Relationship between hybrid performance and parental genetic distance

 

 

Heterosis, or hybrid vigor, is a phenomenon that has been recognized for over a century and refers to the increased performance of crosses compared to their parental components. Heterosis is exploited for agriculture in the form of hybrid breeding. Despite these efforts, the genetic and molecular causes of heterosis are still not fully understood. Nevertheless, several competing but mutually nonexclusive genetic models to explain heterotic effects have been proposed and experimentally validated.

According to quantitative genetic theory, genetic distance between parents at heterotic quantitative trait loci is required for heterosis, but how heterosis varies with genetic distance has remained elusive. Experimental studies have often found a positive association between heterosis and genetic distance; however, it has remained unclear whether heterosis increases continuously with genetic distance or whether there is an optimum genetic distance after which heterosis declines again.

Among the initial studies, the work from Moll et al. from 1962 and 1965, with two contrasting results, has formed the basis for controversial discussions in the past decades. In the 1st study (1962), they observed the highest grain yield heterosis in the crosses with the presumed greatest genetic divergence of the parental varieties. Whereas, later (1965) they observed a decrease in heterosis in the hybrids from the widest crosses. Reif et al. (2003) observed an increasing heterosis with genetic distance and no decrease even under their maximum genetic distance, which they attributed to the similar adaptation of the parental component lines and the lack of extremely distant crosses.

An interesting study recently suggested that even within a species there is an optimum genetic distance between the parents of a hybrid that maximizes the fitness by balancing the benefits of heterosis and the harm of genetic incompatibility. The authors deduce that dominance more likely results from intragenic interactions between an ancestral allele and a derived allele and is therefore expected to rise linearly with genetic distance of the parents. In contrast, the other genetic effects contributing to heterosis, which are overdominance, underdominance as well as positive and negative two-locus epistasis, are more likely to occur between two derived alleles and consequently to increase in proportion to the squared genetic distance between parents.

In conclusion, hybrid breeding requires genetic distance between the parental components and thus between the heterotic groups in a breeding program. However, heterotic effects can be positive as well as negative, and thus, hybrid performance is not maximized by maximizing the genetic distance between parental components of a hybrid but by optimizing it.

References

1. Würschum, T., Zhu, X., Zhao, Y., Jiang, Y., Reif, J.C., & Maurer, H.P. (2023). Maximization through optimization? On the relationship between hybrid performance and parental genetic distance. Theoretical and Applied Genetics, 136(9), 186.

2. Moll, R., Salhuana, W.S., & Robinson, H.F. (1962). Heterosis and genetic diversity in variety crosses of maize. Crop Science, 2, 197–198.

3. Reif, J.C., Melchinger, A.E., Xia, X.C., Warburton, M.L., Hoisington, D.A., Vasal, S.K., Srinivasan, G., Bohn, M., & Frisch, M. (2003). Genetic distance based on simple sequence repeats and heterosis in tropical maize populations. Crop Science, 43(4), 1275-1282.

4. Wei, X., & Zhang, J. (2018). The optimal mating distance resulting from heterosis and genetic incompatibility. Science Advances, 4(11), 5518.