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VII. Use of Selected Materials
RED CLOVER BREEDING AND GENETICS
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
N. L. TAYLOR AND R. R. SMITH
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
RED CLOVER BREEDING AND GENETICS
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.
N . L. TAYLOR AND R . R. SMITH
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.
C. CULTIVAR OR HYBRID RELEASE
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.
RED CLOVER BREEDING AND GENETICS
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