heinrich grausgruber

Zuchtmethodik und Quantitative Genetik UE / H. Grausgruber

Universität für Bodenkultur Wien

Department für Angewandte

Pflanzenwissenschaften und

Pflanzenbiotechnologie

Combining ability

Heinrich Grausgruber

Source: Riedelsheimer et al. (2013)

Zuchtmethodik und Quantitative Genetik UE / H. Grausgruber 2

INTRODUCTION

The identification of the best performing genotypes for cultivar release or use in future crosses as

seed or pollen parental line are two major tasks in plant breeding programmes.

The best performing breeding lines are identified in METs (↑ stability analysis). Parental line

selection is based upon the evaluation of the COMBINING ABILITY of a genotype which can be

determined in specific MATING DESIGNS and evaluates a genotype based on the performance of its

offsprings. The particular mating designs allow the partitioning of the genetic influence into additive

and non-additive components.

The determination of the combining ability is of specific importance in the breeding of hybrid and

synthetic cultivars for the evaluation of inbred lines and varietal components.

Combining ability can also be used to evaluate cross combinations in self-pollinating crops (pure

line breeding), however, it is of less relevance in this case.

COMBINING ABILITY can be determined only in particular MATING DESIGNS on the

PERFORMANCE OF the OFFSPRING.


Simplified example

Baking volume of wheat (Triticum aestivum)

Cultivars Amadeus, Exquisit, Leopold and Capo have one parent (i.e. Pokal) in common – based on

the performance of the cultivars the combining ability of the parent other than Pokal can be

determined

→ the highest baking volume is recorded for Exquisit. Therefore, Agron has the highest combining

ability with respect to baking volume


Types of combining ability

The terms general combining ability (GCA) and specific combining ability (SCA) were first

introduced in 1942 by George F. Sprague & Loyd A. Tatum.

Sprague GF, Tatum LA (1942) General vs. specific combining ability in single crosses of corn. J. Am. Soc. Agron. 34: 923-932.

The original principle of a directed selection of breeding lines based on the performance of the

offspring, however, was recognized already as early as 1850 by the French sugar beet breeder

Louis de Vilmorin (↑ Vilmorin isolation principle).

General combining ability (GCA)

GCA as the average performance of a genotype in a series of hybrid combinations. It is calculated

(for a specific trait) as the (positive or negative) deviation of the mean offspring performance of a

genotype from the grand mean of all offsprings included in the particular mating design. GCA is

mainly caused by additive effects.

Specific combining ability (SCA)

SCA is defined as the deviation of the performance of hybrid combinations from the performance

expected on the basis of the GCA of the parental inbred lines.

In hybrid breeding the particular combination of inbred lines out of many possible combinations is

selected which exhibits the highest F1 performance. Therefore, inbred lines are selected as

parental lines based on the highest SCA. SCA is determined by dominance, over-dominance and

other non-additive effects.


Determination of combining ability – Mating designs

Polycross & Topcross are used to determine GCA, Diallels or Factorials (M×N mating) are used for

the determination of SCA. The latter mating designs allow also the calculation of GCA.

To reduce labour associated with a lot of test crosses molecular markers are nowadays used to

determine the genetic diversity between parental lines to predict heterosis and combining ability.

Polycross

... mainly used for the determination of GCA of open pollinated forage crop with the possibility to

multiply vegetatively

→ used in the improvement of open pollinated populations or the production of synthetics of

forage crops.

Implementation:

Test material is first tested and selected for per se performance and afterwards grown on an

isolated polycross field. As the different components are cross pollinating it has to be considered

that flowering time of all components is synchronized and that replications of one and the same

genotype are not close to each other (restricted randomisation of the repeated plants of the same

genotype).

→ Every genotype should be pollinated from another genotype to the same extent.


Self fertilisation should be disabled by the use of self incompatibility. In case of facultative cross-

pollinating species the percentage of self-pollination should be the same across all used parental

components.

Seeds of single plants are harvested and the bulked seeds of a particular genotype are used to

test the performance of the offspring. GCA is calculted and the components with the highest (or

lowest – depending on the trait!) GCA values are selected and used to build up a new improved

population variety (or synthetic variety).

Vegetative propagation of the components over the year of offspring performance testing

facilitates the inclusion of polycross testing in a breeding programme.

If a synthetic variety is composed of only a few components (clones) the selection based on GCA

is most probably not optimal. To a high probability intercrossing of genetically similar plants will

happen and inbreeding depression can appear.

The single components should have also a high per se performance to realize a high general

varietal ability.

• The polycross test in cross-fertilized plants serves to recognize desirable genotypes of

MOTHER PLANTS or MOTHER CLONES by studying their individual progenies after open

intercrossing.

• The applicability of the polycross test depends on the possibility of simultaneously preserving

the genotypes of the tested plants. VEGETATIVE PROPAGATION is ideal for this purpose.


Polycross of red clover (Source: Grausgruber 1988, Saatbau Linz, Breeding station Reichersberg)


Topcross

... for the determination of GCA of inbred lines if a high number of germplasm has to be tested,

e.g. maize breeding. A high amount of inbred lines are first selected based on the topcross test

derived GCA before the combinations with the best SCA are determined by further mating

designs.

Implementation:

Inbred lines are not pollinated by an undefined pollen donor (↑ polycross) but by a particular

pollen donor, the so-called Tester. Inbred lines and tester are grown isolated side by side in rows.

Seed parents (maternal inbred lines) are either emasculated or male sterile and can, therefore, be

pollinated only by the test line.

Hybrid seed is harvested and tested for performance the next season. The performance of the F1-

hybrids is used for the calculation of the GCA.

Based on the GCA the best inbred lines are used in the further breeding steps.


Test line:

Important for selection efficiency is the selection of the test line. The test line should not mask the

inbred lines to be tested in the most important traits. A weak test line can better differentiate with

respect to the per se performance of the inbred lines.

Flowering synchronicity between inbred lines and test line(s) is important, however, can also be

improved if multi-rows of the test line are sown at different sowing dates.

Inbred lines can be tested either early (‘early testing’) when heterozygosity is still high or later in a

more homozygous stage. Application of DH method (doubled haploids) allows the immediate

testing of homozygous inbred (DH) lines.


Poly- and Topcross test in the breeding of hybrid

(left) and/or population (synthetics) (right) varieties

(Source: Becker 1993)


M×N mating system

M×N diallel, M×N mating, factorial

→ mating design including M seed parental lines and N pollen donor lines inter-crossed

systematically (M×N possible combinations).

→ Determination of both GCA and SCA

Diallels

Contrary to the Factorial all seed parental lines are also used as pollen donor lines.

Full diallels can be laborious if the number of lines to be tested is high, therefore, modifications

were developed to determine GCA and SCA also on modified diallels.


Diallel crosses after Griffing

Generally a diallel cross including p parental lines results in p2 cross combinations

= p inbred lines

+ p×(p-1)/2 crosses

+ p×(p-1)/2 reciprocal crosses

According to Griffing (1956) four different methods of diallel crosses can be differentiated:

METHOD 1 (complete diallel) - p2 combinations

→ includes inbred lines, F1 hybrid lines and their reciprocal cross and, therefore, allows the

determination of GCA, SCA, reciprocal effects and heterosis.

METHOD 2 (half diallel) – p(p+1)/2 combinations:

→ includes parental inbred lines and one set of F1 hybrids and, therefore, allows the calculation of

GCA, SCA and heterosis.

METHOD 3 - p×(p-1) combinations:

→ includes only F1-hybrids and reciprocals and, therefore, allows the calculation of GCA, SCA and

reciprocal effects.

METHOD 4 - p×(p-1)/2 combinations:

→ includes only on set of F1-hybrids and, therefore, allows only the calculation of GCA and SCA

→ Methods 3 and 4 are also called modified diallels.


Analysis of a Griffing-Diallel, Method 4, Modell I (fixed effects)

Total variance is divided into GCA, SCA and residual error part (ANOVA of field trial) → effects are

tested for their significance

Basic model:

Not significant SCA variance means that each hybrid combination can be predicted from the grand

mean of the mating design and the GCA effects of the parental lines, that means that the best

hybrid can be created by the cross of the two parental lines with the highest/lowest (best) GCA

values.


To determine the SCA an expected value for the performance of a particular cross is calculated

based upon the gran mean of the mating system and the GCA values of the two respective parental

lines.

The SCA is the deviation of the (true) observed value from that expected value.

Across the mating system the sum of GCA and SCA equals zero.


Heterosis

Deviation of the heterozygous F1 hybrid from the mean performance of the two homozygous

parental lines.

If F1 hybrid will be selfed over further generations INBREEDING DEPRESSION will appear, the

genes of the parental lines will be recombined and new combinations could be selected. The mean

performance across all offsprings, however, will be the same as the mean performance of the

original two parental lines.

Heterosis can be higher for quantitative traits (e.g. yield) if various favourable alleles are combined.

For quality traits which are often determined by single or few genes heterosis is usualy lower. To

realize high heterosis the crossing of genetically diverse parental lines is usually recommended.

↑ heterotic groups in hybrid breeding


Heterosis = mid parent heterosis

Heterobeltiosis = better parent heterosis


Literature

BAKER, R.J., 1978: Issues in diallel analysis. Crop Sci. 18:533-536.

BHULLAR, G.S., GILL, K.S., KHEHRA, A.S., 1979: Combining ability analysis over F1-F5 generations in diallel crosses of bread wheat.

Theor. Appl. Genet. 55:77-80.

CHRISTIE, B.R., SHATTUCK, V.I., 1992: The diallel cross: design, analysis and use for plant breeders. In: Janick J (ed.), Plant Breeding

Reviews 9, 9-36. John Wiley & Sons Inc., New York.

GRIFFING, B., 1956: Concept of general and specific combining ability in relation to diallel crossing systems. Aust. J. Biol. Sci. 9:463-493.

HAYMAN, B.I., 1954: The analysis of variance of diallel tables. Biometrics 10:235-244.

JINKS, J.L., 1954: The analysis of heritable variation in a diallel cross of Nicotiana rusticana varieties. Genetics 39:767-788.

RUCKENBAUER, P., TANASCH, L., 1975: Möglichkeiten und Grenzen dialleler Kreuzungsanalysen für die Wahl der Kreuzungseltern in der

Kreuzungszüchtung. Bericht 26. Züchtertagung, 229-241. BAL Gumpenstein.

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