Recurrent Selection

Walter R. Fehr and Walter P. Suza

Readings:

Introduction

The chapters you have been assigned for this section provide an overview of the methods used for recurrent selection. It is important to understand the alternative methods in preparation for the first chapters in Principles of Cultivar Development.

Synthetic cultivars

The most extensive use of recurrent selection is for the development of synthetic cultivars, particularly for forage and turf species. In these species, phenotypic selection alone or in combination with genotypic selection is a common practice. The general principles associated with phenotypic and genotypic selection will be illustrated with two cultivars.

The development of FreedomMR represents an example of the use of only phenotypic selection for cultivar development [JPR 2:205-207 (2008)]. The following steps were used by the breeder. The text in the shaded boxes are comments that are provided to assist in understanding the process and the alternatives that the breeder could have considered. These comments are not from the articles.

Cycle 0 population: The synthetic cultivar Freedom was the cycle 0 population used for development of FreedomMR.

The initial population used for recurrent selection is referred to as the cycle 0. Each subsequent cycle of selection is identified with a consecutive number.

Cycle 1: To develop the cycle 1 population, approximately 10,000 plants of the cycle 0 population were germinated in the greenhouse where they were exposed to natural infections of powdery mildew.It is common for breeders to evaluate large numbers of individuals each cycle for recurrent phenotypic selection. Each plant is a different genotype. The individual plants (genotypes) are commonly referred to as clones.

About 40% of the plants that were susceptible to the disease were discarded. The remaining 60% of the plants were transplanted to the field. About 8.7% of the plants in the field were discarded because they were susceptible. The remaining resistant plants were allowed to intermate by insect pollination.By allowing only resistant plants to intermate, the frequency of favorable alleles in the cycle 1 population would be greater than if the resistant and susceptible plants had intermated and seed was harvested from only the resistant ones. Details of this principle can be found on pages 174-178 of chapter 15 and is the subject of the homework problem for Lesson 13.

The seed from the resistant plants was harvested in bulk with a combine and represented the cycle 1 population.

The nursery in which selected clones are intermated is referred to as a polycross, as discussed in lesson 4. In a polycross nursery, the breeder has the option of growing a single plot of each plant or clonally propagating each plant into multiple replications. The breeder can choose to harvest the seed from all the clones in bulk or can harvest the clones individually and bulk equal quantities of seed from each. Details on alternative procedures that can be used for a polycross can be found on pages 181-188 of chapter 15.

Cycle 2 to cycle 5: The same procedure used for cycle 1 was repeated for cycles 2 to 5. The percentages of resistant plants selected varied among the cycles.

Final cultivar: The resistant plants from the cycle 5 population were selected further for absence of pubescence. It is assumed that the resistant plants without pubescence were allowed to intermate in a polycross to produce the breeder (Syn 1) seed of the cultivar FreedomMR.

The development of Warrior indiangrass is an example of the combination of phenotypic and genotypic selection [JPR 4:115-122 (2010)].

Cycle 0 population: The cycle 0 population was the cultivar Oto, which had a genetic background that traced to accessions from native prairies in southern Nebraska and eastern Kansas.

It is not unusual for breeders of forage and turfgrass species to obtain parent plants from commercial plantings, including pastures and golf courses.

Cycle 1: The first cycle of selection involved the evaluation of 146 half-sib families.

For species in which hybridization occurs by open pollination through wind or insects, half-sib families are formed in a polycross. For the development of Warrior, at least 146 individual clones from the cycle 0 population would have been planted in a polycross. The breeder has the option of replicating the clones in the polycross, as described on pages 421-422 of chapter 33. The clones intercrossed naturally by wind pollination. The seed harvested from each of the 146 clones in the polycross had a common parent, referred to as the tester. The 146 clones themselves served as the tester because their pollen was the source of the male gametes responsible for fertilization to produce the half-sib seed that was harvested. The subsequent evaluation of the half-sib seed was to determine the general combining ability of the 146 clones for quality and yield.

The evaluation of half-sib families for the first cycle of selection as done in this example is less common that conducting recurrent phenotypic selection first, followed by evaluation of half-sib families in later cycles. By conducting recurrent phenotypic selection first for highly heritable traits, the traits of individual clones evaluated as half-sib families may be superior to those selected from populations in which recurrent phenotypic selection was not performed.

The progeny from each of the 146 clones was planted in two replications in the field and data were collected for quality and yield.

During the time that the field test was underway, each of the 146 clones was grown in the field in what is referred to as a maintenance nursery. If a clone is one selected for future use, the vegetative tissue used for propagation is taken from the plant in the maintenance nursery.

There were 29 of the 146 clones selected based on their performance. Two cuttings from each of the 29 clones were used to plant a polycross with two replications.

By clonally propagating the selected individuals, all of the female and male gametes involved in production of the cycle 1 seed came from selected individuals. This principle is described on pages 106-110 of chapter 8 in Principles of Cultivar Development, Vol. 1.

The 29 clones intercrossed by wind pollination, the seed from each clone was harvested separately, and an equal quantity of seed from each clone was bulked to form the cycle 1 population.

Cycle 2: Restricted recurrent phenotypic selection (RRPS) was used to develop the cycle 2 population.

The features of RRPS are described on page 181 of chapter 15 in Principles of Cultivar Development, Vol. 1. They include the use of gridding and the intercrossing of only selected individual clones.

      1. Gridding: In the case of Warrior, there were 53 rows in the field, each of which had 14 plants. Approximately three plants in each row with the best visual characteristics were harvested for yield and other traits. The key principle is that the plants in one row of the grid were not compared to plants in other rows of the grid. After evaluation, 39 of the plants were selected for crossing.
      2. Intercrossing: After evaluation, 39 of the plants from the cycle 1 population were grown in a polycross. The seed harvested from the polycross was the cycle 2 population.

Cycle 3: RRPS again was used to obtain the cycle 3 population. The 38 selected plants were grown in a polycross. The seed obtained from the polycross constituted the cultivar Warrior.

Clonal, pure-line, and hybrid cultivars

Recurrent selection for these three cultivars types is used to improve populations from which superior individuals can be selected. Unlike synthetic cultivars, the improved population is not used as a cultivar per se.

The type of recurrent selection used for the three cultivar types is influenced by the feasibility of obtaining self-pollinated seed, the impact of inbreeding depression, the feasibility of obtaining hybrid seed, and the role of combining ability in assessing the genetic potential of an individual.

Clonal cultivars

Recurrent phenotypic selection is the most common method of recurrent selection used for species that are grown commercially as clonal cultivars. Combining ability is not a factor in identifying individuals that will perform well as a clonal cultivar, which minimizes the value of testing half-sib or full-sib families. Inbreeding depression limits the value of testing selfed progeny.

Pure-line cultivars

The use of self-pollinated individuals and their progeny is the most common method of recurrent selection for pure-line cultivars because self-pollinated seed is readily obtained. Recurrent phenotypic selection can be used for quantitative traits with high heritability, if it is feasible to obtain enough hybrid seed for the next cycle of selection when intermating selected individuals. Half-sib selection is possible, if enough seed can be obtained from an individual when crossed to a tester. Genetic male sterility has been used in some cases to facilitate the production of hybrid seed by open pollination for recurrent phenotypic and half-sib selection, as described in chapter 16. Production of hybrid seed when crossing two individuals is a limitation for use of full-sib selection.

Hybrid cultivars

All of the methods of recurrent selection are technically possible for improving populations from which inbred lines are obtained for use in hybrids. A comparison of the methods for genetic gain will be discussed in Principles of Cultivar Development under the subject of maximizing genetic gain.

 

Review Questions 1

  1. You have developed a random mated population of maize. In screening the population for resistance to aphid resistance, you find 64% susceptible and 36% resistant individuals. Resistance is controlled by the dominant allele P. Resistant and susceptible plants can be identified without error.
    1. What is the frequency of the alleles P and p in the cycle 0 population?
    2. You eliminate susceptible individuals from the cycle 0 population after flowering. What would be the genotypic and phenotypic frequencies in the cycle 1 population?
    3. If you self-pollinate the resistant S0 plants in the cycle 1 population, what would be the expected frequency of S0:1 lines that are heterogeneous for resistance? What would be the genotypic and phenotypic frequencies for resistance within a heterogeneous line?
  2. Assume that you are working with the same population as in question 1, but are able to eliminate susceptible individuals from the cycle 0 population before flowering.
    1. What would be the genotypic and phenotypic frequencies in the cycle 1 population?
    2. If you self-pollinate the resistant S0 plants in the cycle 1 population, what would be the expected frequency of S0:1 lines that are heterogeneous for resistance? What would be the genotypic and phenotypic frequencies for resistance within a heterogeneous line?

 

Review Questions 2

  1. Identify an article in any issue of the Journal of Plant Registrations for two plant species, other than those discussed above, in which recurrent selection was used to develop a cultivar or germplasm. Describe season-by-season what was done, beginning with the formation of the cycle 0 population and ending with seed from the last cycle of selection. If you are unsure of some aspects of the procedure that was used by the breeder, provide reasonable assumptions of what might have been done. Be sure to include the reference for each of the two articles.

 

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