Clonal cultivars
Walter R. Fehr and Walter P. Suza
Readings:
- Chapter 33 [PDF], Principles of Cultivar Development. Vol. 1: Theory and Technique, by Walter R. Fehr (Access the full book)
- Chapter 3 [PDF], Hybridization of Crop Plants, by Walter R. Fehr and Henry H. Hadley (Eds.) (Access the full book)
There are three basic steps in the development of a cultivar that is planted commercially by the use of clones.
Step 1. Develop a segregating population.
The cultivars or experimental clones used as parents are highly heterozygous and homogeneous. When two parents are crossed, it does not matter how many plants of each parent are used because all the plants of a parent are genetically identical. When two parents are crossed, every locus of a parent that is heterozygous will segregate, which means that every gamete produced by a parent is genetically different. Consequently, when the gametes of the two parents unite, every hybrid seed is genetically different. Although the hybrid seed can be referred to as the F1, it represents the segregating population that the breeder generally will use for selection. Therefore, the breeder attempts to obtain as many hybrid seed of a cross as are desired for sampling the genetic variation of the segregating population. Accidental self-pollination generally is not a concern because the resulting plants will have inbreeding depression and will be discarded due to inferior performance.
Plant species that reproduce by apomixis represent a unique class of clonal cultivars because they are propagated commercially by seed. The two types of apomixis are facultative and obligate. Facultative apomicts, like Kentucky bluegrass, seed is produced asexually and sexually. The asexual seed is genetically identical to the mother plant on which it is borne. The sexual seed is genetically heterogeneous because it results from the union of gametes from heterozygous plants. Obligate apomicts, like buffelgrass, rarely produce any seed sexually. Additional information about the two types can be found in chapter 2 of Principles of Cultivar Development and pages 41–62 of Hybridization of Crop Plants.
The sexual reproduction of facultative or obligate apomicts is necessary to form a segregating population. Seed harvested from the female parent will include both asexual and hybrid seed. When the seed is planted, the individuals that are identical phenotypically to the female parent are considered to have arisen through apomixis and are discarded. Those that are different from the female parent are assumed to have arisen through sexual reproduction and are used for selection.
The difficulty in crossing parents of obligate apomicts is the limited amount of sexual reproduction. Methods of overcoming this problem are discussed in pages 57–60 of Hybridization of Crop Plants.
Step 2. Select superior plants (clones).
Every hybrid plant obtained from the cross of two clonal cultivars is a potential new cultivar. The first phase of selection is to identify the individual hybrid plants that have the characteristics desired in a new cultivar. The next phase is to clonally propagate the selected individuals for more extensive evaluations that occur over multiple locations and years. An example of this evaluation process is outlined for potato on pages 386–387 of Principles of Cultivar Development, Vol. 2.
Step 3. Production and commercial distribution of seedstock
This will be covered in more detail below. An example of the process of developing a clonal cultivar is provided for sugarcane in the following presentation by Sheilah Oltmans-Deardoff:
Step 4: Preparation of seedstock for commercial planting
The focus here is on the production and commercial distribution of seedstock. The term “seed” refers to vegetative propagules, such as tubers and rhizomes, or true seed of species that reproduce by apomixis, such as Kentucky bluegrass or buffelgrass.
One of the challenges in the multiplication of clonally propagated cultivars is the potential for the seedstock to carry diseases, particularly viruses. Diseases can be transmitted by contact with an infected clone or the tools used for propagation. To prevent this from happening, various methods of managing seedstock have been adopted.
Example 1: Potato
The testing of potato seedstock (tubers) to detect the presence or absence of transmissible viruses, also known as virus-indexing, is a requirement in North America. An excellent summary of the propagation procedures used to generate disease-free potato seed-stock was developed by staff at Oregon State University, archived here: Propagation Procedures, Foundation Potato Seed Program [PDF].
One explanation for the absence of viruses in meristem tissue is their lack of vascular bundle cells, which eliminates the mode of transport of the virus and restricts their spread. Another explanation is that cells in meristem tissues divide more rapidly than the rate of virus replication. Thus, new meristems tissue escapes virus infection.
Other methods of eliminating viruses in potato tubers involve heat treatment or a combination of both meristem culture and heat treatment. Rare mutations can occur during the tissue culture process. To ensure that plants produced by tissue culture are of high quality and uniformity as their parents, molecular markers such as SSR can be used for cultivar identification.
Example 2: Sweet potato
The production of virus-free sweet potato involves the use of meristem culture to produce plants that are later tested for the presence of viruses by grafting them onto a virus-sensitive (indicator) relative of sweet potato known scientifically as Ipomea setosa. After grafting, the indicator plant that is used as the rootstock is monitored for disease symptoms. If no symptoms are observed on the indicator plant, the meristem- cultured plants used as the scions are considered virus-free.
The procedure is described in greater detail by Xu et al (2024).
Example 3: Banana
All the banana diseases are thought to spread through the exchange of vegetative stock (suckers) among farmers. Virus-indexing for banana involves the collection of suckers from the field and growing them in disease monitoring facilities. The plants are screened for diseases and, if negative, are used for tissue culture production of disease-free stocks. Genetic tests using DNA markers may be applied to establish that the plants produced through tissue culture are the same genetically as their parent. Virus- and disease-free stocks are subsequently distributed to farmers.
For more information about the production of disease-free banana seedstock, see this report on Managing Banana and Citrus Diseases [PDF].
Example 4: Kentucky bluegrass
‘Awesome’, JPR 5:5-10 (2011)
Awesome was derived from a cross between ‘Limousine’ as the female parent and Midnight’ as the male parent. Limousine seed was sown in flats and allowed to germinate in the greenhouse. The seedlings from the greenhouse were transferred to a field nursery. Plants that looked different from Limousine were flagged and their seed was harvested individually for subsequent evaluation. Awesome is different from Limousine with respect to size, shape and color of its seedheads. The average level of apomixis in Awesome is about 95%. Awesome is protected under the United States Plant Variety Protection (PVP) Act. A description of seed production of Kentucky bluegrass is provided in this presentation: Kentucky_bluegrass_seed_prod [PPT]
Applied Learning Activity 4
Answer the following questions for each of the four cultivars listed below:
- Paspalum – ‘Aloha’, JPR 5:22-26 (2011)
- Redbud – ‘Ruby Falls’, Hort. Sci. 45:146-147 (2010)
- Strawberry – ‘Valley Sunset’, Hort. Sci. 45:663-665 (2010)
- Sweet potato – ‘Liberty’, Hort. Sci. 46:125-129 (2011)
- What were the characteristics that the breeder wanted in the new cultivar when the breeding program was designed?
- What were the parents used to form the segregating population?
- Outline season-by-season how the selection process was carried out from the time individual plants from the segregating population were grown until the final selection of the cultivar was made. When indicated by the author, describe the number of individuals from the population that were evaluated each season, whether individual plants or plots with multiple plants were used for evaluation, and the number of locations and replications used for testing.
- What were the characteristics of the individual that led to its release as a clonal cultivar?
Applied Learning Activity 5
For the following four cultivars, provide the following information:
- Paspalum – ‘Aloha’, JPR 5:22-26 (2011)
- Redbud – ‘Ruby Falls’, Hort. Sci. 45:146-147 (2010)
- Strawberry – ‘Valley Sunset’, Hort. Sci. 45:663-665 (2010)
- Sweet potato – ‘Liberty’, Hort. Sci. 46:125-129 (2011)
- What part of the plant is used for commercial propagation?
- What is the method used to multiple and prepare the seedstock for distribution?
- Who distributed the seedstock commercially?
- What legal protection was sought for the cultivar, if any.
References
Brede, A. D. 2011. Registration of Turf-Type Kentucky Bluegrass Cultivars ‘Awesome’, ‘Blue Velvet’, ‘Courtyard’, ‘Ginney’, and ‘Perfection’. J. Plant Reg. 5:5-10.
Fehr, W. R. (ed). 1987. Principles of Cultivar Development. Vol 1. Theory and Technique. McGraw-Hill, Inc., New York.
Fehr, W.R., Hadley, H.H. 1980. Hybridization of crop plants. American Society of Agronomy, Madison, Wisconsin.
Jackson, D. M., H. F. Harrison, J. A. Thies, J. R. Bohac. 2011. ‘Liberty’ Dry-fleshed Sweetpotato. Hort. Sci. 46:125-129.
Jamieson, A. R., K. R. Sanderson, J. Privé, R. J. A. Tremblay. 2010. ‘Valley Sunset’ Strawberry. Hort. Sci. 45:663-665.
Scully, B. T., R. T. Nagata, D. M. Sistrunk, R. H. Cherry, G. S. Nuessly, K. E. Kenworthy, J. DeFrank. 2011. Registration of “Aloha” Seashore Paspalum. J. Plant Reg. 5:22-26.
Werner, D.J., L. K. Snelling. 2010. ‘Ruby Falls’ and ‘Merlot’ Redbuds. Hort. Sci. 45:146-147.
Xu, Ying, Li-Xiang Wang, Chen Chen, Shao-Shan Ma, Rui Zhou, and Ai-Sheng Xiong. 2024. Virus-Free Sweet Potato Industry: Development Status and Production Suggestions. Horticulturae 10(9): 979. https://doi.org/10.3390/horticulturae10090979
