Ecological and evolutionary approaches to improving crop variety mixtures 

Wuest et al., 2021

Variety mixtures can provide a range of benefits for both the crop and the environment. Their utility for the suppression of pathogens, especially in small grain crops, is well established and has seen some remarkable successes. However, despite decades of academic interest in the topic, commercial efforts to develop, release and promote variety mixtures remain peripheral to normal breeding activities. Here we argue that this is because simple but general design principles that allow for the optimization of multiple mixture benefits are currently lacking. We therefore review the practical and conceptual challenges inherent in the development of variety mixtures, and discuss common approaches to overcome these. We further consider three domains in which they might be particularly beneficial: pathogen resistance, yield stability and yield enhancement. We demonstrate that combining evolutionary and ecological concepts with data typically available from breeding and variety testing programmes could make mixture development easier and more economic. Identifying synergies between the breeding for monocultures and mixtures may even be key to the widespread adoption of mixtures—to the profit of breeders, farmers and society as a whole.

Re-partitioning the problem of combining multiple disease resistances. a, Typical behaviour of population-level resistance in mixtures as the fraction of the susceptible variety changes . Dashed line, expected pathogen pressure; solid line, observed pathogen pressure. b, Partitioning the large problem of breeding for a genotype resistant to multiple pathogens into smaller problems of assembling multiple ‘pathogen specialists’ into a mixture. The perfect genotype (red) may never be found, yet complementary combinations (purple, light and dark blue combinations) should be relatively abundant. c, Hypothetical numerical example of how partitioning resistances against multiple pathogens can break a large hard problem into multiple smaller problems. Here, resistance against three pathogens is conferred by recessive alleles at one, two and three loci, respectively, and combined in a single cross. Screening for favourable allele combinations may occur by molecular or phenotypic means. Hereby, identifying the ‘perfect individual’ homozygous for all six recessive alleles in an F2 population (centre) would require the sowing and screening of around 9,431 plants (the population size required to achieve >90% chance of finding at least one perfect genotype). Combining the alleles into either one of two mixture components requires the screening of two much smaller populations (~378 (left) or ~806 (right) plants). Circle sizes indicate the relative populations sizes needed and question marks denote either recessive or dominant alleles.

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