Heterosis and Hybrid Crop Breeding: A Multidisciplinary Review

Abstract

Although hybrid crop varieties are among the most popular agricultural innovations, the rationale for hybrid crop breeding is sometimes misunderstood. Hybrid breeding is slower and more resource-intensive than inbred breeding, but it allows systematic improvement of a population by recurrent selection and exploitation of heterosis simultaneously. Inbred parental lines can identically reproduce both themselves and their F₁ progeny indefinitely, whereas outbred lines cannot, so uniform outbred lines must be bred indirectly through their inbred parents to harness heterosis. Heterosis is an expected consequence of whole-genome non-additive effects at the population level over evolutionary time. Understanding heterosis from the perspective of molecular genetic mechanisms alone may be elusive, because heterosis is likely an emergent property of populations. Hybrid breeding is a process of recurrent population improvement to maximize hybrid performance. Hybrid breeding is not maximization of heterosis per se, nor testing random combinations of individuals to find an exceptional hybrid, nor using heterosis in place of population improvement. Though there are methods to harness heterosis other than hybrid breeding, such as use of open-pollinated varieties or clonal propagation, they are not currently suitable for all crops or production environments. The use of genomic selection can decrease cycle time and costs in hybrid breeding, particularly by rapidly establishing heterotic pools, reducing testcrossing, and limiting the loss of genetic variance. Open questions in optimal use of genomic selection in hybrid crop breeding programs remain, such as how to choose founders of heterotic pools, the importance of dominance effects in genomic prediction, the necessary frequency of updating the training set with phenotypic information, and how to maintain genetic variance and prevent fixation of deleterious alleles.

Keywords: autogamous; dominance; genomic selection; heterosis; inbreeding depression; reciprocal recurrent genomic selection.

FIGURE 1

Illustration of the partitions of population-level heterosis by Lamkey and Edwards (1999). Arrows indicate random mating or random crossing, and lines indicate selfing to homozygosity. Note that only dominance, additive × dominance, and dominance × dominance effects can contribute to baseline heterosis, but dominance, additive × dominance, dominance × dominance, and additive × additive effects can contribute to panmictic-midparent heterosis, inbred-midparent heterosis, and F₂ heterosis. Equations are further described in Supplementary Table 1.

FIGURE 2

Graphical overview of traditional and genomics-assisted hybrid crop breeding pipelines.