Breeding of cross pollinated crops

Introduction:

Breeding of cross pollinated crops is different from that of self-pollinated crops as they have different population structures. Cross pollinated crops show inbreeding depression ranging from low (cucurbits) to severe (carrots) (Acquaah, 2012). We use individual plant selection in case of self-pollinated crops but in case of cross pollinated crops, individual plant selection is not effective and seldom used because segregation causes progeny to deviate from the parental type. In cross pollinated crops mass selection and the closely related procedure of progeny selection and the line selection are much more closely used then single plant selection.  In selecting breeding techniques for cross pollinated crops scientist and breeders must be very careful as these crop populations are highly heterozygous and heterogeneous. There are mainly two broad method of the improvement and breeding of cross pollinated crops they are population improvement and production of hybrid and synthetic varieties.

Population improvement:

Population improvement method of breeding cross pollinated crops is further categorized into two groups i.e. Breeding method without progeny test (mass selection) and Breeding method with progeny test (progeny selection and recurrent selection).

Mass selection:

In mass selection desirable individual plants are chosen, harvested, and the seeds composited without progeny test to produce the following generation. Selection of the plant is purely based on the maternal parent only and there is no control over the pollination, mass selection amounts to be a form of random mating with selection (Pertanian, 1977). The main purpose of mass selection is to increase the proportion of the superior genotype in the population. The efficiency mass selection depends on gene number and the heritability. Mass selection has been effectively in increasing gene frequencies for characters which are easily seen or measured (Chakraborty & Dasgupta, 2011).

Merits of mass selection:

• Mass selection has been effective in increasing gene frequencies for characters which are easily seen or measured
•  Mass selection has been useful in developing varieties for special purpose and in changing the adaptation of varieties to fit them in new environment.
• It is simple breeding program which is based on phenotype and the selection cycle is very short.
• The new varieties developed from mass selection is similar to the original population. Therefore extensive multi-location yield trial may not be required before its release as a new variety.

Demerits of mass selection: 

• Mass selection has not been effective in modifying characters such as yield that are governed by many genes and cannot be accurately judged on the basis of the appearance of single plants.
•   Inability of identified superior genotype from the phenotypic appearance of a single plant 
• Uncontrolled pollination which leads to the pollination of superior plant by both superior and inferior pollen parent 
• Strict selection leads to the reduced population size which leads in turn to inbreeding depression.

Fig 1:Steps involved in Mass selection of cross pollinated crops, From B.D. Singh 1990

Progeny selection:

Progeny selection is the most effective way of distinguishing among the single plant whose superiority is environmentally induced and those whose superiority is from superior genotype. In this method we grow small number of progeny (10 to 50) of each individual plant selected in the previous generation. Various mating system can be used in progenies to be tested. The most common method is to harvest open pollinated seeds from the selected plants and is used to establish the progeny plot. This is also known as ear-to –row selection and most commonly used in maize.

Merits of progeny selection:

• Progeny selection is based on the progeny test, not only on the phenotype of the plant, which more accurate reflection of superior genotype. Thus progeny selection is more effective than mass selection in identifying superior genotype.
• Excessive inbreeding can be avoided by providing sufficient care while selecting large number of superior plants.   
• The selection procedure involved is relatively simple and easy to perform.

Demerits of progeny selection:

The main demerit of progeny selection is that there is no control on the pollination. The plants of superior progeny are also pollinated by the plants of inferior progenies.As compared to the mass selection, progeny selection is complicated and the cycle requires 2-3 years which is time consuming.

 

Fig 2: Steps involved in progeny selection of cross pollinated crop species, From B.D. Singh 1990

Recurrent Selection:

The procedure followed in recurrent breeding methods were followed from 1920s, as breeding scheme similar to recurrent selection was suggested in 1919 by Hanyes and Garber and independently by East and Jones in 1920s, but it was given name of recurrent selection only in 11940s. The first detailed description of this type of breeding scheme was published by Jenkins in 1940. This system as used in maize has the following operational features.

• Plants from the heterozygous source are self-pollinated and at the same time evaluated for the some desirable character or characters.
• Plants with the inferior performance for the character or characters under improvement are discarded.
• The superior plants are propagated from the selfed seeds.
• All possible intercrosses among these superior progenies are made by hand or if this is impractical, the intercrosses are made by open pollination among the selected progenies.
• The resulting intercross population serves as source material for the additional cycle of selection and intercrossing.

Recurrent selection was meant to include reselection generation and generation, with interbreeding of selects to provide for genetic recombination. Recurrent selection are further divided into four group they are simple recurrent selection (SRS), recurrent selection for general combining ability(RSGCA), recurrent selection for specific combining ability(RSSCA), reciprocal recurrent selection (RRS).

3.1.3.1 Simple Recurrent selection:

Simple recurrent selection is the type of selection which does not include tester. It is also known as phenotypic recurrent selection. This method is the extension of the mass selection.

For the first time the experimental evidence on effectiveness of the simple recurrent selection in changing gene frequency was presented in 1950 by Sprauge and Brimhall, who studied the oil content in the corn kernel.

The main features of simple recurrent selection are given below.

i. The tester is not used in this scheme.

ii. It does not measure the combining ability.

iii. The selection is based on phenotype or simple test.

iv. This method is useful only for those characters which have high heritability.

3.1.3.2. Recurrent selection for general combining ability (RSGCA):

In RSGCA, a number of plants which appears superior to the breeders are selected from the source population. These selected individuals (SO) are selfed and also crossed to a heterozygous tester stock to identify the SO with the good general combining ability. The selected individuals are propagated from the selfed seed, intercrossed in all possible combination, and a composite of intercrossed seed is used to establish a population for further selection.

The main features of this scheme are given below:

i. This method is used for genetic improvement of quantitative characters.

ii. The selection is made on the basis of test cross performance.

iii. A heterozygous tester with broad genetic base is used for testing general combining ability. Generally, an open pollinated variety is used as a tester.

iv. This method is used for improving general combining ability of a population for a character.

v. This method is more effective with incomplete dominance and less effective with over dominance.

vi. This method is used for the improvement of those characters which are governed by additive gene action.

3.1.3.3. Recurrent selection for specific combining ability (RSSCA):

This type of recurrent selection was proposed by Hull in 1945 on the assumption that an important part of heterosis results from the nonlinear interaction of genes at different loci, from interaction between alleles at the same locus or from both cause in combination. Thus specific combining ability is used to find such lines which could bring high heterosis after a suitable cross. Tester used in this type of selection is homozygous so it is also known as half sib recurrent selection with homozygous tester (Shimona, 2020).

The main features of RSSCA are given below:

i. This method is used for the genetic improvement of polygenic character

ii. Selection is made on the basis of test cross performance.

iii. A homozygous tester with narrow genetic base is used for testing specific combining ability. In other words, an inbred is used as a tester.

iv. This method is used for improving specific combining ability of the population for a character.

v. This method is more effective with over dominance and less effective with incomplete dominance.

vi. This scheme is used when a character is governed by non-additive (dominance and epistasis) gene action.

vii. This method requires three seasons or years for completion of each cycle of selection.

Fig 3: Recurrent selection for specific combining ability (RSSCA), From B.D. Singh 1990

3.1.3.4. Reciprocal recurrent selection (RRS):

RRS was proposed in 1949 by Comstock, Robinson, and Harvey as a procedure that would be useful in selecting simultaneously for both general combining ability and for specific combining ability. This method involves two heterozygous source population, A and B, which should preferably be genetically unrelated. A number of plants from source A are selfed pollinated and crossed with a sample of plants from source B. in similar fashion, a number of plants from source B are selfed and crossed with a sample of plant from source A. selection is based on the experimental comparison of test cross progenies in replicated yield trials. The plant selected are then inbreed from S1 progenies derived from the selfed seed of the SO plants. The two resulting population A' and B' serve as source population to initiate the next cycle.

Sufficient plants should be selected in each generation to hold inbreeding within the two sources to an acceptable level. Otherwise the within-group variability on which selection operates may be reduced to the point where progress will be adversely affected.

Merits of recurrent selection:

• Recurrent selection is an efficient breeding method for increasing the frequency of superior genes in a population for various economic characters. Thus it is an important method of population improvement.
• Repeated intermating of heterozygous progeny provides greater opportunities for recom-bination to occur. Thus this method helps in breaking repulsion phase linkages.
• This method also helps in maintaining high genetic variability in a population due to repeated intermating of heterozygous plants/populations.
• The selection is made on the basis of test cross performance (except in simple recurrent selection) and only selected plants are allowed for intermating.

Demerits of recurrent selection:

• This method is not used directly for the development of new varieties. The new varieties are developed by the use of end product in hybridization. This is only a method of population improvement.
• This method involves lot of selection, crossing and selfing work.
• This method permits selfing which leads to loss of genetic variability.

Fig 4: Steps involved in RRS of cross pollinated crop species, From R. W. Allerd, 1960.

2.2 Hybrid and Synthetic varieties:

Production of hybrid and synthetic varieties is the method of breeding of cross pollinated varieties which is popularly used now a days for the improvement of the cross pollinated crops.

2.2.1 Hybrid variety:

The term hybrid variety is used to designate F1 population that are used for commercial planting. These F1 population can be obtained by crossing clones, open-pollinated varieties, inbred lines, or other populations that are generally dissimilar in their genotype (Brown, Caligari &Campos, 2014). Hybrid exploits heterosis if they are feasible. Heterosis is be exploit greatly in maize.

Operations in the producing hybrids:

The operations that are carried out for the production of the hybrid varieties are i)Selection of desirable plants in open-pollinated populations ii)Selfing of these selected plants through several generations to produce homozygous inbreed lines iii)Crossing of chosen lines.

Developments of inbreeds:

Inbreeds of any crop is developed by the self-fertilization of that crop. The population through which inbreeds are selected is known as source population. These source population are generally open pollinated varieties. The plants to be selfed are selected for their vigor, standing ability, freedom from disease and other desirable characters.

Merits of hybrid varieties:

• Hybrid varieties exploits heterosis to the greater extent since they exploits both general combining ability and specific combining ability of a crop.
• Production of hybrid varieties opens new area for the development of the crop and provide farmer with new better variety that are resistant to various pest, diseases and better yielding at that particular agro-ecological domain.
• Hybrid varieties can be produced in both open and cross pollinated crops. Thus it helps to recombine suitable genes for the production of suitable variety.
• Aesthetic plants beauty can be changed by the production of hybrids since hybrid can produce plants with changes in color, number and size of leaves, stems, flowers, fruits and seeds. Greater variability.

Demerits of hybrid varieties:

• For the production of hybrid varieties, first inbreeds have to be developed which requires a lot of time and experience personal to perform the task.
• Seed of hybrids have to be changed year after year and seed is costly to farmers. Farmers can't save seeds of hybrid varieties for the next season.
• In many cases hybrid varieties though high yielding, but are poor in organoleptic and nutritional aspect as compared to the original traditional varieties.
• Production of hybrid seeds at small farm is practically not possible for cross pollinated crop species as these species requires large isolation distance.

Fig 5: Steps involved in production of hybrid varieties of cross pollinated crop species, From B.D. Singh 1990

2.2.2. Synthetic varieties:

Hybrid varieties exploits heterosis to a greater extent but their seed can't be save for raising next season crop. This problem can be solved by the production of the synthetic varieties. Synthetic varieties seem to offer a good opportunity for the controlled utilization of an appreciable amount of heterosis.

Hayes and Garber (1919) were apparently the first to suggest the probability of the commercial utilization of synthetic varieties. A synthetic variety is synthesized from genotype which have been tested combining ability. Only genotypes which combines well with each other in all combinations are put into a synthetic variety.

The operations involved in the production of the synthetic varieties are i) Evaluation of lines for GCA ii) Production of synthetic varieties iii) Multiplication of synthetic varieties.

Merits of synthetic varieties:

• It utilizes heterosis in cross pollinated crops in which floral structures causes difficulties in the pollination control. It particularly used in the improvement of forage crop species.
• Production of synthetic varieties have great value in those region where the cost of the hybrid seed is high relative to the value of the expected crop.
• The grater variability of the synthetic varieties permits more flexibility to meet the changeable growing condition of the marginal area.
• The performance of synthetic varieties can be adversely affected by lines with the relatively poor GCA.

Demerits of synthetic varieties:

• The performance of synthetic varieties are lower than that of hybrid varieties because hybrid varieties exploits both GCA and SCA whereas synthetic varieties only exploits SCA.
• Synthetic varieties can be produced and maintained only in the cross pollinated crops whereas hybrid varieties can be produced in case of both self and cross pollinated crops.
• The performance of synthetic is adversely affected by lines with relatively poorer GCA.

Fig 6: Steps involved in production of synthetic varieties of cross pollinated crop species, From B.D. Singh 1990

3. Conclusion:

For breeding cross pollinated crops, breeders used mainly different methodology of population improvements. These methods are relatively easy to perform and requires less labour and technical knowhow. Under population improvement the most widely used method is mass selection because of its relative ease to breeders. Other methods under population improvement are the improvement of mass selection or the modified form of the mass selection.

In resent time production of hybrid and synthetic varieties have become popular. Hybrid varieties are high yielding than traditional varieties, old landraces, pure line varieties and exploits both GCA and SCA of crop involved in the hybridization program. Synthetic varieties are also becoming popular because they are widely used in the improvement of those cross pollinated crops whose pollination control is difficult. Mainly forage crop improvement program produces synthetic varieties.