Top 3 Breeding Methods Used for Cross-Pollinated Crops

This article throws light upon the top three breeding methods used for cross-pollinated crops. The methods are: 1. Mass Pedigree Method 2. Inbreeding 3. Recurrent Selection.

1. Mass Pedigree Method:

In this method of breeding, the best individuals with desired characters are selected on the basis of phenotypic performance in a source population. Open-pollinated seeds of the selected individual plants are divided into two halves. Second year replicated progeny row trial is conducted using one set of half seeds from each plant.

On the basis of the progeny performance, the best parental individuals are identified. The remnant half seeds from the superior parental plants are mixed and grown in isolation for random mating during the third year.

This method of breeding is equivalent to ear-to-row selection in context of maize originally proposed by C.G. Hopkins at the Illinois Agricultural Experiment Station in 1896 to improve protein and oil content of maize. This method has been named as mass-pedigree method by S.S. Rajan in India. This very method is called line breeding when selection is based on progeny tests and a group of progeny lines is composited.

2. Inbreeding:

The mating of individuals more closely related than individuals mating at random is known as inbreeding. The lines produced by continued inbreeding are known as inbred lines. Self-fertilization is the most intense form of inbreeding.

In plant breeding nearly homozygous lines are produced by continued self-fertilization accompanied by selection for five to six generations. This can be used as the method of breeding only in those crops, which do not show any loss of vigour due to inbreeding, like cucurbits.

The three important uses of inbreeding in cross-pollinated crops are as follows:

(i) To attain uniformity in plant characters.

(ii) To improve yield etc. by individual plant selection as in cucurbits in which there is no inbreeding depression.

(iii) To develop suitable inbred lines in production of hybrids and synthetics.

Synthetic Variety:

The term ‘synthetic variety’ has come to be used to designate a variety that is maintained from open pollinated seed following its synthesis by hybridization in all combinations among a number of selected genotypes, which have been tested for combining ability.

The components of a synthetic variety could be inbred (usually), clones, mass selected populations or various other materials. The component units are maintained so that the synthetic may be reconstituted at regular intervals.

The inbreeds to be used as component lines are chosen on the basis of combining ability tests. The component inbred are crossed in all possible combinations. This inter-crossed seed is called as Syn 0.

Equal quantity of seed from all crosses is composited and the mixture is allowed open-pollination in isolation and seed is harvested. This becomes Syn 1 generation. In absence of reconstitution of a synthetic at regular intervals, the population becomes an open-pollinated variety.

The testing for combing ability is the decisive criterion for a synthetic variety by which it can be distinguished from a conventional variety of a cross-pollinating species, which originates in a continuous selection of individuals and subsequent progeny tests. The greater variability caused by crossing several components with high general combining ability makes the synthetic varieties more adaptable compared to conventional varieties.

Similar to hybrids, the yield of a synthetic variety generally also decreases after the Syn 2, until an equilibrium is reached which, in partially self-fertile species, depends on selfing rate and inbreeding (minimum depression), but also on the number of components used in the Syn 0.

The performance of Syn 1 can be estimated by the formula:

The yield to be expected, usually increases with the number of components until an optimum is reached. The question regarding the most favourable number of genotypes the Syn 0 should be composed of cannot be clearly answered, because the evidence from research and practice is too divergent.

The yield can be increased by:

1. An increase of the mean performance of the F1 combinations.

2. An increase of the mean performance of the parents (inbred lines, clones).

3. An optimum combination of the components.

Therefore, it is obvious that I₀ plants should already be tested for their combining ability and plants or lines should be used as components, the inbreeding depression of which is not as strong as in the I₅. In order to maintain performance in subsequent generations, mass selections have been found to be sufficient in maize.

The performance of synthetics can be improved by one further breeding cycle. It consists of selection of genotypes from the synthetic variety, their testing by a dialled, and combination of genotypes with the highest combining ability for a new synthesis (= recurrent selection).

Composite Varieties:

Concept of composite varieties (in maize) originated in India. Composite varieties are generally derived from the varietal crosses in advanced generation. These are usually developed from open-pollinated varieties or other heterozygous populations or germplasm which have originally not been subjected to inbreeding or have not been elaborately tested for their combining ability.

Usually, they involve open pollinated varieties, synthetics, double crosses, etc., selected for yield performance, maturity, resistance to diseases and pests. These composites often show a high order heterosis in F₁’s when widely diversed populations are crossed. Advanced generations of such heterotic crosses often show stabilized yields. General combining ability and additive gene effects play predominant role in exploitation of these populations.

The details of the steps involved in development of composite variety are as follows:

(i) Screening of diverse germplasm by evaluation at multi-locations/years to identify the sources having adaptability, desirable agronomic attributes and resistance to major diseases and tolerance/resistance to serious insects.

(ii) Making of all possible crosses among selected superior genotypes or top crossing with a varietal complex of screened base varieties.

(iii) Conducting multi-location test with the F₁ and F₂ generations of varietal crosses and selection of F₂‘s showing desirable agronomic features along with least decline in F₂.

(iv) Evaluation of selected F₂ populations and identification of the best one as practically composites are constituted by compositing seeds of various populations and allowing the mixture to stabilize under open pollination along with some mild selection in isolation. The constituent entries may not be maintained for reconstituting the composite. Composite may serve as a base population for developing inbred lines.

Hybrids:

The term hybrid variety is used to designate F₁ populations that are used to commercial planting. The F₁‘s are obtained by crossing genetically unlike parents. The pioneering work on hybrid maize was done by G.H. Shull (single crosses) and D.F. Jones (double crosses). Other innovative researchers in this area have been E.M. East, H.K. Hayes, F.D. Richey and M.T. Jenkins and others.

Types of Hybrids:

(i) Single cross:

A single cross is a hybrid progeny from a cross between two unrelated inbred.

= A x B

(ii) Three-way cross:

A three-way cross is the hybrid progeny from a cross between a single cross and an inbred.

(A x B) x C

(iii) Double cross:

A double cross is the hybrid progeny from a cross between two single crosses.

(A x B) x (C x D)

(iv) Modified single cross:

A modified single cross is the hybrid progeny from a three- way cross which utilizes the progeny from two related inbred as the seed parent and an unrelated inbred as the pollen parent.

(A x A’) x B

(v) Double modified single cross:

A double modified single cross is the hybrid progeny from two single crosses, each developed by crossing two related inbred.

= (A x A’) x (B x B’)

(vi) Modified three-way hybrid:

A modified three-way hybrid is the progeny of a single cross as female parent and another single cross between two related inbred.

= (A x B) x (C x C’)

(vii) Top cross hybrid:

This is inbred x variety hybrid. Following top cross hybrids may be formed:

(a) Inbred line x variety

(b) Inbred line x experimental hybrid

(c) Inbred line x synthetic variety

(d) Inbred line x family

(viii) Double top cross hybrid:

A double top cross hybrid is the progeny of a single cross and a variety. Such hybrids have been produced on commercial scale in India and China.

Methods of Inbred Line Development:

(i) Standard selfing method:

Self-Pollination of individual plants within single plant progenies grown is the most common procedure used to develop inbred lines. This breeding procedure has two important problems.

(a) Vigour of the lines is decreased with inbreeding because of loss of favourable dominant allelles and any heterozygous loci that have over-dominant effects. Many lines are so poor in seed yield, pollen production, etc., that they cannot be used in a programme to produce single cross hybrid seed.

(b) Effective selection within the row for the plants that have desired agronomic traits becomes minimal in generations beyond S₃. Therefore, some breeders use only two or three generations of self-pollination with subsequent reproduction by sib-mating within progenies.

(ii) Special techniques:

These techniques involve the doubling of haploids derived from either maternal or paternal gametes. Somaclonal variations from inbred lines offer another opportunity. However, due to low frequency of haploids and doubling of haploids to the diploid state, these methods are still not an important component of most breeding programmes.

Besides, these two methods, there are three other methods where along with developing inbred lines, there are opportunities to improve them simultaneously.

These are as follows:

(iii) Pedigree method:

In this method, a pair of elite lines that complement one another are crossed to produce the F₂ generation and pedigree selection is practiced by sampling the F₂ population. F₂ populations of the single crosses are the most frequently used source populations for line development.

(iv) Backcrossing:

This is a modification of pedigree method. Modifications of backcross method have also been suggested for example, in convergent improvement by Richey, there is parallel improvement of two inbred lines by the reciprocal addition of dominant favourable genes present in one line and lacking in the other line.

In this method, two inbred A and B are crossed. The F1 is backcrossed with A followed by selection of desirable traits of B and F₁ is also backcrossed with B where selection for desirable traits of A is made. After about three backcrosses and selection, selfing is done to fix the selected genes. This method is useful for improving such characters as vigour, resistance to diseases, pests and lodging.

(v) Gamete selection:

This scheme devised by L.J. Stadler in 1944, is based on the premise that if superior zygotes occur with a frequency of p², superior gametes would occur with a frequency of p. The procedure involves crossing an elite line with a random sample of pollen of plants from a source population.

The resulting F₁ plants and the elite line are testcrossed to a common tester and F₁ plants are also selfed. Testcross progenies are evaluated in a replicated trial. The test crosses of F₁ plants that exceed the elite line by tester are presumed to have obtained superior gametes from the source population. Superior gametes are recovered as F₂ self’s.

Selection and selfing are continued till desirable homozygosity/uniformity is attained. Though gamete selection is not used as extensively as pedigree and backcross methods, it does have some intrinsic features, and consequently, is included in some breeding programmes.

Testing of Inbred:

The general combining ability of the inbred is tested by making all possible crosses [n (n – 1)/2 ] in a diallel fashion or else top cross test is carried out. Specific combining ability is also estimated.

Time of Testing:

One procedure is to test the inbred for hybrid performance/combining ability in about the fifth generation of selfing when the number of selected lines is greatly reduced. This breeding system assumes favourable relationships of plant and other traits of inbred lines (traits as selection criteria during inbreeding) with combining ability for grain yield.

A second system of inbred development is based on an evaluation for hybrid performance in the early generation of self- pollination, e.g., testcrosses of the S₀ plants or S₁ lines. Genotypes that are identified for above average performance in these tests are continued in the selfing and selection nursery.

This procedure has been called as ‘early testing’ originally proposed by M.T. Jenkins in 1935. The assumption is that the combining ability of a line is determined early in its development and will change relatively little in subsequent generations of inbreeding and selection.

By early testing, the breeder is in a position to discard some lines that are inadequate in hybrid performance and wasteful expenditure on these lines is avoided. However, probably most breeders use a method that is intermediate between these two systems. In this approach, first hybrid evaluation are of S₂ or S₃ lines.

Combination of Inbred in Hybrids and Prediction of Double Cross Performance:

Single Crosses: n (n – 1) / 2

Three-way crosses: n (n – 1) (n – 2) / 2

Double crosses: n (n – 1) (n – 2) (n – 3) / 8

where, n = Number of inbred

If there are 4 inbred, A, B, C and D, the performance of the double cross (AxB) (CxD) is predicted as follows:

(i) Based on mean performance of all possible six single crosses:

(A x B) + (A x C) + (A x D) + (B x C) + (B x D) + (C x D)/6

(ii) Based on mean performance of the four non-parental single crosses:

(A x C) + (A x D) + (B x C) + (B x D)/4

(iii) Based on mean performance of four lines over a series of single crosses:

(A x E) + (A x F) + (A x G) + (A x H) + (B x E) + (B x F) + (B x G) + (B x H)+………. +(D x H)/n

(iv) Based on mean performance of top-crosses of the four inbreeds:

(A x variety) + (B x variety) + (C x variety) + D x variety/4

Of the above four methods, method (ii) is found to give more accurate results.

Genetical Basis of Heterosis:

The phenomenon of hybrid vigour, expressed particularly in the first generation (F₁) following the crossing of cultivars or inbred lines, has been known for more than a hundred years. The term heterosis, coined by G.H. Shull in 1909 suggests a mechanism based on heterozygosity and therefore, is not fixable in the homozygous state. Two hypotheses have been put forward to explain heterosis.

They are as follows:

(i) Over-dominance Theory:

This was proposed by G.H. Shull and E.M. East independently in 1908. According to this hypothesis, hybridity/heterozygosity is superior to either homozygote and this state of heterozygosity has a stimulating effect upon the physiological activities of the organism leading to superiority of Aa over AA or aa.

(ii) Dominance Theory:

This hypothesis was proposed by C.B. Davenport in 1908, A.B. Bruce in 1910 and F. Keeble and C. Pellew in 1910. According to this hypothesis, each dominant allele contributes equally to heterosis and the recessive alleles contribute nothing. It is also assumed that the dominance is complete.

If all the dominant alleles are concentrated in one parent, and the counterpart recessive alleles in another parent, the F₁ will be equal to the parent having all the dominant alleles.

For example:

In the above cross, if each dominant allele, contributes 1 unit and the recessive allele, 0 unit, then the P₁ will have a value of 4 and P₂, a value of 0. F₁ will have a value of 4.

However, there are situations, where F₁ is superior over the better parent.

These cases under dominance theory can be explained assuming that dominance and recessive alleles are distributed in both the parents as given below:

In this cross, the phenotypic value of P₁ is 3, that of P₂ is 1 and F₁ has a score of 4 which is superior to the better parent. Under this model it should be possible to derive a pure line from the F₁ which should be equal to F₁ in the performance and thus this heterosis will be fixable.

However, it is generally agreed that heterosis is not fixable in the homozygous state. This would be the case if hybrid vigour were due to true over-dominance or due to tight linkage in the repulsion phase at some incompletely dominant loci.

Much evidences suggest that apparent over-dominance is, in fact, due to non-allelic interaction and linkage disequilibrium and that heterosis is mainly a result of the bringing together of unidirectionally dominant alleles distributed between the parental line. Under this, heterozygosity is not an essential prerequisite for high performance, uniformity and stability of performance.

3. Recurrent Selection:

Recurrent selection is a method of breeding designed to concentrate favourable genes scattered among a number of individuals by selecting in each generation among progeny produced by matings inter-se of the selected individuals (or their selfed progeny) of the previous generation.

Based on the ways in which plants with desirable characters are identified, recurrent selection has been divided into four types.

These types are:

(i) Simple recurrent selection or recurrent selection for phenotype

(ii) Recurrent selection for general combining ability

(iii) Recurrent selection for specific combining ability

(iv) Reciprocal recurrent selection

In simple recurrent selection a number of plants are self-pollinated in a source population in first year. At maturity superior plants based on phenotypic performance are selected. In second year, seeds produced by self-fertilization of the selected plants are planted and crossed in all possible combinations and the produce is bulked.

This completes original selection cycle. Since selection is based on the phenotype of the plant, it is useful only for characters with high heritability. In those cases, where it is possible to identify the desired selections before flowering as in case of cauliflower, cabbage, etc., inter-crosses of selections may be made in the first year of each cycle and the second year may be eliminated from each cycle.

Thus, strictly speaking, selfing is not an integral component of simple recurrent selection, rather it is done only to prevent crossing from the inferior pollen grains before the plants reach to selection stage.

In recurrent selection for general combining ability, a three year cycle is involved. In first year a number of plants are self-pollinated and crossed to a broad based heterozygous tester stock to identify the S₀ plants with good general combining ability. In second year, the crosses are evaluated to identify those that are superior. Self’s of first year are kept in reserve.

In third year, the reserve selfed seeds are grown out, inter-crossed in all combinations, and a composite of inter-crossed seeds is used to establish an improved population for further selection. This procedure developed as a direct outgrowth of studies of early testing first proposed by M.T. Jenkins in 1935.

Recurrent selection for specific combining ability was proposed by F.H. Hull in 1945. This method of selection is same as that of recurrent selection for general combining ability except that the tester selected is a narrow base an inbred line. The recurrent selection for general and specific combining ability is equivalent to half sib progeny test.

Reciprocal recurrent selection proposed by R.E. Comstock, H.F. Robinson and P.H. Harvey in 1949 aims at simultaneous improvement of two heterozygous and heterogenous populations (designated as A and B).

A serves as tester for B and B serves as tester for A. This method is as effective as recurrent selection for gca when additive gene action predominates, and is as effective as recurrent selection for sea when non-additive effects are of major importance.

The steps are as follows:

I. Season:

Selected plants of population A are self-pollinated and crossed to plants of population B. Likewise plants are selected and self-pollinated in B and outcrossed to plants of population A.

II. Season:

Test cross progenies of both the populations are evaluated in replicated trial. Superior progenies are identified on the basis of performance in this trial.

III. Season:

Selfed seed from plants with superior test cross progenies are grown population wise separately and inter-crossed to reconstitute two populations which will be now called as A’ and B’. This completes one cycle and additional cycle (s) may be initiated.