Understanding the classics: the unifying concepts of transgressive segregation, inbreeding depression and heterosis and their central relevance for crop breeding

Abstract

Transgressive segregation and heterosis are the reasons that plant breeding works. Molecular explanations for both phenomena have been suggested and play a contributing role. However, it is often overlooked by molecular genetic researchers that transgressive segregation and heterosis are most simply explained by dispersion of favorable alleles. Therefore, advances in molecular biology will deliver the most impact on plant breeding when integrated with sources of heritable trait variationand this will be best achieved within a quantitative genetics framework. An example of the power of quantitative approaches is the implementation of genomic selection, which has recently revolutionized animal breeding. Genomic selection is now being applied to both hybrid and inbred crops and is likely to be the major source of improvement in plant breeding practice over the next decade. Breeders' ability to efficiently apply genomic selection methodologies is due to recent technology advances in genotyping and sequencing. Furthermore, targeted integration of additional molecular data (such as gene expression, gene copy number and methylation status) into genomic prediction models may increase their performance. In this review, we discuss and contextualize a suite of established quantitative genetics themes relating to hybrid vigour, transgressive segregation and their central relevance to plant breeding, with the aim of informing crop researchers outside of the quantitative genetics discipline of their relevance and importance to crop improvement. Better understanding between molecular and quantitative disciplines will increase the potential for further improvements in plant breeding methodologies and so help underpin future food security.

On heterosis, inbreeding depression and general combining ability in annual caraway (Carum carvi)

“…Hence, we can select for higher essential oil content within the given gene pool. Usually, quality traits do not show strong inbreeding depression and heterosis (Mackay et al 2021). Our results are in accordance with this general observation because across both years we found no significant difference between inbred lines and corresponding F 1 populations.…”

“…Possibly, inbreeding depression was mainly generated in the subsequent step/steps of inbreeding. Finally, we can argue that the existence of inbreeding depression as reverse side of heterosis (Mackay et al 2021) can be considered evident, if we can provide evidence for heterosis.…”

“…Research in many species revealed that heterosis might be widely explained by additive dominance effects, whereas overdominance or epistasis merely play additional roles. (Kaeppler 2012;Mackay et al 2021;Zhou et al 2018). By recurrent selection within the gene pool of high yielding caraway breeding material, favorable dominant alleles could be accumulated.…”

Genomic and phenotypic characterization of finger millet indicates a complex diversification history

“…Our data provide a firmer basis for the future development of finger millet. The information can, for example, help guide the determination of heterotic groups to produce potentially hybrid finger millet varieties in the future (Labroo et al, 2021;Mackay et al, 2021). Hybridization between 'East Africa' and 'Nepal' or 'Southern Africa' accessions may also generate useful genetic variation for use in the genetically narrow East Africa region.…”

Mitigating tradeoffs in plant breeding

“… Mackay et al. (2021) consider that plant breeding “works” due to both transgressive segregation and heterosis as a result of the dispersion of favorable alleles.…”