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VI. Implications of Quantitative Genetics to Breeding Methodology
R. H. MOLL AND C. W. STUBER
graphic range desired in the improved strain. The relevant issues here involve phenotypic stability (Section IV, B). As the range of diversity of
environments is expanded for which a superior strain is sought, genotype-environmental interactions become more serious, and selection of
strains with satisfactory stability becomes more difficult.
Procedures most appropriate for a given breeding problem depend to
a large extent on whether the breeder is seeking an improved variety, a
pure line, or an F, hybrid. Questions concerning the desirability of F, hybrids for commercial use depend to some extent upon the amount of
heterosis relative to improvements possible by capitalizing upon transgressive segregation. However, the ultimate decision of whether or not the
breeding objective is an F, hybrid often rests on many other factors, such
as the availability of sterility mechanisms to facilitate cross-pollination, the
desirability of the genotypic control afforded by F, hybrids, and various
Once the decision is made to develop F, hybrids, the breeder must
choose the most appropriate selection procedures. Comparisons between
different kinds of selection procedures in Section V are most important and
suggest that, in the initial stages at least, the more simple methods of
intrapopulation selection may be quite satisfactory. More complex methods
of interpopulation selection may become advantageous in later stages, after
significant improvements have been achieved.
A broad range of genetic diversity is available in all major crop species.
However, breeding procedures for plant improvement have severely limited
germplasm diversity in materials available for commercial production. In
fact, some crops are being grown almost in monocultures over large areas.
This creates a serious hazard, in that the crop becomes extremely
vulnerable to disasters, as was evidenced by the 1970 southern corn leaf
blight epidemic in the United States (Committee on Genetic Vulnerability
of Major Crops, 1972).
Because of the narrowing germplasm base in breeding populations, new
emphasis is being directed toward the use of broad-based genetic populations, in which recurrent selection is being initiated. The first step in the
development of such populations is the selection of superior genetic material. For many species, this will involve an evaluation of exotic as well
as local or native materials. As Eberhart et al. (1967) suggest, two or three
years spent introducing and evaluating exotic germplasm in addition to
the local sources may often produce greater results than a ten-year program
recycling the local materials. Eberhart ( 1971) and Goodman ( 1965 ) have
demonstrated the benefits from incorporating exotic germplasm into United
States maize breeding materials.
Large numbers of collections of germplasm are available in most economically important species; therefore, some type of systematic plan for
evaluation is required. For preliminary screening, general combining ability
is the most important consideration. Therefore, the screening might be accomplished by making testcrosses of collections to a locally adapted line
or variety. Agronomic notes can be recorded in the testcross nursery, with
yield comparisons among the testcrosses and other performance evaluations
conducted in following seasons. As Comstock and Moll (1963) suggest,
adequate information for selection of the best entries should be possible
with results from a single season, if evaluations are made at several
Whether the final goal is to develop one or two breeding populations,
some technique should be used to ensure thorough recombination as the
selected entries are being composited. Eberhart et al. (1967) proposed
a method specifically designed for corn, which might be modified for use
in other plant species. In their method, individual entries are planted in
a replicated manner and detasseled. Rows of bulked seed of all entries
are planted between ranges of the individual entries to provide pollen. At
harvest, ears of each entry are saved and bulked over replications to represent that entry in the following season. If desired, selection might be imposed, and only ears from the best plants of each entry would be saved.
Individual entires are handled in a similar manner for a minimum of four
generations with the variation among entries decreasing as recombination
progresses. The total number of ears saved in each generation should be
reasonably large (possibly 800-1 000) to minimize loss of favorable genes.
Compositing is more difficult in most self-pollinating species. However,
the presence of genetic male-sterility mechanisms in many of these species
affords an opportunity for recombination and adaptation of recurrent selection procedures to improvement problems (Brim and Stuber, 1973; Doggett, 1972; Doggett and Eberhart, 1968; Gilmore, 1964). To minimize
the contribution of the cytoplasm of the male-sterile source in the composite, heterozygous fertiles from a maintainer line can be used as male
parents in the synthesis of the initial population. (This normally will require hand pollinations,) The genetic contribution of the male-sterile genotype can be minimized by subsequent backcrossing. This should be followed by several cycles of intermating, in which only male steriles are saved
to plant the succeeding cycles. If pollen is transmitted primarily by insects,
mating may not be random. Therefore, a sampling scheme, such as a grid
R. H. MOLL AND C. W. STUBER
system, to divide the intermating nursery into subblocks should be imposed
before harvesting male-sterile plants (Brim and Stuber, 1973).
After the intermating cycles are completed, recurrent selection schemes
(discussed in Section V) can be initiated. Genetic male-sterility systems
can be used effectively to provide recombination between cycles of selection, particularly in self-pollinating species.
C. TESTINGAND EVALUATION
Before initiation of a recurrent selection program, it is desirable to have
some measure of the response that can be expected per unit of time. Decisions concerning the selection scheme to be used and the selection intensity
to be imposed are influenced by the magnitude of genetic variances. However, it may not be necessary to devote time and resources to produce
precise estimates of variance components for crops that have already been
investigated thoroughly. As pointed out previously, the magnitude of additive variance for particular traits appears to be similar among populations
of the same kind. Therefore, the breeder may rely on information from
investigations in related populations to aid in practical decision making.
Quantitative genetic studies of a wide range of crop species have indicated that the additive genetic (or general combining ability) component
is usually more important than the nonadditive (or specific combining ability) component, and that epistatic variance components can be ignored
in predictions of selection response in many cases. Therefore, the assumption of predominantly additive genetic variance in a breeding population
should be reasonably safe.
If the breeder feels a definite requirement for variance component estimates before initiating a selection program, then he should be certain that
the estimation experiments are adequate to provide reliable estimates of
the kind required. Estimation of genetic variances requires the use of
appropriate mating and environmental designs. Dudley and Moll ( 1969)
compared various designs and suggested that the most preferable design
is the simplest one that will provide the required information. Results from
numerous studies in corn (Moll and Robinson, 1967) indicate that 256
progenies (each with two common ancestors, e.g., full-sib families) would
be a minimum to estimate additive and dominance variance components,
and these progenies need to be grown in at least two environments. Adequate seeds for this number of progenies may be difficult to produce in
many self-pollinating species, and experimental procedures using inbred
relatives (Stuber, 1970) may be more appropriate to provide the estimates
Genetic variances can be estimated during the evaluation phases of the
selection program, if the selection scheme includes some type of family
structure. After two or three cycles of selection have been completed,
reasonably precise estimates of these variances should be available. These
estimates can then be used to predict further selection response. As indicated previously (see Section V), genetic variances in corn and tobacco
have not changed significantly over several selection cycles for traits with
low heritabilities such as yield. Therefore, predictions made from early
cycle variance estimates should be reasonably reliable over several cycles
for such traits.
Although prior estimates of genetic variances are desirable for a population improvement program, the choice of selection scheme will be dictated
primarily by the breeder’s specific objectives, the mode of reproduction
of the species, and resources available. In a recurrent selection program,
decisions concerning the number of parents selected for each cycle of intermating and the selection intensity have far reaching effects as they relate
to long- and short-term gains. Progress over the short term may be the primary aim of the plant breeder; however, conservation of the genetic potential of a population over the long term should maintain a high priority
in a breeding program. Most breeding programs involve both short-term
and long-term goals, and replicated selection similar to that proposed by
Baker and Curnow (1969) may provide useful flexibility, Short-term objectives can be realized best with very intense selection, but unless population sizes are large, the rate of inbreeding would prohibit long-term goals.
Thus, the replicates that showed greatest improvement could provide the
sources for short-term objectives, and replicates could be intermated to
minimize inbreeding in more advanced selection cycles.
Theoretical developments are available to provide the breeder some
guidelines in making these decisions regarding population size. Robertson
(1960, 1963) showed that expected total advance and “half-life” of recurrent selection processes are proportional to effective population size ( N ) ,
an that in long-term selection programs, N should be as large as possible.
Rawlings (1970) proposed an effective population size of 30-45, with a
selection intensity of 0.10 as a reasonable compromise to satisfy both longand short-term objectives. Similar conclusions were reported by Baker and
Curnow ( 1969 ) .
Although maintenance of a control (or check) population may be difficult to justify for the empirical plant breeder, it is difficult to assess the
progress achieved without appropriate points of reference. An inbred line
or single cross will provide a constant genotype as a control population;
however, a genetically homogeneous population normally shows more interaction with environments than a genetically heterogeneous population
R. H. MOLL AND C. W. STUBER
(Sprague and Federer, 1951). Therefore, the original random mating population in which selection was initiated will probably provide the best source.
With cold storage facilities, this population can be maintained over long
periods of time with little chance for significant changes caused by natural
selection. However, even heterogeneous populations interact differently
with changing environments. This is evidenced by the observation that
many open-pollinated varieties of corn produce relatively poorly when subjected to high plant densities and high fertility regimes. Therefore, an
element of caution must be exercised with the use of any control
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THE DEVELOPMENT OF TRlTlCALE
F . J . Zillinsky
International Maize and Wheat Improvement Center (CIMMYT).
Mexico City. Mexico
I. Historical Review ................................................
A . The Development of Octoploid Triticale ..........................
B. The Development of Hexaploid Triticale ..........................
I1. Breeding and Research in Eastern Europe ...........................
A . Hungary ....................................................
B. Russia . . . . . . . . . . . . . .
111. Breeding and Research in
tern Europe ..........................
A . Sweden .....................................................
B. Spain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C . Germany . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I V. Breeding and Research in North America ............................
A . United States ........... ...................................
B. Canada . . . . . . . . . . . . . . .
V. Triticale Improvement at CIMMYT ................................
A . The Establishment of an International Base ......................
B. Breeding Program ............................................
VI . Recent International Developments ................................
A . Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B . Industrial and Nutritional Quality ...............................
C . Recent Cytological Research . . . .
D . Nomenclature ................................................
E . General Comments ............................................
I . Historical Review
A . THE DEVELOPMENT
The history of triticale extends back almost a century . It is highlighted
by a series of contributions from many scientists in several countries across
three continents (Fig. 1 ) . Historical reviews have been prepared by numer315
F. J. ZILLINSKY
FIG.1. Scientists engaged in research on triticale, taken at the International Maize
and Wheat Improvement Center (CIMMYT) Headquarters, El Batan. Reading left
to right: N. E. Borlaug, CIMMYT; L. H. Shebeski, University of Manitoba;
A. Kiss, Hungary; A. Muntzing, Lund, Sweden; E. SAnchez-Monge, Madrid, Spain;
K. D. Krolow, West Berlin, Germany; E. Larter, University of Manitoba; F. J.
ous authors, but those Muntzing (1973a) and Briggle (1969) have been
used freely in the preparation of this manuscript.
Triticale is an artificially created derivative of a cross between wheat
and rye and possesses the chromosome complements of both parental species. There are two main groups of triticale: the octoploid triticales, which
are amphiploids of hybrids between hexaploid wheats and rye; and hexaploid triticales, which are amphiploids of hybrids between tetraploid wheat
and rye. Recently tetraploid forms have been reported (Krolow, 1973).
The first report of hybrids between wheat and rye was published by
Wilson in 1875. The hybrids were highly sterile and did not reproduce.
Rimpau, a German scientist, obtained a fertile, true-breeding strain from
a cross between bread wheat and rye in 1891. It was not until 1935 that
this strain was proved to be an amphiploid with 2n = 56 chromosomes
(Lindschau and Oehler, 1935; Muntzing, 1936).
According to Muntzing ( 1973a) an unusual outcrossing phenomenon
was observed in 1918 by Meister at the Saratov Experiment Station in
Russia. Thousands of natural wheat-rye hybrids occurred in wheat plots
which had been adjacent to rye plots the previous year. He reproduced
plants from these hybrids for several generations and eventually obtained
true-breeding, more or less fertile derivatives. In 1930 Meister gave a
botanical description of the new species and named it Triticum seculotricum surutoviense Meister. Lewistsky and Benetzkaja (193 1 ) produced cytological evidence that the new forms produced by Meister from the bread