essfully 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 the line performance in RCB trials. Line 5, possessing the higher stability of performance, retained the average 20% superiority over Celia in the two years (Table 4), whereas Line 3 only during the first year. This emphasizes the importance of giving increased weight to the genetic component of homeostasis any time the breeder has to decide among entries with equal Equation A values.
|Rank no.||Design code no.||CH Value||Equation A value||Rank no.||Design code no.||CH Value||Equation A value||Rank no.||Design code no.||CH Value||Equation A value|
The efficiency of selection is also evidenced as the CH and equation values of the original cultivar are up to three and four times lower, respectively, than the top selected lines. Notably, a high CH value does not necessarily correspond to a similarly high Equation A value, as is the case with Line 16. This line, although similar in homeostasis to Line 3, had a lower equation value, which was reflected in its inferior performance at the RCB trials of the following season.
Two unappreciated sources of genetic variation exist within elite cultivars . The first stems from the negative correlation between yielding and competitive ability, which leads to a gradual cultivar degradation because of the gradual proliferation of low yielding-strong competing plants (yC) at the expense of high yielding-weak competing plants (Yc). This happens during the standard process of cultivar reproduction under dense stand [15,41]. The second overlooked source is the one produced by the interaction between genotype and environment, since the genome is capable of releasing adaptive variation by triggering off genetic or epigenetic mechanisms in response to environmental changes [27,28,29,30,31,32,33,34,35,36]. All the above justify the application of nonstop selection on the basis of Equation A to exploit this seemingly endlessly released variation for breeding purposes. In this way, the crop yield genetic potential of Celia, instead of remaining at the level of 12 Equation units, could have 