p yield potential into two components: (1) the plant yield potential, and (2) the plant stability of performance. The invention of the honeycomb selection designs permits the formulation of two parameters measuring the two components with high precision and accuracy.
3. The realization of accurate and precision whole-plant field phenotyping for crop yield potential.
4. The possibility of applying ultra-high selection pressures (1 to 0.5%, meaning less than 1% plants retained after the comparative evaluation) and the resulting attainment of high advance through selection without sacrificing accuracy of selection. Although in theory , the higher the selection pressures, the higher the expected progress through selection, in conventional breeding practice, the working selection pressures are commonly at the level of 10% or more.
5. The growing of plants in the absence of competition cancels the undesirable correlation between the CV and the mean that confounds predictions and permits the use of CV to select for stability of performance.
6. The predictive power of the two equations is further increased due to the possibility to select in the same generation plants characterized by high and stable crop yield potential, as opposed to progeny testing requiring at least two successive generations.
7. The increase of the genetic gain per annum from the average 1% genetic gain attained by conventional breeding, to a two-digit percentage.
8. The production of density-neutral cultivars possessing a number of advantages, including avoidance of reseeding in case of non-satisfactory germination and increased tolerance to drought stress through use of lower seeding rates.
9. The increase of the effectiveness of molecular breeding by enabling to rank an unlimited number of plants for crop yield potential with equal accuracy of assessment.
10. The unit of selection in honeycomb breeding is the individual plant, while the unit of analysis in molecular breeding is also the individual plant genome. This offers the possibility to successfully marry phenotyping with genotyping and to bridge the so-called genotype-phenotype gap .
3. Results and Discussion
3.1. Intra-Cultivar Selection for Yield and Stability within Elite Cotton Cultivars
The first set of experiments lasted four years and focused on one leading cotton cultivar, Celia, after a preliminary evaluation of seven cotton varieties using a replicated-7 honeycomb selection design (Figure 1). This first-year trial allowed selection of 30 out of the 133 total plants representing Celia based on the Plant Yield Index. In the next year, the 30 plants along with the original cultivar formed 31 progeny lines and were grown as a replicated-31 honeycomb selection design (Figure 2) with 28 plants (replications) per line. The next two years the top selected lines were tried in standard Randomized Complete Block (RCB) trials, along with the original cultivar, to assess selection outcome and genetic gain.
Table 1 shows the ranking of the 31 lines of the replicated-31 honeycomb selection design based on the values of Coefficient of Homeostasis, ranging from 3.2 to 23.7. Importantly, the higher CH values belong to Lines 5 and 3, which outperformed others in the RCB trials of the subsequent years (Table 3, Table 4), testifying to the importance of stability as a genetic component in the selection process. The corresponding Equation A unitless values are the mean values of the five best plants per line. The wide range of these values and the fact that Lines 3 and 5, which excelled in subsequent RCB trials (Table 3, Table 4), also possess the highest Equation values, testify to its high resolving and predicting power, unraveling a tremendous variation for crop yield genetic potential among progeny lines. Further, although Lines 5 and 3 possess almost equal Equation A values, they differ in the values of the two components. Line 5 was superior in homeostasis, i.e., in stability of performance, and inferior in yield potential, whereas the opposite was the case for Line 3. This difference was reflected in th