Wednesday, March 23, 2011

Genetics

Wheat genetics is more complicated than that of most other domesticated species. Some wheat species are diploid, with two sets of chromosomes, but many are stable polyploid, with four sets of chromosomes  or six

  • Eionkron wheat is diploid .
  • Most tetraploid wheats are derived from wild emmer, T. dicoccoides. Wild emmer is itself the result of a hybridization between two diploid wild grasses, T. urartu and a wild goatgrass such as Aegilops searsii . The unknown grass has never been identified among now surviving wild grasses, but the closest living relative is Aegilops speltoides.The hybridization that formed wild emmer  occurred in the wild, long before domestication, and was driven by natural selection.
  • Hexaploid wheats evolved in farmers' fields. Either domesticated emmer or durum wheat hybridized with yet another wild diploid grass  to make the  wheats, spelth wheat and bread wheah These have three sets of paired chromosomes, three times as many as in diploid wheat.
The presence of certain versions of wheat genes has been important for crop yields. Apart from mutant versions of genes selected in antiquity during domestication, there has been more recent deliberate selection of alleles that affect growth characteristics. Genes for the 'dwarfing' trait, first used by japanes breed widingto produce short-stalked wheat, have had a huge effect on wheat yields world-wide, and were major factors in the success of the green revulation in Mexico and Asia. Dwarfing genes enable the carbon that is fixed in the plant during photosynthesis to be diverted towards seed production, and they also help prevent the problem of lodging. 'Lodging' occurs when a ear stalk falls over in the wind and rots on the ground, and heavy nitrogenous fertilization of wheat makes the grass grow taller and become more susceptible to this problem. By 1997, 81% of the developing world's wheat acreage was planted to semi-dwarf wheats, giving both increased yields and better response to nitrogenous fertilizer.

Wild grasses in the genus Triticum and related genera, and grasses such as rye have been a source of many disease-resistance traits for cultivated wheat breeding since the 1930s


Heterosis, or hybrid vigor , occurs in common  wheat, but it is difficult to produce seed of hybrid cultivars on a commercial scale because wheat flowers are complete and normally self polinate. Commercial hybrid wheat seed has been produced using chemical hybridizing agents; these chemicals selectively interfere with pollen development, or naturally occurring cytoplasmic male sterility systems. Hybrid wheat has been a limited commercial success in Europe , the USA and South Africa F1 hybrid wheat cultivars should not be confused with the standard method of breeding inbred wheat cultivars by crossing two lines using hand emasculation, then selfing or inbreeding the progeny many  generations before release selections are identified to be released as a variety or cultivar. made by crossing the wild goatgrass wheat ancestor Aegilops tauschii and various durum wheats are now being deployed, and these increase the genetic diversity of cultivated wheats.

stomata are involved in both uptake of carbon dioxide gas from the atmosphere and water vapor losses from the leaf due to water transpiration. Basic physiological investigation of these gas exchange processes has yielded valuable carbon  based methods that are used for breeding wheat varieties with improved water-use efficiency. These varieties can improve crop productivity in rain-fed dry-land wheat farms.




In 2010, a team of scientists announced they had decoded the wheat genome for the first time This announcement was widely misreported as representing a finished genome sequence. In fact, sequence data was produced which allows the identification of wheat genes, but the data was not assembled to represent the map of the genome.

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