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Line development and hybrid evaluation (example maize)

Line development and testing within the heterotic pools

Source population

Open-pollinated cultivars, synthetic cultivars, single crosses, back-crosses, dou-
ble crosses, related line crosses, and exotic germplasm can be used as source
population. The requirements for source material in hybrid breeding are a high
performance level and broad genetic variation for all agronomically important
traits such as yield, resistance and quality.

Line development

The aim during the development of inbred lines is to achieve homozygosity while
ensuring plant vigor and per se performance, which usually decreases with the
level of homozygosity. The following methods are used to develop inbred lines:

 Bulk method

Genetic variation is created by arti cial hybridization between chosen parents.
The F1 to F 5 is then grown as bulk population wherein no conscious selection is imposed. It is assumed that the frequency of adapted genotypes to the environment, in which the bulk populations are grown (i.e., stress environments), will increase. It is therefore important that the bulks are grown in representative environments.
Individual plants showing desirable characteristics are selected at the F6 stage. From each plant, an ear row is grown and the produce from the best lines/rows are bulk harvested, for initial yield trials. More advanced yield trials are grown from bulk harvest of desirable populations.
The major advantage of the bulk method is that conscious selection is not attempted until plants have been selfed for a number of generations. Additionally it is the cheapest method for producing inbred lines. The disadvantage is however the time from initial crossing until nal yield trials. Additionally natural selection in the bulk populations is not always that which is favorable for growth in agricultural practice.

Double haploids are also grown in bulks as the time from crossing to evaluation is minimized.

 Pedigree method

The scheme begins by hybridization between the chosen homozygous parental lines, and segregating F2 populations are obtained by sel fing the heterozygous F1 lines.
Single plant selection is carried at the F2 through to the F6 generation. Single plants are selected from among the segregating F2 population and are grown in ear rows at the F3 generation. The best single plants are selected from the better plant rows and these are grown in plant rows again at the F4 stage. This process is continued until F6 where neat homozygous lines are reached. As inbreeding reduces the genetic variation within families and increases the genetic variation among families, the efficiency of selection among lines increases with increasing homozygosity while it decreases within lines.
This scheme is very laborious and expensive as a considerable area and amount of record keeping is required. Selection is conducted on single plants where error of observation is very high especially for quantitatively inherited traits.

The pedigree method might be the most widely used breeding scheme.

 Bulk/ pedigree method

The scheme begins by hybridization between the chosen homozygous parental lines, and segregating F2 populations are obtained by sel fing the heterozygous F1 lines.
Individual plants are selected in the segregating F2 populations and are grown in ear rows at the F3 stage. Selected F3 populations are bulk harvested and preliminary yield trials are grown at the F4 stage by planting the bulked F3 seed. F5 and F6 bulk seed yield trials are grown by planting bulked seed from the previous year trial. At the F6 stage single plant selections are once again made from nearly homozygous lines. Progeny of these plant selections are grown then as ear rows at F7; second cycle initial yield trials at F8 and more advanced yield trials at F9.

The advantage of the bulk/pedigree method is that inferior lines are identi ed and discarded early in the breeding scheme. However, the number of inbred lines that can be evaluated is reduced. It might be that this is the most commonly used method to produce inbred lines.

modified breeding methods

The modi ed pedigree trial breeding schemes enable yield trials to be grown simultaneously to pedigree selection.
Single plants are selected from amongst segregating F2 populations. One or more single plants are selected from each of the desirable F3 plant rows, and the remainder of the row is bulk harvested. The single plant selections are grown as plant progeny rows at F4. Simultaneously the harvested F3 bulk is planted in a preliminary yield trial. The seed from the bulk yield trial is used to plant a more extensive bulk yield evaluation at the F5 stage. Based on the results from the F4 bulk yield trial the most productive population are identi ed. Single plants are selected from the corresponding plant progeny rows and the remaining row is bulk harvested for a further yield trial the following year at the F5 generation. This process is repeated in the F5 and F6 stage, where near homozygous lines are reached. With this method yield and other quantitatively inherited traits can be evaluated in repeated bulks, while single gene traits can be screened for on a single plant basis.

Backcrossing

Backcrossing is used to transfer one or a few traits/genes from the donor parent to the recurrent and most desirable inbred line. The use of DNA markers has facilitated both the speed and accurate recovery of the recurrent parent, and the reduction of linkage drag. Backcrossing
in hybrid breeding is used for two purposes
1) To introduce a single gene into an already desirable inbred parent
2) To produce near isogenic lines, which can reduce the cost of seeds P1xP2=F1;  F1xP1=NIL1 and F1xP2=NIL2 develop near isogenic lines which can be used as modi ed single cross (NIL1xNIL2).

 Double haploid technology

The population of homozygous lines has to be a random representative of the gamete array possible. The genetic combinations revored from haploid systems may be disproportionally composed from one parent, as there is the possibility that this parent showed a greater propensity for regeneration in culture and would determine a higher frequency of this genome in the population of gametes. Induction of haploid lines can be achieved by an inducer or by anthere culture or intraspecific crosses (such as maize x wheat).

 Line evaluation

The goal is to discard lines with poor combining ability at an early stage (S1 or S2) to reduce the material to be tested in later. Early testing makes only sense when the per se performance of the lines is high. With this the genetic variance does not change as only the heterotic loci are part of the genetic variance. Early generation testing has to be integrated in multi-stage selection.
During the inbreeding process the genotypes are selected for

  • per se performance such as yield, which is important for seed production, or good pollinator properties. For some traits there is a high correlationbetween per se performance and test-cross performance, such as maturity or oil concentration. For yield, the correlation is rather low.
  • GCA via topcross test. The general combining ability (GCA) is estimated by crossing the inbred lines to one common tester (an advanced inbred line of the opposite heterotic pool). The GCA is determined by comparing the performance of each progeny, assuming that the only di erence between the progenies can be attributed to di erent inbred parents. Breeders tend to test more inbreds than to increase the number of test parents, which can reduce the cost of seed production, by convergence, called convergent improvement. The performance of 3-way or 4-way hybrids is predicted rather than tested in the fi eld.
  • { Test S0 plants for GCA: The tester is emasculated and pollinated with the pollen of several S0 plants. In parallel the S0 plants are selfed. According to the test cross performance, the selfed seed is then selected. This procedure is laborious as the pollen has to be collected manually.
  • { Test S1 lines for GCA: The S0 plants are selfed and grown in observation plots. The seed of the S1 lines are used to sow 1) a topcross nursery wherein the S1 lines are emasculated and crossed to a common tester and 2) observation plots wherein the S1 are selfed and evaluated according to their per se performance. Only S1 lines that correspond to selected topcross hybrids will be retained in the breeding program. The test of S1 lines requires one more season. However, a larger amount of seed quantity is available which can be used to evaluate the testcrosses at di fferent locations.
    Selected S2 families are further selfed to the S5 generation where a second round of topcross evaluation is performed. At this stage, any superior inbred lines are then considered for release to the hybrid commercialization side of the breeding activities.

Heterotic pools in maize

Flint: cold tolerant
Dent: Sti stalk and non-sti stalk
Iowa Sti Stalk
Lancaster

Hybrid evaluation

Stage 1: testcross performance of experimental lines in a few locations (5)
Stage 2: hybrid evaluation of selected lines in more hybrid combinations and locations (20)
Stage 3: Hybrid evaluation in 50 locations on research plots in several hybrid combinations
Stage 4: evaluation of best pre-commercial hybrids in 75 research plot locations and 200-500 on farm locations
Stage 5: Hybrid performance verification in 75 reseach plot locations and 300-1500 on farm strop plot tests.

 

 

April 6th, 2013
Topic: Crop Science, Plant breeding Tags: None

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