Plant breeding aims to produce gene combinations that improve crop yield.
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A plant species is not strictly defined by the inability of two individuals to routinely produce offspring that is fertile; interspecific hybridizations do occur naturally and sometimes produce fertile offspring.
The importance of Plant Breeding:Plant breeders created the crop varieties that made it possible to increase harvest yield (tons/hectare). This activity started in earnest in the developed countries in the 1920s and in the developing countries in the 1950s (Green Revolution).What would the world look like (area planted to crops) without plant breeding?
50% of these in-
creases are due
to better varieties
50 % to more
Plant breeders define their selection criteria. New Green Revolution varieties were selected for specific characteristics:
Plants not dependent on day length for flowering (flowers any time of year)
Plants responsive to nitrogen fertilizer
All genetic material was freely
available and exchanged
Even after much breeding certain crops perform best in certain climatic/soil areas. That is where they are grown. The areas for corn (originally from Mexico) and soybeans (domesticated in China) overlap, but sorghum (from Africa) is grown in a different area. Wheat (from the Middle East) is different again (not shown).Yield trials must be done in many different places at once if companies want to sell seeds to farmers in the entire area.
In the northern hemisphere, winter wheats are planted in the
fall, from September through December. Winter wheat sprouts
before freezing occurs, then becomes dormant until the soil
warms up in the spring. Persistent snow cover might be dis-
advantageous; however, winter wheat needs a few weeks of
cold before being able to flower. The wheat grows and
matures until ready to be harvested by early July. Winter
wheats are hard wheats and have more protein than softer spring
wheats. Winter wheats are used for yeast breads.
Spring wheat is any kind of wheat sown in the spring.
Spring wheat has less protein than winter wheat and is
used for cakes and other baked goods.
Introgression or back crossing
With each subsequent backcross generation, there are fewer genes from the red parent. Each time you select in the field for the phenotype you are looking for.
In the 7th generation you end up with 1.5 % of the red genome (400 genes) and with the crucial genes that encode the desired phenotype.
However, because of crossing over it does not go down exactly as 50, 25, 12.5, 6.25, etc in all individuals. % of each genome varies considerably among individuals. Molecular mapping allows you to determine the % and if the locus you are interested in is present. But… how do you choose the right progeny at each step? Molecular maps and marker assisted selection are crucial, but you need to know the markers associated with your trait.
This is an “old-fashioned” linkage map. New linkage maps are made
exclusively with unique short molecular probes called CAPs
(cleaved amplified polymorphic DNAs). Linkage means that when
you make crosses and you look at the progeny, certain traits (or
molecular markers) are linked and do not segregate.
Wheat genotypes on low P calcareous soils
Screening of existing genotypes (landraces) for use in breeding programs. Some traits or phenoptypes can simply be identified and the seeds distributed to farmers. For others, introgression may be necessary.
Soybean genotypes on low P acidic soils
Calcareous (kal′ker·ē·əs)soils have high
levels of calcium and magnesium
carbonate, with a pH of 7.6 to 7.8, which is too high for optimal growth of most crops. Iron is not available at high pH.
Acidic soils with a pH of 5.0 to 5.5 are found in many tropical regions and are often highly weathered. The low pH solubilizes aluminum from the aluminosilicate soil minerals , which is toxic to the plants.
Plant libraries of recombinant inbred lines (RILs)
and introgression lines (ILs) are needed to screen for
phenotypes and exploit structural and functional
Finding interesting genes in wild relatives
One of the many children is bigger!
A plant library of introgression lines (ILs) Introgression of Indica rice chromosome segments into Japonica rice.Only 3 of the 12 chromosomes are shown. A library of 88 “lines” covers the entire rice genome. These lines can be tested in the field, and a specific trait (e.g. drought resistance of Indica rice can be assigned to a specific region of the Indica genome. You won’t know exactly which gene, because each segment has 50-200 genes.
Making such a plant library,
in combination with
is a big job!