Endosperm drives diversity

​Humans’ interest in the corn endosperm use for food, ease of transport of kernels and annual reproduction of corn caused corn to gain diverse genetics.  As cultures developed local uses for corn, they selected endosperm characters to fit their needs. Flinty endosperm types with hard endosperm have Compact cells especially towards the outer layers of the endosperm.  Perhaps favored initially because of having less moisture upon maturity and therefore less damage from freezing and perhaps easier to store. Popcorn is a variant of flint corn in which the concentrated of compact cells in the outer portions of the endosperm surround softer cells with more moisture.  Heating causes expansion of the inner cells leading to the explosion exposing the outer cells.  
 
Human selection of flinty corn led to specific food uses thousands of years ago and to preferences in current dry milling processes.  Similar pressures also led to the softer starch types such as the floury corns in which the endosperm composition led to extremely easy grinding into flour.  This was favored by the Aztecs and Incas because of this characteristic.
Dent corns are an intermediate for the flint and floury corn types.
 
Most of the corn endosperm types are greatly associated with recessive genes influencing the starch metabolism. A recessive gene (su) reduces and delays the metabolism in endosperm that results in sugar to be transformed into starch. The gene sh2 results in even more sugar.  Most corn endosperm starch is composed of a branched, poly-carbohydrate called amylopectin and a non-branched one called amylose. A recessive gene results in all amylopectin and is called waxy.  A different recessive gene results in most amylose starch.
 
Ancient selections for favored endosperm types often included isolation of corn populations probably encouraged by humans’ realization that these types required freedom of contamination from other types.  The consequence of isolation and endosperm selections over multiple environments allowed the selection of diverse genetics for other characteristics of corn.  Today we benefit from this genetic diversity worldwide.