Soybean breeding program at the Institute of Field and Vegetable Crops uses Single Seed Descent method of selection. The single seed descent method was proposed by Brim in 1966, and the procedure has been the predominant method of soybean selection in the U.S. since. Single seed descent makes it possible to produce three generations of self-pollination in a single year using winter nurseries or greenhouses, thus accelerating the development of homozygous lines for the testing of yield in replicated trials. However, due to the unavailability of a winter nursery and a lack of sufficient greenhouse space that would accommodate all of the breeding materials, we at the Institute had to adapt the method to make it suitable for such conditions and is making use of only those aspects that involve the reduction of space and labor while at the same time maintaining a satisfactory level of variability up to the F5 generation.
The single seed descent method is usually not applied until a certain level of homozygosity is reached in the F4 or F5 generation. Selection in the earlier generations can still be done, but on a smaller scale, i.e. it is performed in the sense that pods are not taken from plants that are diseased or lodged or prone to pod splitting, and so on.
Single seed descent requires that only the most basic data be taken down, such as the designation of the cross and what generation it is in. Also, minimal space is required to grow successive generations of single plants when compared to the rows of progenies characteristic of pedigree selection. Another advantage of the single seed descent method is the presence of full variability in each generation. With no selection in the early generations, the amount of variability present among the F5 plants is similar to that found in the F2 generation. Finally, the number of recessive homozygotes increases in successive generations – with the postponement of selection for a recessive trait until the F5 generation, nearly 47% of the plants will be homozygous for such a trait (Wilcox, 1998).
Perhaps the biggest disadvantage of single seed descent consists of the irreversible loss of identity of superior plants from earlier generations. Besides that, a superior plant observed in the F2 generation will be represented by no more than a single plant in the subsequent generations, making it impossible to select a larger number of lines from superior plants. Moreover, plants that would otherwise be discarded will remain in the population up until the F5 generation.
METHOD DESCRIPTION
Selection of Parental Pairs
Each cycle of breeding begins with the selection of parental pairs to be used for obtaining new genetic variability. Choosing parental pairs is the first crucial moment in breeding, because it determines the success of the future selection process. Generally, elite parental lines of different origin have the greatest chance of producing superior progeny. The choice of pairs to be used in crosses depends on many factors – the traits one is trying to improve, the relative importance of other traits in relation to yield, the origin of the lines being used, and the resources the breeder has available to them.
Cross Pollination
About 300 crosses used to be carried out per year at the Institute of Field and Vegetable Crops, but the number has decreased in recent years to 150 crosses per year. It Crossing soybeans is only possible in the morning, before dew formation, as temperature increase and reduced relative air humidity in the late morning hours cause decrease in pollen viability. As a result, crossing soybeans begins at 6 AM, and continues as long as possible.
The primary task of soybean cross-breeding is to collect buds from the male plants. Reproductive buds are identified by the emergence of petals (corolla), supported by sepals (calyx), to the extent that allows determination of petal color.
Cross-breeding requires the selection of buds of unpollinated female plants with developed reproductive organs, which are identified as such by the emergence of petals (corolla). Using binoculars and tweezers, green sepals are unfolded in order to remove petals and stamens, and reveal anthers, whereby anther damage must be avoided and female stigma must remain intact. Anthers are then collected from the buds of male plants and placed onto the open female’s stigma. Finally, female’s petals are closed with sepals, wrapped with the nearest trifoliolate leaf, and tied with a wire tag containing the date of crossing, cross number, and flower number.
The first control is conducted five to seven days after crossing, considered effective if the pod is formed during that period. In case of successful crossing, protection (trifoliolate leaf) is removed and the tag remains at position to indicate pod location. The tag is removed in case of unsuccessful crossing, indicated as such by the absence of pod formation.
Ripe pods are picked and placed into a bag with the label, with all other pods obtained in one cross. During the winter period, pods are hand-picked, seeds are placed in a separate bag with one remaining tag, and used for growing F1 generation the following year.
Growing F1 generation
First filial (F1) generation is sown manually, using 50 cm inter-row and 50 cm intra-row spacing. Some of the crosses are labeled during sowing.
Several controls are carried out during vegetation. Preliminary control is carried out at the first-triofoliate stage, when F1 hybrid seedlings are exposed by the color of their hypocotyl. Plants which are not F1 hybrids are labeled and discarded during harvest. The second control is carried out at the flowering stage, when color of the flowers and trichomes indicates which plants are F1 hybrids. Non-hybrid plants are labeled and discarded during harvest. Harvest is conducted manually at the stage of full maturity, by picking each plant separately. Seeds are then placed in separate bags with an adequate label.
During winter, seeds are prepared for the F2 generation sowing. Undamaged, healthy hybrid seeds, as indicated by the hilum color, are selected, mixed and sown in one row as the F2 generation the following year.
Growing F2, F3 and F4 generation
Sowing F2 generation is carried out mechanically at the plot envisaged in the sowing plan, using 50 cm inter-row and 3 – 4 cm intra-row spacing. Each row is 100 m long and represents one crossing. Cross numbers are indicated on the labels after sowing.
After reaching full maturity, plants are harvested manually by taking one pod off each plant in a row and placing it in a separate bag.
During winter, seeds are prepared for sowing the F3 generation. Selected pods are picked by hand and seeds are selected. If they could not be indicated in the F1 generation, hybrid plants are revealed by the hilum color in the F2 generation. Healthy, undamaged seeds are selected and sown in one row the following year as the F3 generation.
Generation F3 and F4 are grown in a similar way. When F4 generation reaches full maturity, F5 generation is selected. Depending on plant overall appearance, a total of 50 to 150 plants are selected from one combination, and the selected plants are labeled. Each labeled plant is hand picked by the technical field staff, while separate rows are harvested mechanically. Seeds of the selected plants are then placed in separate bags with a label.
Growing F5 generation
The F5 generation is mechanically sown, using 50 cm inter-row and 3 – 4 cm intra-row spacing according to the standard soybean sowing practices. Rows are 3 m long and each row represents one line. Line combinations are labeled after sowing.
Crop control and phenological observations are carried out consistently during vegetation. Color of flowers and trichomes are recorded, disease incidence is estimated by the plant pathologist, and the maturity group is determined towards the end of the vegetation.
Lines are selected for preliminary trials when plants reach full maturity. The selected lines are hand picked and labeled, while separate rows are harvested mechanically. Labeled seeds are then placed in separate bags.
Growing F6 generation (Preliminary Trials)
Preliminary trials are mechanically sown using 50 cm inter-row and 3 – 4 cm intra-row spacing according to the standard soybean sowing practices, into small plots containing four rows four meters long in two replications. One preliminary trial block comprises 37 to 38 lines + 2 to 3 standards of the corresponding maturity group, and PT blocks (not each plot) are labeled after sowing.
Crop control and phenological recordings are carried out consistently during vegetation. Phenological observations record sprouting date, vigor, flower and trichome color, maturity date, and lodging, while plant pathologist estimates the incidence of plant diseases. Maturity groups are determined according to standards at the end of vegetation.
Mechanical harvesting is conducted when plants reach full maturity. Labeled seeds of each line are then placed into separate bags.
Immediately after harvest, bags are delivered to the storage for weight and moisture measurements in order to reduce the moisture content to 14%. These results are the most important criterion in the selection of lines for comparative trials.
Growing F7 and F8 generation (Comparative Trials)
Comparative soybean trials last two years. The same procedure is applied in both years, whereas the first year’s lines are selected for the second comparative trial, and afterwards for the internal macro trial.
Comparative trials are conducted the same way as preliminary trials, only in 4 replications, so that one CT block comprises 17 to 18 lines + 2 to 3 standards of the corresponding maturity group.
The obtained results are used for selection of lines in the second comparative trial or internal macrotrial.
Internal Macrotrial
Lines selected from the second-year CT are sown in six 100m long lines, according to the soybean sowing standards. The breeder can thereby assess genotype performance under specific production conditions. The results obtained from internal MTs are used as guidelines in selecting lines suitable for delivery to the Variety Registration Committee, where the lines are tested for 2 years. At the same time, the prospective cultivars are tested in macrotrials set up at 15 to 20 representative localities. The results are used for determining the lines which should be introduced into production of the released cultivars.
