Breeding methods, Biology tutorial

Mode of reproduction:

The mode of reproduction of a crop finds out its genetic composition that, in turn, is the deciding factor to build up appropriate breeding and selection processes. Knowledge of mode of reproduction is as well necessary for its artificial manipulation to breed enhanced kinds. Only such breeding and selection processes are appropriate for a crop which doesn't interfere by its natural state or make sure the maintenance of such a state. It is due to such reasons that imposition of self-fertilization on cross-pollinating crops leads to the drastic reduction in their performance.

For this teaching aim, plant breeding processes are presented as four categories:

a) Line breeding (or autogamous crops)

b) Population breeding (or allogamous crops)

c) Hybrid breeding (mainly allogamous crops, a few autogamous crops)

d) Clone breeding (vegetative propagated crops).

Self fertilizing crops or autogamous crops:

Certain limitations caused the methods for self fertilization (that is, partial and full self fertilization) to build up in a number of plant species. A few of the reasons why a self fertilizing process of reproduction is so efficient are the effectiveness of reproduction, and also decreasing genetic variation and therefore the fixation of highly adapted genotypes. Nearly no inbreeding depression takes place in self fertilizing plants as the mode of reproduction lets natural selection to occur in wild populations of such plants.

Vital steps in the enhancement of self fertilizing crops are the choice of parents and the recognition of the best plants in the segregating generations. The breeder must as well encompass definite goals with the choice of parents. Self fertilizing is simpler to maintain, however this could lead to misuse of seed.

Several farm and domestic significant, self fertilizing crops comprise rice, maize, Sorghum, Millets, soy beans, cowpea beans, groundnuts, potatoes, tomatoes and so on.

Mass selection:

This process of selection depends mostly on selection of plants according to their phenotype and performance. The seed from chosen plants are bulked for the next generation. This process is employed to enhance the total population by positive or negative mass selection. Mass selection is simply applied to a limited degree in self fertilizing plants and is an efficient process for the enhancement of land races. This process of selection will merely be efficient for highly heritable traits. One shortage of mass selection is the large influence which the environment has on the growth, phenotype and performance of the single plants.

A plant developed by this process will be more uniform than such developed by mass selection as all of the plants in such a variety will encompass the similar genotype. The seed from chosen plants are not added altogether however are kept apart and employed to carry out offspring tests. This is done to study the breeding behavior of the chosen plant.

Stratified mass selection for ear size over 22 cycles has drastically modified plant phenotype in the maize population Zacatecas 58. Plants in the C22 cycle were 50 cm taller, had double the leaf area index, reached anthesis 7 days later and had a 30% higher harvest index than C0. Differences in the growth were detected early in ontogeny. The root growth of C22 surpassed that of C0 and the ratio of shoot dry mass to root dry mass was decreased by around 12%, from 8.0 ± 0.2 to 7.1 ± 0.1. Analysis of yield components revealed that C22 was superior to C0 in grain weight, number of rows per year, number of grains per row and the net yield per unit area. As the two genotypes were phonologically dissimilar, planting density optima are probably dissimilar for each population.

Selection of cross-pollinated crops:

Plant species where normal mode of seed set is via a high degree of cross-pollination encompass features reproductive features and population structure. Survival of self-sterility, self-incompatibility, imperfect flowers and mechanical obstacles make the plant dependent on foreign pollen for normal seed set. Each and every plant receives a blend of pollen from a huge number of individuals each having dissimilar genotypes. Such populations are characterized through a high degree of heterozygosity having tremendous free and potential genetic variation, which is maintained in a steady state by free gene flow among individuals in the populations.

In the growth of hybrid varieties, the aim is to recognize the most productive heterozygote from the population, which then is generated with the exclusion of other members of the population.

Mass selection:

It is the easiest, simplest and oldest process of selection where individual plants are selected based on their phenotypic performance and bulk seed is employed to generate the next generation by mixing it. Mass selection confirmed to be quite efficient in maize enhancement at the initial phases however its efficacy particularly for enhancement of yield, soon came beneath severe criticism which culminated in the refinement of the process of mass selection. The selection after pollination doesn't give any control over the pollen parent as outcome of which efficient selection is limited just too female parents.

The heritability estimates are decreased by half, as only parents are employed to harvest seed while the pollen source is not known after the cross pollination has occurred.

Recurrent selection:

This kind of selection is a refined version of the mass selection procedure and distinct as follows:

- Visually chosen individuals out of the base population experience progeny testing

- Individuals chosen on basis of the progeny test data are crossed by each other in each and every possible manner to produce seed to form the latest base population.

1) Half-sib selection with progeny testing:

Selections are made based on the progeny test performance rather than phenotypic appearance of the parental plants. Seed from the selected half-sibs that have been pollinated through random pollen from the population, is grown in an unreplicated progeny row for the main purpose of selection. A part of the seed is planted to find out the yielding capability, or breeding value, for any character of each plant. The seed from the most productive rows or remnant seed from the outstanding half-sibs is bulked to complete one cycle of the selection.

2) Full-sib selection having progeny testing:

A number of full-sib families, each generated by making crosses among the two plants from the base population are computed in replicated trials. A portion of each full-sib family is saved for recombination. Based on the computation, the remnant seed of chosen full-sib families is employed to recombine the best families.

Breeding of Asexually Propagated Crops:

Asexual reproduction covers up all such modes of multiplication of plants where normal gamete formation and fertilization doesn't occur making these distinctly dissimilar from normal seed production crops. In the absence of sexual reproduction, the genetic composition of plant material being multiplied remains necessarily the same as its source plant.

Clones of mother plants can be made by the exact genetic composition of the mother plant. Superior plants are chosen and propagated vegetative; the vegetative propagated offspring are employed to build up stable varieties devoid of any deterioration due to the segregation of gene combinations. This unique feature of asexual reproduction helped to build up a number of cultivars of fruits and vegetables comprising apples, grapes, pears and peaches.

1) Enhancing asexual plant material via selection:

The selection in such crops is limited to the material introduced from other sources, like field plantations. The enhancement of asexually propagated plants via induced mutations has distinct benefits and limitations. Any vegetative propagule can be treated by mutagens and even a single desirable mutant or a part of a mutated propagule (or chimera) can be multiplied as an enhanced kind of the original variety.

2) Selection of asexual plants:

Selection, in case of the asexual plants, can be stated as the selection of the best performing plant and the vegetative propagation thereof. Since plants are not completely genetically stable, it can be expected that deviations would take place via the years. Selection is therefore an ongoing procedure where deviants are chosen or eliminated from the selection program. The main aim of selection is to better the quality and yield of forthcoming plantations. Different approaches can be followed in the selection method of asexual plants, like mass selection and clone selection from the clone blocks.

In the mass selection there are various factors which should be considered when choosing plants in a mother block, example: vineyard. Time of selection is a big factor, as you have to choose when most of the features of the plant are clearly showing. With asexual perennials the best time is just before harvest. For the best outcomes the chosen plant should be computed throughout the next season, when growth-abnormalities, leave disfigurations and virus symptoms are best visualized. Mass selection is done yearly on the similar plant for a minimum of three years. A plant which doesn't conform to the needs in any given year of the selection cycle is discarded from the program.

3) New clone development:

The growth and registration of new clones occur by means of local clone selection in the old plantations, and also the importation of high quality clones from abroad, for local computation. A clone is the vegetative offspring of one particular mother plant; it doesn't show any genetic, morphologic or physiologic deviations from the mother plant. Evaluation occurs by the different chosen clones after selection. A few plants reproduce by (more or less strict) self-fertilization where pollen from a plant will fertilize reproductive cells or ovules of the similar plant. Other plants just (mostly) let cross-pollination where pollen from one plant can only fertilize a different plant. 

Asexual propagation (or vegetative propagation) can as well take place in plants (example: runners from sweet potato Ipomea batatas plant or suckers from Plantains or bananas Musa spp.) which provides a new plant that is genetically similar to its parent plant. All such differences change the way plant breeders work. Apomixis is the phenomenon that seeds are generated, however in a necessary asexual manner, so that parent and offspring belong to one clone simply as in the case of normal asexual propagation.

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