Tải bản đầy đủ - 0 (trang)
VII. Use of Selected Materials

VII. Use of Selected Materials

Tải bản đầy đủ - 0trang



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




FIG. 1. Genotypic expectations of S-alleles in the production of doublecross hybrid red clover.

S-allele genotypes maintained vegetatively for use in crossing (Leffel, 1963;

Johnston et al., 1968). Results of control of crossing by the S-allele system

conform to theoretical expectations according to a gene marker analysis conducted by Anderson el al. (1972). One of the most difficult problems is maintenance of parental lines, which are low in vigor and easily infected with viruses.

Leffel (1963) suggested that seed maintenance of parental lines might be

feasible if mutation to the self-fertility allele (Sf)does not occur (Fig. 2). Leffel

and Muntjan (1970) elaborated the seed maintenance scheme as follows: I. plants

are vegetatively increased and are selfed at high temperature. Each I1 line is

maintained and increased by the composite sib method. All seed produced by this

sib-mating would be heterozygous-that is, S1S2.The I,sib, population would be

sib-mated with bees at high temperatures, perhaps in large greenhouses or in the

southwestern United States. The expected segregation of S-alleles in the I,sib2

generation would be the same as that in the I1 generation. The F, single-cross

S,Ss x S5S, would result from the cross of two lines in the I,sib, or subsequent

alternate generations. Each of the two lines would be self- and sib-incompatible

in the normal environment. A heritable degree of PSC of about 8-10% probably

would be necessary.

Because of the difficulties of isolating lines for seed maintenance with 8-10%

PSC, Duncan et al. (1973) proposed vegetatively increasing superior I, plants



with high PSC. These I, clones then would be isolated under high-temperature

field conditions, and self-seed would produce the 12. Selfed seed from two I1

lines (12’s)would be mixed and sown to produce single-cross seed. Single-cross

seed from different clonal sources would be mixed and sown for the production

of double-cross hybrid red clover.

Implicit in the seed maintenance scheme is the stability of S-alleles. Anderson

er al. (1974) observed a change to a new S-allele during inbreeding and cautioned

that continued seed maintenance by PSC and sib-mating in alternate generations

may cause an increase in frequency of new S-alleles; thus two isolated inbreds

would sib-mate as well as cross.

Evidence exists that hybridization of inbred lines without prior selection for

combining ability does not result in superior hybrids (Anderson et al., 1972;

Taylor and Anderson, 1974). A test of combining ability in 10 I, single crosses of

red clover showed that the best and poorest performing single crosses for persistence were, respectively, 197% and 33% of the check cultivar, KENSTAR

(Cornelius et al., 1977). Predicted performance of all possible double crosses of

the 10 I, clones indicated that the best and poorest could be expected to be 169%

and 61%, respectively, of the check. Cornelius et al. also found that genetic

variance among modified single-cross hybrids (both I, parents from a single I.

clone) is considerably greater than it is among double-cross hybrids. They concluded that high-seed-yielding and vigorous I I clones should be isolated for

comparison of their performance both in modified single crosses and in double



FIG. 2. Genotypic expectation of S-alleles in the production of

I,-sib, single-cross hybrids.



Theoretically, advanced generations of single- or double-cross hybrids would

be expected to decline in forage yield. Smith (1978) examined advanced generations of 15 single crosses derived from a diallel cross of Ilsibl lines of six red

clover clones and observed an average of 4% decline in the Syn 2 and 9% in the

Syn 3. However, some families did not decline in yield in advanced generations.

He concluded that, with enough breeding effort, it should be possible to develop

high performing combinations that will retain their performance in advanced

generations of seed increase.


In many countries, performance testing of cultivars prior to release is required.

In the United States, testing is required by the state experiment stations and the

federal government prior to release of publicly developed cultivars (Fergus and

Hollowell, 1960). Privately developed cultivars are tested by the commercial

companies themselves, but no uniform standards have been established. A cultivar review board has been suggested for red clover cultivars prior to certification by the Association of Official Seed Certifying Agencies (AOSCA, Seed

Certification Handbook, 1971). Cultivars may be registered by the Crop Science

Society, provided certain criteria, primarily uniqueness, are met (Garrison,

1968). Breeders’ rights to released cultivars also may be protected under the

Plant Variety Protection Act (PVPA, Anonymous, 1973) administered by Federal Seed Regulatory officials, often acting in cooperation with state regulatory

officials. A cultivar may be protected under PVPA either by private litigation or

under the seed certification option. The certification option requires that the

protected cultivar be sold by cultivar name only as a class of certified seed. In

this option, the Federal Seed Regulatory Office assumes the responsibility for

enforcement. The primary requirement for plant variety protection is distinctiveness from other red clover cultivars in existence. KENSTAR and REDLAND are

examples of protected cultivars.

VIII. Maintenance of Genetic Stability during Seed Multiplication

Frequently, to increase rapidly and to maintain seed supplies of new cultivars,

the seed must be produced outside the area of forage adaptation. High-seedyielding areas include the western United States, Canada, and Israel. Because

most red clover cultivars are heterogeneous and heterozygous, and the environments of the seed- and forage-producing areas are likely to be different, opportunity exists for shifts away from the bred characteristics. Trueness-to-type

tests must be conducted, comparing the increased seed with the original.



Causal factors involved in genetic shifts are not completely understood. Apparently, a shift toward earliness and winter susceptibility occurs with increases

at southern locations (Bula et al., 1965, 1969). According to Taylor et al.

(1966), early-flowering genotypes, particularly at southern locations, produce

the most seed, shifting the cultivar toward an earlier type. Earliness and lack of

persistence have been shown to be related (Taylor et al., 1966). Other factors,

such as lack of attention to land history, volunteer seedlings, lack of isolation,

and seed mixtures in harvesting and seed-cleaning equipment, cannot be ruled

out. Differential survival of genotypes due to diseases in diverse environments

also is a factor, particularly in old stands.

Prevention of genetic shifts is important because of the necessity of continuing

seed production in high producing areas. Steps that have been taken to prevent or

minimize shifts include:

(1) Limiting the number of generations of seed increase (example,


(2) Restricting the area of seed increase to more northern locations (example,


(3) Clipping the first growth to allow more equal flowering of genotypes. This

practice reduces seed yield in the western United States (Dade, 1966; Rincker et

al., 1977) but is a prevalent practice in the forage-producing area of the eastern

United States (Taylor et al., 1966).

(4) Preventing seed production on first-year stands (Example, KENSTAR).

Some evidence indicates that differences in seed production exist between stands

that have or have not been subjected to freezing (Bula et al., 1965, 1969).

(5) Restriction to less heterogeneous cultivars (Example, TEPA vs. ALASKALAND, Dovrat and Waldman, 1966). Apparently, the less diverse cultivars

are less subject to changes in population structure.

(6) Strict adherence to AOSCA certification requirements, consisting in land

history, isolation, control of volunteering, and seed mixtures.

In summary, most investigations, while indicating the possibility of genetic

shifts in red clover cultivars, also suggest means of limiting shifts. It appears

likely that seed increases can be continued outside the area of forage adaptation

without serious loss of desirable agronomic characteristics.


Akerberg, E. 1974. Hereditas 77, 177-182.

Anderson, L.B. 1960. N . 2. J . Agric. Res. 3(4), 680-692.

Anderson, L.B. 1973. N . 2. J . Agric. Res. 16(3), 395-398.

Anderson, M . K . , Taylor, N.L., and Kirthavip. R . 1972. Crop Sci. 12, 240-242

Tài liệu bạn tìm kiếm đã sẵn sàng tải về

VII. Use of Selected Materials

Tải bản đầy đủ ngay(0 tr)