The inception of agriculture approximately 10,000 years ago shifted the predominant homo sapiens lifestyle from nomadic groups of hunter-gatherers, to larger social groups that ‘put down roots’ and formed a stable community. For the first time in history, agriculture generated a net food surplus, removing the need to follow seasonal patterns of wild plants and animals, as well as freeing up a proportion of the community to specialise their workforce and drive further advancement.
Given the extensive history and influence of agricultural practices in human society, it’s amazing to think that selective breeding was only properly realised during the British agricultural revolution in the mid 18th century, and the underlying mechanism of Mendelian inheritance only discovered in 1866. And it has only been relatively recently that significant gains have been achieved in the agricultural industry.
The agricultural powerhouse of the 20th century has conclusively been America, with their agricultural output more than quadrupling between 1930 and 2000. Additionally, the area of farmland and cost of food products decreased over this period, indicating vastly improved farming efficiency (1).
Just as agriculture itself initially freed up a proportion of the population to specialise, the same occurred through technological advancements in America throughout the 20th century, with 41% of the workforce employed in agriculture in 1900 dropping to 1.9% in 2000 (2).
Early in the 20th century, breeding theorists suggested that moving away from the non-directed open pollination strategies would result in significant increases in crop yield. Breeders started self-fertilizing (selfing) high performing plants to create inbred lines with two homozygous copies of their genome that were almost genetically identical from parent to offspring. Crossing these different inbred lines together produced beneficial hybrid offspring (heterosis).
Heterosis, or hybrid vigor, is the phenomenon of improved survival and yield of hybrid offspring from homozygous inbred parents. Explanations for this observation include the dominance hypothesis, where inbred parents that have two identical copies of each allele have a higher proportion of double recessive alleles, typically resulting in disadvantageous traits. However, crossing different parental lines masks recessive alleles in one parent with a dominant allele in the other parent. In addition, overdominance proposes that the combination of one dominant and one recessive allele is better than the homozygous combination of either parent (3).
Early in the adoption of this new breeding strategy, parental lines had not been optimised enough to produce a sufficiently beneficial offspring, necessitating the development of double cross hybrids. While double cross breeding required an extra reproductive step, they exhibited a larger pool of genetic variation, initially performing better than single cross hybrids. However, with years of iterative inbreeding and hybrid crossing, parental lines had advanced in performance enough to enable single cross hybrid breeding to be viable, saving time and farmland acreage (Figure 2).
Single cross breeding generated a new variety of plants, specifically plants that had improved resource allocation through enhanced fertilizer responsiveness and increased yield of grain per unit of biomass (short stature wheat), which ultimately had significant improvements on crop yield. Breeding innovations were accompanied by technological advancements in agrichemicals, such as herbicides, pesticides and fertilizers, equipment mechanisation and improved irrigation system, establishing the green revolution of the 1960s.
The central paradigm of conventional agricultural breeding programs are phenotypic-centric, with a predominantly agnostic view on genetics. The pool of hybrid offspring need to be grown to grain-bearing maturity to assess their performance (phenotype). This is a spatially intensive and time consuming venture, with breeding programs lasting up to 8 years of cyclic inbreeding and yield growth trials before a commercial seed product is generated, at a success rate <0.01% (6).
UP NEXT: Additional challenges with conventional breeding programs and the technological advancements being developed to overcome these issues.
(1) Gardner, Bruce. “U.S. Agriculture in the Twentieth Century”. EH.Net Encyclopedia, edited by Robert Whaples. March 20, 2003. URL http://eh.net/encyclopedia/u-s-agriculture-in-the-twentieth-century/
(2) Dimitri, C., Effland, A., & Conklin, N. C. (n.d.). The 20th Century Transformation of U.S. Agriculture and Farm Policy. Retrieved from http://ageconsearch.umn.edu/record/59390/files/eib3.pdf
(3) Crow, J. F. (2001). Heterosis. In S. Brenner & J. H. B. T.-E. of G. Miller (Eds.) (p. 933). New York: Academic Press. https://doi.org/10.1006/rwgn.2001.0611
(4) Broman, K. W. (2005). The Genomes of Recombinant Inbred Lines. Genetics, 169(2), 1133 LP-1146. Retrieved from http://www.genetics.org/content/169/2/1133.abstract
(5) Troyer, AF. 2006. Adaptedness and Heterosis in Corn and Mule Hybrids. Crop Science 46:528-543
(6) Butruille, D. V. (2006). Applying Molecular Marker information and Data Mining to a Commercial Breeding Pipeline. https://pdfs.semanticscholar.org/presentation/ff37/fccf1937da3f2008c79751e450e7ff0a3724.pdf