Understanding the symbols used to describe the progeny generated following hybridization and self-pollination is critical for clear communication between/among researchers in the breeding community. The two symbols used by plant breeders to enable such communication are F and S, where F is derived from the word filial, defined as the sequence of generations after the mating of two parents. S is the symbol used to denote generations of self-pollination. Different systems exist in the use of the F and S symbols to describe breeding groups – populations, individuals, and inbred lines – making it the more crucial to avoid misleading outcomes by clearly stating the meaning of symbols, and thus underscores that advantages and disadvantages exist in the use of each system.

Using F# and S#

Symbology

A. Using “F_#” and “S_#” to designate the number of generations of selfing or sib-mating:

In plant breeding, the ‘F’ symbol is used to denote the filial (i.e., family) generation of offspring following a cross between two or more parents. The subscript (#) represents the specific generation (F_#). F₁ is the first generation following a cross and subsequent generations are designated F₂, F₃, F₄, etc., based on the number of generations the offspring are self-pollinated or sib-pollinated (i.e., pollinated by a sibling plant in the same progeny row). Pollination can occur naturally or artificially if it is imposed by the breeder. If both of the parents of a cross are homozygous then the F₁ offspring will be homogeneous (i.e., all plants will be uniform and genetically the same) and heterozygous at individual loci. If either or both of the parents are heterozygous then the F₁ offspring will be heterogeneous (non-uniform). In a complex cross involving more than two parents, even if the parents are homozygous), the F₁ generation will be heterogeneous and heterozygous.

Single Gene Example With Two Homozygous Parents

In the example above (Fig. 1) , both parents were developed by the breeder after undergoing several generations of selfing so that they are homozygous at all loci. The cross between these two unrelated parents produces F₁ progeny that are all uniformly heterozygous (Aa), and F₁ progeny population (all F₁ from this cross) will be homogeneous since each F₁ will be ‘Aa’ type at this locus.

Self-Pollination

When an F₁ plant is self-pollinated or when two F₁ plants are crossed with each other, F₂ seed is produced. If the parents were homozygous then the F₂ generation is the first generation when the offspring are heterogeneous (i.e., segregating for different parental alleles). The F₂ generation is typically the generation when selection for simple traits begins. Self-pollination of F₂ plants produces F₃ plants, self-pollination of F₃ plants produces F₄ plants, and so on as shown in Fig. 2.

Cross-Pollinated Species

In cross-pollinated species, ‘S_#’ is used instead of ‘F_#’. The symbol S₀ can be used to describe the progeny from a single cross between two homozygous parents as either:

1. Similar to F₁ (in self-pollinated) which indicates that the plant was not derived from self-pollination or
2. Similar to F₂ (in self-pollinated) which indicates that the population is formed by random mating and is therefore heterogeneous and heterozygous

Therefore, it is important that the breeder clearly describes what she/he is referring to in a particular situation and then be consistent in usage.

Using F_x:y and S_x:y

Breeding Lines

B. Using “F_x:y” or “S_x:y” to describe breeding lines according to the generation they were derived:

Breeding lines (or genotypes) are derived from individual plants at various generations. An F_2:4 line refers to an F₄ line that was derived from a single F₂ plant. The F₂ plant was selfed to produce F₃ seeds, which were then grown in a single F₃ progeny row, self-pollinated, and then harvested as a F₄ bulk of many or all of the F₃ plants in this row. Based on this scheme each individual F₂ plant gives rise to a genetically distinct F_2:4 line. These lines are also described as “F₂-derived lines in the F₄ generation”, or simply “F₂-derived F₄ lines”.

Summary

An F_3:4 line refers to an F₄ line (or progeny row) created from a single F₃ plant growing in an F₃ progeny row that was produced from the seed of a self pollinated single F₂ plant. An individual plant was selected from an F₃ row to produce F₄ seed and this seed when grown represents an F_3:4 line. These are also described as “F₃-derived lines in the F₄ generation”, or simply “F₃-derived F₄ lines”. The difference from F_2:4 lines is because the first subscript designates the generation of the last individual-plant selection (Fig. 3).

To summarize:

F_x:y or S_x:y , describes ‘x’ as the generation where single plant was harvested separately to give rise to the derived line, and y represents the current generation of inbreeding of the plants within this derived line.

Writing a Standard Pedigree

Symbology

C. Example of writing a standard pedigree:

Each organization follows a different standardized system for recording pedigrees. In this section, we will describe the system adapted from Purdy et al (1968) modified and used by wheat breeders at CIMMYT (CGIAR institute). Depending on the crop you work on and where you are employed you may use a modified system.

The female parent is designated by listing it first (starting from the left) followed by the male parent (on the right). For example, A is the female parent and B is the male parent in an (A x B) cross. An (A x B) cross can also be written as A/B.

If an F₁ (A/B) plant is pollinated with parent C, and the F₁ is used as the female and C as the male, the resulting three-way cross would be designated as A/B//C. Subsequent crosses with parental materials D, E, F, and G used sequentially (all as males) are indicated using a number to record the cross order in the following way: A/B//C/3/D/4/E/5/F/6/G.

Example

If the example above is changed to use D & F as female parents, with E and G remaining as males, the cross would be recorded as follows:

• Step 1: A/B is the first cross,
• Step 2: A/B//C is the second cross, where A/B is the female.
• Step 3: D/3/A/B//C is the third cross, with D as female, and A/B//C as male.
• Step 4: D/3/A/B//C/4/E, with E as male, and the 4-parent cross as the female. NOTE: bold and underline text is for information and instructional purpose only. In writing a pedigree, you will not have to bold text. One will simply write the pedigree as D/3/A/B//C/4/E

The inclusion of “5/F” as the female and 6/G as a male completes the pattern.

Backcross Pedigree

In multiple backcrosses, the sequence of these letters from left to right corresponds to the sequence in which the backcrosses are made. Backcross pedigrees include an asterisk (*) and a number indicating the dosage of the recurrent parent. The asterisk and the number are placed next to the crossing symbol (/) that divides the recurrent and donor parents. The following are examples of pedigree formats involving backcrosses:

• A is the recurrent parent: A*2/B of the initial cross and has been used as a parent two times. Therefore, A*2/B indicates one backcross or a BC₁ cross.
• B is the recurrent parent: A/3*B, and has been used as a parent three times. Therefore, A/3*B indicates a BC₂ cross.

A*2/B is therefore A//A/B and indicates that A was used as a female in both F₁ and BC₁.

A/3*B could be B/3/A/B//B and indicates that A was used as a female, F₁ was then used as female, and BC₁ was used as male.
A/3*B could be A/B//B/3/B and indicates that A was used as a female, F₁ was then used as female, and BC₁ was used as a female.

The F#: derived symbols as previously described for regular crosses will follow the BC_# designation. For example, BC₁F_2:4 or BC₂F_2:4.

Identity Number

Assigning an Identity Number to Each Cross or Backcross

Every cross should receive a unique ID number that will allow everyone in the breeding group to recognize the year the cross was made (e.g., 2014), a cross number (e.g., 1001), and perhaps the target purpose of the cross (e.g., HO for high-oil, or abbreviation for another specific trait or market segment).

Using BCID

Recording Selection History

D. Recording selection history using a Breeder’s Cross IDentification (BCID) designation:

Every F₁ plant, segregating line, or advanced line in a program is assigned a Breeders’ Cross IDentification (BCID) and a selection history. This selection history records the process of selection, which describes where and how the initial cross was made and where and how subsequent selection steps occurred for each generation of selection.

Example

An example of this system is provided below (using CIMMYT’s wheat breeding program).

Each BCID begins with a letter designation for the origin of the cross (e.g., CM = crusa Mexicana; Spanish for ‘Mexican cross’). This is followed by an indication of the kind of cross (e.g., BW = bread wheat x winter wheat, SS = spring x spring wheat; SW = spring x winter wheat), an abbreviation of the year when the cross was made (e.g., 00 = 2000), an abbreviation of the location where the cross was made (e.g., Y = Yaqui Valley), and finally a sequential number representing the order in which that cross was made within the crossing cycle (e.g., 0124). The table below shows the letter codes used to indicate the locations in Mexico where crosses were made and the different environmental conditions where CIMMYT breeders carry out selection in wheat. Note that more than abbreviations than shown in table are used to describe locations or nurseries.

Location Codes

Location codes for other countries were determined by the cooperators/breeders in those countries to ensure that everyone was aware and compliant. Hypothetical examples of BCIDs and selection histories are presented below.

• Example 1: BCID = CMBW08Y0199, a single cross was made in Mexico (“CM”) in 2008 between a bread and winter wheat at location ‘Y’ and this was cross# 0199 that year. The pedigree method of selection was followed for the development of the genotype. The selection history for this example indicates that in the F₂, the 35^th plant was selected at location “Y”, and in the F₃ generation this was the 15^th plant selected at location “M”, etc. Finally, in the F₇ the “0M” indicates that a single plot was grown at location “M” and harvested in bulk (i.e., all plants in the plot were harvested into one bag or packet). This created the genotype CMBW08Y0199-35Y-15M-7Y-5M-12Y-0M.
• Example 2: BCID = CMBW08Y0124, a single cross was made in Mexico (“CM”) in 2008 at location “Y”, with 0124 designating that this cross was number 0124 in the series of crosses made at that location and year. The selection history reflects that a modified pedigree/bulk selection method was used. In the F₂ the “81Y” indicates that this genotype was the 81st individual plant among those selected at location “Y.” The F₃ designation of “010M” indicates that 10 plants were selected and harvested in bulk from the F₃ progeny row grown at location “M.” Seed from the bulked F₃ progeny row was planted at location “Y” in the F₄ and 10 plants were selected and harvested in bulk. Similar scheme was used in F₅. In the F₆ a single plant was selected from this genotype (15th plant) at location “Y” and constituted the seed for the next generation. In the F₇ (or more appropriately, F_6:7) all plants in the progeny row at location “M” were harvested in bulk, as shown by the designation “0M”. This created the genotype CMBW08Y0124-81Y-010M-010Y-010M-15Y-0M.
• Example 3: BCID = CMSS07Y051, which describes that a single cross was made in Mexico (“CM”) in 2007 at location “Y”, with 051 designating that this cross was number 051 in the series of crosses made at that location and year. The cross was of type “SS”, spring wheat x spring wheat. The selection history indicates four generations (F₂-F₄) of selection in which 30 plants were bulked from the progeny row (or plot) for each season at either the “M” or “Y” locations. In the F₅ generation the genotype selected was the 53rd plant from the bulk plot at the “Y” location. In the F₆ a complete plot bulk was harvested at location “M”. This lead to the creation of the genotype CMSS07Y051-030Y-030M-030Y-53Y-0M.

Additional Notes

After the BCID, the selection history is presented in which the numbers identify the number of individual plant(s) selected and the letter indicates the location where selection took place and/or under what specific conditions selection was conducted.

The zero-letter combinations (e.g., 0Y, 0M, etc.) are reserved for populations harvested in bulk during that generation (i.e., the entire plot was cut and threshed as one unit). A zero followed by a number (e.g., 05…, 010…) and then by a letter indicates that the modified pedigree/bulk selection method was used in which a certain number (e.g., 5 or 10) of selected heads are bulk (0) harvested. The location where the selection was made and, in some cases, the special type of selection performed, is indicated by a letter code.

References

Purdy, L.H., W.Q. Loegering, C.F. Konzak, C.J. Peterson, and R.E. Allan. 1968. A Proposed Standard Method for Illustrating Pedigrees of Small Grain Varieties. Crop Science 8:405-406.