Pure-Line Cultivars

There are four steps in the development of pure-line cultivars.

Step 1: Develop a segregating population.

The first step in developing a pure-line cultivar is to develop a segregating population. The most common type of population is obtained by crossing two elite parents. A three-parent cross or backcross population is used when it is unlikely that a two-parent cross will have an adequate frequency of segregates with one or more of the traits desired in the new cultivar. The use of more complex populations generally is limited to situations in which the breeder chooses to do recurrent selection for multiple cycles for improvement of a particular trait.  For example, recurrent selection for improved resistance to iron chlorosis on high pH soils was carried in soybean. The initial cycle 0 population was obtained by intermating 10 elite lines and 10 plant introductions with the highest level of resistance to iron chlorosis available. Recurrent selection by the use of S_0:1 lines was successful in developing genotypes that had an exceptionally high level of resistance to iron chlorosis.

In making crosses between pure-line parents, it generally is assumed that the parents are homozygous and homogeneous for quantitative traits.  Consequently, the breeder will only use as many plants of each parent for crossing as is necessary to obtain the desired number of hybrid seed.  The number of hybrid seed will depend on the type of population that is developed.  For a two-parent cross, all the hybrid seeds are considered genetically identical; therefore, the breeder will obtain enough to generate the size of the F₂ population that is desired.  For a three-parent or backcross population, the hybrid plants from the initial two-parent cross is crossed to a third parent or to the genotype used as the recurrent parent.  In the second cross, the breeder will try to obtain as many hybrid seed as possible to sample the segregating gametes from the two-parent F₁ individuals.

Step 2: Develop pure lines.

You need to review and fully understand the alternative methods of developing pure lines that were discussed in the Inbreeding chapter in Plant Breeding Methods.  As indicated in those lessons, the method used by the breeder will be determined by the resources available and their personal experience

The generation when lines are derived for testing as potential cultivars is an important decision that must be made by the breeder.  The advantage of deriving lines in an early generation, such as the F₂ or F₃, is that it takes fewer years to develop a cultivar.  The disadvantage is that frequency of lines with the desired level of homogeneity will be less in early generations than for more advanced generations.  The generation of deriving lines often is influenced by the breeding schedule that commonly includes the use of the local environment and off-season nurseries.  For example, if crosses are made in the local environment, the breeder may be able to grow the F₁ and F₂ generations in an off-season nursery, followed by selection among F₃ plants in the local environment for subsequent testing as F₃-derived lines.

An example of the process of developing a pure-line cultivar is provided for soybean in the following images:

Step 3: Testing of pure lines as potential cultivars

The third step in the development of pure-line cultivars is the testing of lines until one is identified that merits release as a cultivar.   Initial evaluation of pure lines for yield and other traits generally is done in a limited number of replications and environments with a relatively small plot size.  The objective of the initial evaluation is to discard lines that the breeder feels are too inferior to warrant further evaluation.  The number of lines evaluated and the percentage of lines selected depends on the quality of the parents used to form the population, the number of traits that a line must have to be selected, the frequency of lines with an adequate level of homogeneity, the stringency of the breeder in making selections, and the number of lines that can be tested the next season.  Practical experience with a crop is needed to adequately manage these variables effectively.

For most self-pollinated species, the seed that is used for planting a field test is the self-pollinated seed harvested from the previous season of testing. This assumes that the equipment used for harvest is adequately self-cleaning to minimize seed mixtures from one plot to the next.  Specialized harvest equipment is available for this purpose.

Step 4: Seed purification and multiplication for a new cultivar

An important decision that the breeder must make is when to begin seed purification and increase of lines that have the potential to become new cultivars. The least expensive strategy is to delay the process until the testing is completed and the new cultivar has been chosen for release.  If this strategy is used, it delays the time when seed of the new cultivar will be available to the producer.  Most breeders prefer to have seed available to producer as soon as possible, which means they carry out seed purification and multiplication concurrently with field testing.  The disadvantage of this procedure is that funds will be spent on seed multiplication of lines that ultimately are not released.

The two methods used to produce breeder seed of a pure-line cultivar are mass selection and progeny testing.

Mass selection

Seed of a line inspected for uniformity is planted in an increase.  The seed commonly is obtained from the previous year of yield testing and may or may not be inspected for uniformity before it is planted.  The plants in the increase are inspected for uniformity, off-types are removed, and the remaining plants are harvested in bulk to obtain breeder seed.  The advantages of mass selection are that it is requires only one season and a minimum amount of labor and expense.  The disadvantage is that the genetic and phenotypic uniformity of the breeder seed generally is less than when progeny testing is used. A description of the use of mass selection for breeder seed production was provided for the spring barley cultivar Transit (JPR 5:270-272).

Progeny testing

When progeny testing is used for breeder seed production, individual plants are selected from a line, a progeny row is grown of each, and the rows are evaluated for uniformity and similarity to each other for phenotypic traits.  The selected progeny rows may be harvested in bulk or each may be harvested separately and inspected for seed traits before bulking.  If the breeder chooses, seed from progeny rows may be harvested separately and each may be grown separately a second season to further examine them for uniformity before they are bulked.  The advantage of progeny testing is greater genetic and phenotypic uniformity of the breeder seed than is only mass selection is used.  This can be important when a cultivar must meet standards for genetic purity for the seed certification.  The disadvantages of progeny testing are that it takes at least two seasons and requires more labor and expense than mass selection. A description of the use of progeny testing for breeder seed production was provided for the lentil cultivar Essex (JPR 5:19-21).

The following four pure-line cultivars are described in Journal of Plant Registrations.

• ‘Transit’ spring barley – JPR 5:270-272 (2011)
• ‘CL151’ rice – JPR 5:177-180 (2011)
• ‘Snowglenn’ winter durum – JPR 5:81-86 (2011)
• ‘Bailey’ peanut – JPR 5: 27-29 (2011)

For each cultivar, answer the following questions.

1. What type of cross (single cross, three-way cross, or other) was used to obtain the segregating population in which selection was practiced? Why do you think the breeder chose this type of cross?
2. What type of parents (experimental line, cultivar, or other) was used to develop the population?
3. In what generation was the cultivar derived?
4. Outline season-by-season the method used to derive the line. If the details are not clear, use your best judgment in describing what likely was done.  End your answer when the line is ready for testing to determine its merits as a new cultivar.

 Answer the following questions for each of the four pure-line cultivars below.

• ‘Dan’ winter hulless barley – JPR 5: 1-4 (2011)
• ‘Avalanche’ navy bean – JPR 5:170-176 (2011)
• ‘Barlow’ hard red spring wheat – JPR 5: 62-67 (2011)
• ‘ Merl’ soft red winter wheat – JPR 5: 68-74

1. Describe season-by-season the number of locations and replications used for testing from the time the cultivar was derived until it was selected for release.
2. Describe the method the breeders used to prepare breeder seed.
3. What type of intellectual property protection, if any, was used for the cultivar?

References

Blanche, S. B., X. Sha, D. L. Harrell, D. E. Groth, K. F. Bear, L. M. White and S. D. Linscombe. 2011. Registration of ‘CL151’ Rice. J. Plant Reg. 5:177-180.

Brooks, W. S., M. E. Vaughn, C. A. Griffey, W. E. Thomason, J. J. Paling, R. M. Pitman, W. Dunaway, R. A. Corbin, J. C. Kenner, E. G. Hokanson, H. D. Behl, B. R. Beahm, S. Y. Liu, P. G. Gundrum, A. M. Price, D. E. Brann, D. L. Whitt, J. T. Custis, D. E. Starner, S. A. Gulick, S. R. Ashburn, E. H. Jones Jr., D. S. Marshall, M. O. Fountain, T. D. Tuong, D. P. Livingston, R. Premakumar, M. J. Kurantz, F. Taylor, R. A. Moreau, and K. B. Hicks. 2011. Registration of ‘Dan’ Winter Hulless Barley. J. Plant Reg. 5: 1-4.

Fehr, W. R. (ed). 1987. Principles of Cultivar Development. Vol 1. Theory and Technique. McGraw-Hill, Inc., New York.

Griffey, C. A., W. E. Thomason, R. M. Pitman, B. R. Beahm, J. J. Paling, J. Chen, P. G. Gundrum, J. K. Fanelli, D. W. Dunaway, W. S. Brooks, M. E. Vaughn, E. G. Hokanson, H. D. Behl, R. A. Corbin, J. E. Seago, B. C. Will, M. D. Hall, S. Y. Liu, J. T. Custis, D. E. Starner, S. A. Gulick, S. R. Ashburn, E. H. Jones Jr., D. L. Whitt, H. E. Bockelman, E. J. Souza, G. L. Brown-Guedira, J. A. Kolmer, D. L. Long, Y. Jin, X. Chen, and S. E. Cambron. 2011. Registration of ‘Merl’ Wheat J. Plant Reg. 5:68-74.

Hall, M. D., W. Rohrer-Perkins, C. A. Griffey, S. Y. Liu, W. E. Thomason, A. O. Abaye, A. Bullard-Schilling, P. G. Gundrum, J. K. Fanelli, J. Chen, W. S. Brooks, J. E. Seago, B. C. Will, E. G. Hokanson, H. D. Behl, R. M. Pitman, J. C. Kenner, M. E. Vaughn, R. A. Corbin, D. W. Dunaway, T. R. Lewis, D. E. Starner, S. A. Gulick, B. R. Beahm, D. L. Whitt, J. B. Lafferty, and G. A. Hareland. 2011. Registration of ‘Snowglenn’ Winter Durum Wheat. J. Plant Reg. 5:81-86.

Isleib, T. G.,  S. R. Milla-Lewis, H. E. Pattee, S. C. Copeland, M. C. Zuleta, B. B. Shew, J. E. Hollowell, T. H. Sanders, L. O. Dean, K. W. Hendrix, M. Balota, J. W. Chapin. 2011. Registration of ‘Bailey’ Peanut. J. Plant Reg. 5: 27-39.

Mergoum, M., S. Simsek, R. C. Frohberg, J. B. Rasmussen,T. L. Friesen, T. Adhikari. 2011. ‘Barlow’: A High-Quality and High-Yielding Hard Red Spring Wheat Cultivar Adapted to the North Central Plains of the USA. J. Plant Reg. 5:62-67.

Obert, D. E.,  A. Hang, G. Hu, C. Burton, K. Satterfi eld, C. P. Evans, J. M. Marshall, and E. W. Jackson. 2011. Registration of ‘Transit’ High β-Glucan Spring Barley. J. Plant Reg. 5:270-272.

Osorno, J. M., K. F. Grafton, G. A. Rojas-Cifuentes, J. R. Gelin, A. J. Vander Wal. 2011. A New Navy Bean for the Northern Plains: Registration of ‘Avalanche’. J. Plant Reg. 5:170-176.

Vandemark, G. J., K. E. McPhee, and F. J. Muehlbauer. 2011. Registration of ‘Essex’ Lentil. J. Plant Reg. 5:19-21.