Vasilia A. Fasoula

Principles underlying genetic improvement for high and stable crop yield potential

Abstract This paper explores the genetic basis of high and stable crop yield, and delineates the conditions that provide the link between genotypic and phenotypic superiority and, consequently, improve selection efficiency in plant breeding. Response to selection is viewed as the result of the conversion, through intralocus recombination, of two heterotic nonadditive allelic loci into a single super additive locus, and fixation of the latter. This concept connects the important phenomena of vigor, degeneration, and response to selection through a unifying genetic basis. Heterozygosity of nonadditive allelic loci leads to the nonheritable repulsion-phase vigor. Heterozygosity of a super additive locus leads to the partially heritable coupling-phase vigor. Homozygosity of the super additive locus leads to the fully heritable inbred vigor, which is responsible for genetic gain. A reliable explanation of heterosis is the one that considers repulsion- and coupling-phase vigor as being mutually essential and inter-convertible. Response to selection is the outcome of the evolution of hybrid to inbred vigor. The superiority of inbred vigor is due to the improved additive allelic complementation, that at the same time enhances epistatic interactions. The full exploitation of the above concept leads to maximization of homeostatic crop yield and can be successfully achieved using the principles of honeycomb breeding. These include clarification of the negative role of competition on: (1) crop yield, and (2) selection efficiency, and explain why crop yields are maximized when cultivars are monogenotypic. The unit of evaluation and selection becomes the individual plant grown at the critical distance, where the range of phenotypic expression is enlarged, and the negative effect of competition on selection efficiency is eliminated. Honeycomb breeding uncouples the reliable selection for yield and stability from the visual evaluation that predominates during the critical early generations of selection, and offers the transition from single-trait evaluation to whole-genome phenotypic evaluation. The concept of whole-genome phenotypic evaluation recognizes that genes controlling crop yield concern the genome as a whole and belong to three categories: (1) genes that control yield potential per plant and expand the lower limit of the optimal plant density range; (2) genes that confer tolerance to biotic and abiotic stresses and expand the upper limit of the optimal plant density range; (3) genes that control cultivar responsiveness to inputs. The outcome of selection, based on whole-genome phenotypic evaluation during all generations of a breeding program, is high yielding, stable, and density-independent cultivars.

Development of crop cultivars by honeycomb breeding

The ability of agriculture to adapt to environmental changes and to address main issues of food quality and environment protection is a fundamental factor in achieving sustainability. Low yield capacity of contemporary sustainable farming systems, however, is a major obstacle to future growth of sustainable agriculture. In addition, increasing pressure is placed for higher food supply due to the projected population increase. To overcome these barriers and stimulate the wide adoption of sustainable agriculture, ample supply of cultivars that satisfy the requirements for sustainability without compromising productivity is essential. Otherwise, the viability of sustainable agriculture is unsound. Moreover, plant breeding has to be a non-stop process supporting agriculture because of the ongoing climate changes. The studies of the effects of competition on crop yield and selection efficiency unravelled important findings for plant breeders. Firstly, the uppermost cultivar type is the mono-genotypic and particularly the highest evolutionary grade of ‘pure line’. Secondly, single plant selection is effective only when it is realized in the absence of competition for growth resources. Honeycomb methodology, by considering as a major principle the application of selection in the absence of competition, counteracts the disturbing effects of competition on selection effectiveness. Furthermore, the honeycomb experimental designs cope with the confounding implications of soil heterogeneity. These two findings help breeders to consider the individual plant as an evaluating and selection unit. As a consequence, the development of pure line cultivars that fully meet the needs of a sustainable agriculture is possible. Most importantly, honeycomb breeding exploits effectively not only favourable but marginal environments as well through the development of density-neutral cultivars. Marginal environments are exploited optimally when lower plant populations are used. It is of essence to realize that without the ability of exploiting successfully marginal environments which represent the majority of the production environments globally, sustainability in agriculture becomes problematic.

SSR-Marker Analysis of the Intracultivar Phenotypic Variation Discovered within 3 Soybean Cultivars

Genetic variation within homogeneous gene pools in various crops is assumed to be very limited. One objective of this study was to use 144 simple sequence repeat (SSR) markers to determine if the single-plant lines selected at ultra-low plant density in honeycomb designs within the soybean cultivars Benning, Haskell, and Cook had unique SSR genetic fingerprints. Another objective was to investigate if the variation found was the result of residual genetic heterozygosity that could be detected in the original gene pool where selection initiated. Our results showed that the phenotypic variation for seed protein content and seed weight has a genotypic component identified by the SSR band variation. The 7 lines from Haskell had a total of 63 variant alleles, the 5 lines from Benning had 34 variant alleles, and the 7 lines from Cook had 34 variant alleles, therefore, possessing unique genetic fingerprints. Most of the intracultivar SSR band variation discovered was the result of residual heterozygosity in the initial plant selected to become the cultivar. More specifically, 82% of the SSR variant alleles were traced in the Benning Foundation seed source, 93% in the Haskell seed source, and 82% in the Cook seed source. The remaining variant bands (18% for Benning, 7% for Haskell, and 18% for Cook) could not be detected in the Foundation seed source and were likely the result of mutation or some other mechanism generating de novo variation. These results provide evidence that genetic variation among individual plants is present even in homogeneous gene pools and can be further utilized in breeding programs.

Application of prognostic breeding in maize

Abstract. Innovative approaches and new efficiencies in plant breeding are required to accelerate the progress of genetic improvement through selection. One such approach is the application of prognostic breeding, which is an integrated crop-improvement methodology that enables selection of plants for high crop yield potential by evaluating its two components: plant yield potential and stability of performance. Plant yield and stability are assessed concurrently in each generation by utilising the plant prognostic equation. The genetic material used for this study was 2350 F2 plants (C0) of the commercial maize hybrid Costanza. The study presents the results of the application of prognostic breeding for 6 years in two contrasting environments (A and B), starting from C0 and ending in C5. It utilises ultra-high selection pressures (1.5% to 0.5%) to isolate superior lines with crop yield comparable to Costanza, and estimates the annual genetic gain accomplished through application of this selection strategy. Application of prognostic breeding led to the isolation of superior lines whose productivity was comparable to Costanza. The productivity gap between Costanza and the best selection was reduced from 87% (C0) to 0.5% (C5) in trial 1 (environment A), from 87% (C0) to 2% (C5) in trial 2 (environment B) and from 70% (C0) to 1% (C3) in trial 3 (environment B). Genetic gain was much higher (up to 50%) in the early cycles C0–C2 of prognostic breeding and smaller in cycles C3–C5. The best lines selected were evaluated in randomised complete block trials across both environments and 2 years. Across years, the top two lines in environments A and B averaged 87% and 91% of the Costanza yield, respectively, and they had higher prolificacy (greater number of ears per plant) than Costanza. Across all cycles, the average annual genetic gain ranged from 23% to 36% in the different trials, providing evidence that selection efficiency can be significantly maximised by using this breeding strategy.