Genetic "Improvement" in Maize

When we were asked to create a blog on some topic of Evolutionary Biology, I immediately thought back to one of my first year lab practicals where we demonstrated Mendelian & Population Genetics using Zea mays (maize) cobs. This got me thinking to how and where this corn came from. I found a journal article by Yamasaki et al., (2005) which explained that this maize was domesticated from teosinte (Z. mays subsp parviglumis) between 6000 and 9000 years ago in southern Mexico. Due to its adaptability and variability over a wide range of environments, this original maize, named ‘Landraces’, spread across America.

Over time, improvement of maize crops has occurred due to the selection of key morphological and agronomic traits that have been enhanced/controlled through the selection of specific alleles within the original maize genes. This resulted in loss of genetic diversity and inbred lines of maize currently used in hybrid maize production. It was found that artificial selection to improve the crops not only affected obvious morphological and developmental differences such as enhanced productivity and performance (yield, resistance to biotic and abiotic stresses) but also specific biochemical pathways (such as enzymes used in the starch synthesis pathway).

 A large-scale screening was conducted to discover the genes responsible for maize domestication and improvement. During the domestication of Zea mays, the genes that had been heavily selected for (for desirable traits) are now the ones that lack the most genetic diversity. Whereas in neutral genes (genes not modified or influenced by human interference), higher genetic diversity is retained and is expected to be reduced only by bottleneck effects, as shown by figure 1 (Yamasaki et al., 2005). By identifying the selected genes, the reconstruction of the gene selection history of maize can be ‘re-created’ with focus on candidate genes for maize improvement. In theory, artificially selected genes can be screened for in any animal or plant domesticate, although much depends upon the relative levels and patterns of gene diversity in neutral, selected and wild taxon genes. This holds potential for many future developments in reverting genetic bottlenecks and increasing genetic diversity within traits that have been heavily selected for due to domestication.