Pure Line

A pure line is a population that originates through inbreeding or self-fertilization, descendants always equal to each other and to the progenitors […]

Concept of Pure Line

A pure line is a population that originates through inbreeding or self-fertilization, descendants always equal to each other and to the progenitors for a certain characteristic.

In the early 20th century, the Danish botanist Wilhelm Johannsen defined pure line as a group of genetically identical individuals, descendants of a single self-fertilized individual. From 1900 to 1907, Johannsen used a variety of beans, Phaseolus vulgaris var. nana to perform a series of experiments (such as the pea, self-fertilization is a natural process in beans). The conclusions that resulted were:

  • The selection of a pure line corresponds to the process of separation of pure line(s) of an original mixed population (i.e. in which there is genetic variability);
  • Individuals of a pure line have the same genotype;
  • Due to the previous point, the selection within a pure line is not effective;
  • Variations that appear within a pure line are due to environmental factors.

Pure line selection

Pure lines are obtained through repeated self-fertilizations of heterozygous individuals or through a sequence of inbreeding in a population initially heterozygous.

The self-fertilization of autogamous species allows obtaining homozygous individuals after 6 to 8 generations. After the 5th generation, about 97% of the genes that were originally heterozygous (in the mother plant) will be homozygous.

Inbreeding also allows reaching an almost complete homozygosity, however, the process is more time consuming. Approximately 20 generations of crosses between siblings are required to obtain a pure line in mammals, for example.

The first step is to select the “parents” with the wanted feature(s) and to cross them. Only the descendants with the feature(s) are retained for new crossing. The process is repeated over several generations until a pure line with individuals who exhibit all the wanted feature(s) is obtained.

Some advantages of pure lines selection

  • Easy and low cost crop improvement method ;
  • Improved and uniforms varieties both in aspect as in performance (higher-yielding, more disease-resistant, etc.);
  • In a laboratory context, the use of a pure line allows to detect any mutation or abnormality.

Some disadvantages of pure lines selection

  • Weak adaptive capacity;
  • Very narrow genetic base;
  • Requires time and space.

Applications of the pure lines selection

  • Allows standardization of products of crops (colour, size, shape, texture, etc.);
  • Allows to obtain different purified varieties;
  • Can be performed after hybridization for segregation of different populations;
  • Pure line individuals can be used in crossing programs for pedigree certification;
  • Pure lines are used in research in biology, medicine and biochemistry to study mutations and other biological mechanisms.
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References:

  • Archunan, G. (2004). Genetics. 1st ed. New Dehli: Sarup & Sons.
  • Bonneuil, C. and Thomas, F. (2009). Gènes, pouvoirs et profits. 1st ed. Versailles: Quae.
  • Müller-Wille, S. (2007). Hybrids, pure cultures, and pure lines: from nineteenth-century biology to twentieth-century genetics. Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences, 38(4), pp.796-806.
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