

testing of I, clones because of segregational effects during the inbreeding process.

Julen (1974) reported high heritability values for crude protein content of red

clover progeny from a diallel among high and low parents. Selection should be

effective in improving protein content, but this could lead to decreased digestibility, particularly of the leaves. High heritability values were observed for qualitative characters and low values for quantitative characters in tetraploid red clover

(Jaranowski and Broda, 1977).



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N . L. TAYLOR AND R. R. SMITH



D. BACKCROSS BREEDING



Backcross breeding has not been utilized much in red clover, perhaps because

of the lack of suitable recurrent parents and the possibility of selection for

desirable characters during breeding by other procedures. The only program to

the authors' knowledge is the transfer of mildew resistance to the cultivar

KENSTAR (Taylor and Anderson, 1974). LAKELAND as the donor parent was

crossed with each of 10 clones of Kenstar in 10 separate cages. Seed was

harvested only from the clonal parent in each cage, thus ensuring that cross seed

was produced. The F, and five subsequent backcross generation plants were

screened as seedlings for mildew resistance in a greenhouse according to the

method of Hanson (1966). Since mildew resistance is inherited as a dominant

gene (Stavely and Hanson, 1967), it was necessary to sib-mate the fifth

backcross generation, reselect for mildew resistance, and then to progeny-test the

selected plants. Heterosis was exhibited after intermating the 10 improved clones

and reconstituting the synthetic. Subsequent cultivar evaluation indicated complete transferal of mildew resistance and restoration of forage yields to the level

of the original synthetic. Forage yields of advanced synthetic generations were

not determined (Taylor and Anderson, 1974).



E. INBREEDING AND HETEROSIS



Perhaps the most important advance in the inbreeding of red clover is the

increase in self-seed set by high-temperature treatments. Pseudo-selfcompatibility (PSC) increases when red clover clones are grown at 32-38°C

(Leffel, 1963; Leffel and Muntjan, 1970). In an excised stem technique, self-seed

production ranged from 2 to 20% for 10 clones maintained at 40°C (Kendall and

Taylor, 1969; Kendall, 1973). The technique was as follows: Stems were cut at a

length of 14 cm when petal color first became visible in the flower bud, or when

most florets were suitable for pollination. Usually, about 5 to 10 stems were

placed in glass bottles containing an aqueous solution of 2.5% sucrose and 22

ppm of boric acid. During the period of anthesis, the culture bottles were partially submerged in a water bath at 25"C, which was held in an incubator at 40°C.

After anthesis, the flower heads were selfed by hand manipulation (rolling) or by

tripping the individual flowers with a dissecting needle. During seed development the media and plants were held at 20°C. Another satisfactory technique is to

leave the flower heads intact and insert young heads in an incubator maintained at

40°C. Holes suitable for insertion of heads are drilled in the sides of the incubator.

Although PSC is increased by high temperature, it apparently is an inherited

characteristic and is trqsmitted from clones to progenies (Brandon and Leffel,



RED CLOVER BREEDING AND GENETICS



143



1968). PSC also decreases with inbreeding, so that it is difficult to maintain inbred

lines beyond two generations of selfing (Duncan et al., 1973). It is now apparent

that PSC under high temperatures can supply sufficient self-seed for most breeding purposes.

Inbreeding of red clover leads to a decrease in vigor similar to that in other

cross-pollinated diploid plants (Fergus and Hollowell, 1960). Considerable variation exists, however, and some inbred families contain plants that equal or

exceed their noninbred parents (Taylor et al., 1970). In general, inbred lines are

difficult to maintain because of lack of vigor and virus susceptibility.

Red clover also may be inbred by sib-mating for identification and elimination

of aberrant types. Among genotypes isolated in this manner are chlorophyll

deficiencies, annual growth habit, aneuploid lines with two supernumerary

chromosomes (2n = 16), and spontaneous polyploids (2n = 21, 35, and 42)

(Strzyzewska, 1974).

Inbreeding also has been conducted with tetraploid red clover, as discussed in

Section V,C. Self-fertility of tetraploid cultivars ranges up to 13% and declines

in subsequent generations. Inbreeding depression may be up to 30-40% of the

fresh forage weight and 23-2696 for seed yield, but, as in diploids, it varies

among inbred lines (Laczynska-Hulewicz, 1963).

The primary reason for inbreeding red clover is to obtain inbred lines, which,

when hybridized, will give a maximum expression of heterosis. Heterosis also

may be obtained by crossing noninbred cultivars. Manner (1963, 1966) reported

that bulk crosses gave higher green matter yields than both the mother and the

parental means. Control of pollination in this case is not complete, since a part of

the seed is derived from intracultivar crosses. Novoselova and Malasenko (1967)

and Bekuzarova and Mamsurov (1974) reported that hybrids obtained from crosses of geographically distant forms resulted in heterosis expressed by higher seed

and forage yields, leaf size, and winter-hardiness. Shcheglov and Zvyagina

(1975) also reported heterosis in six of nine F, hybrids of late-maturing cultivars

and cytoplasmic male-sterile genotypes. Generally, crosses of inbred parents will

result in heterosis, and the vigor of the F, will on the average equal that of the

noninbred parent. Some combinations may be significantly superior to the

noninbred parents (Taylor et al., 1970; Krstic, 1972). Utilization of heterosis,

then, involves a search for superior combining ability (discussed in previous

section).

A secondary reason to inbreed red clover is to obtain homozygous S-allele

genotypes necessary for control of crossing (see Section VII,B,2).

F. TISSUE CULTURE



The use of tissue culture for breeding and improvement of red clover has been

limited. However, recent research on growth media and redifferentiation



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N. L. TAYLOR AND R . R . SMITH



suggests that procedures are now available for variant selection, meristem culture, and embryo culture (Bingham et a l . , 1977; Beach and Smith, 1978; Phillips

and Collins, 1977, 1978; Ranga Rao, 1976). Zakrzewski and Zakrzewska (1976)

reported the use of red clover callus tissue as a medium for propagating the stem

nematode. Freeing breeding clones of viruses, and long-term cold storage of

meristematic tissue of valuable genotypes, appear to be two of the most immediate practical uses of tissue culture.

VII. Use of Selected Materials



The ultimate goal of any plant breeding program is to develop and make

available to the grower germplasm that is superior in performance to that being

used at the current time.



A. GERMPLASM RELEASES



Often material is developed by the breeder that possesses unique characteristics, such as insect or disease resistance, or persistence, but that has not been

developed into a cultivar. Breeders may officially release and provide limited

amounts of seed of these populations to other breeders for further selection and

use. Formal release procedures are similar to those for a cultivar, except that

extensive testing and maintenance of a continual supply of initial seed are not

required. A specific example is the release of the hybrid between Trifolium

pratense and T . difisum Ehrh. (Taylor and Anderson, 1973b).

Germplasm of other seed lots is interchanged among breeders on an informal

basis. It is commonly understood that any user or developer of cultivars from either

formally or informally shared germplasm will recognize the organization and

breeder responsible for developing the initial germplasm.

B . CULTIVAR DEVELOPMENT



Older cultivars were developed by direct use of specific ecotypes or selection

among open-pollinated progenies of parent material that had been subject to

natural selection. In recent years the cultivars are the result of breeding programs

that apply conscious selection for the choice of parental material. Cultivars

originating from these programs are synthesized from the selected parent material. In recent years emphasis has been placed on the development of hybrids in

red clover.



RED CLOVER BREEDING AND GENETICS



145



I . Synthetic Cultivars

The recombination of selected clones or lines into synthetic cultivars is an

effective method of utilizing stocks with superior characteristics. The initial

clones or lines (Syn 0) are used to produce the F, (Syn 1). Because of limited

seed quantities in Syn 1, the material may be advanced by random mating to the

Syn 3 or Syn 4 generation before it is available to the farmer. Factors such as

yeild or performance of parent clones or lines, number of parent clones or lines

combined, yield of F1 crosses, and extent of natural cross-pollination may affect

performance of the synthetic cultivar. The maximum performance should be

realized in the Syn 1 generation, with a decline in subsequent generations depending upon the above factors.

In the absence of natural selection, the yield of a synthetic cultivar should not

decline beyond the Syn 2 generation. However, if natural selection is important,

the cultivar may be maintained in a limited generation program. ARLINGTON

(Smith et a l . , 1973), KENSTAR (Taylor and Anderson, 1973b), and NORLAC

(Folkins et al., 1976) are recent examples of cultivars that have been developed

as synthetics. ARLINGTON is the result of intercrossing six lines, and

KENSTAR and NORLAC are advanced generations of intercrossing 10 and 11

initial clones, respectively.

2 . Hybrids

Combining highly selected materials into single- or double-cross hybrids

maximizes genetic gain, since both additive and dominance genetic variance are

utilized. Genetic male sterility is one method of controlling crossing in red

clover, and several genetic male steriles have been isolated (Smith, 1971;

Macewicz, 1976a, b; Taylor et a l . , 1978). Smith concluded that the male-sterile

gene could be employed with the self-fertility gene (Sf) in a hybrid breeding

program. It is doubtful, however, that genetic male sterility could compete with

other systems for controlling crossing in red clover.

In common with other crops, cytoplasmic male sterility (CMS) may be more

useful for hybridization of red clover than genetic male sterility. Shcheglov and

Zvyagina (1975) and Zvyagina (1973) have reported CMS induced by colchicine. In crosses with late-maturing cultivars, most of the forms used as male

parents were total or partial fertility restorers.

Considerable research has been conducted on the possibility of using the

S-allele system for the control of pollination for single- and/or double-cross

hybrid red clover. One method for control of crossing and theoretical expectations of S-alleles in I, single and double crosses are shown in Fig. 1 (Anderson et

al., 1972). I,, clones are inbred one generation by PSC, producing homozygous



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VI. Alteration of Populations through Selection and Hybridization

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