The evolution of maize (corn) from teosinte

left to right: teosinte - F1 hybrid -- maize

 Teosinte is a much branched large cane-like grass that grows in Mexico.  It has a small seed head with 5-12 kernels instead of the 500 kernels of maize ears.  The seed coats are hard and indigestible for animals.   It is the ancestor of our modern maize plants, created by Native American breeders thousands of years ago, but it doesn’t look much like maize at all.  Plant geneticists have traced the breeding of this crop via genetic analysis.  It is an interesting example of how far morphological evolution can go with just a few genes changed, in this case by human hands.

 Genetic analysis answered questions concerning the evolution of each part of the plants.  The change in ear morphology was determined to be due to approximately five genes by crossing teonsinte and maize and taking it to the F2 generation, where only 1 in 500 progeny produced heads that looked like either parent.  This is close to the expected 1/1024 that one would expect if there were 5 genes responsible (a 3:1 phenotypic ratio (1/4) for each gene, to the fifth power).  So the wild difference in the ears was due to just 5 genes.  Next the branching habit of the two plants is very different – a shrub versus a single tall cane.  The gene responsible, tb 1,  is expressed in the lateral growth nodes and inhibits branching.  The amino acid sequence encoded  is the same for the alleles of both plants; only the promoter sequences differ.  Maize expresses twice as much tb 1.  Next, a single allele of another gene, tga 1, is responsible for the hard fruit case. 

 

Corn cobs hang around a long time.  I can remember seeing ancient cobs on a long hike in Utah to an ancient Native American encampment.  It was strange being far back in the wild, with no other people, looking at pottery shards and old cobs about the size of hot dogs.  They have found cobs as old as 4400 years.  DNA analysis of these showed the presence of the modern tb 1 allele, but a modern allele for another gene, su 1, which allows for tortilla production, was not present.  There are many other genes with lesser effects that have maize-specific alleles compared to teosinte.  The thought is that their effects were revealed when the important mutations occurred with tb 1, tga 1 and su 1.  Then, the farmers could select the best plants in their eyes and these lesser genes could have their best alleles selected for.

 

Even though this was human selection, natural selection could operate the same way.  Is there really that much difference?  The alleles appear, they confer benefit or advantage, and the progeny multiply.  And just a few genes can make a big difference, which would allow other, lesser genes to evolve as well.

 

 

[This is part of my book summary on Barton et al.'s Evolution]

 Image:  John Doebley http://teosinte.wisc.edu/images.html