Adaptedness and Heterosis in Corn and Mule Hybrids

The origin of U.S. Corn Belt corn (Zea mays L.), heterotic groups, and heterotic patterns becomes less obvious with more cycles of breeding. Heterosis is poorly understood; simple curiosity cries out for more information. I endeavor to shed light on the effect of adaptedness and heterosis on U.S. Corn Belt corn. I relate pertinent happenings in the phenomenal increase in U.S. corn production. I briefly review the origins of Northern Flint and Southern Dent races of corn and two major, persistent open-pollinated cultivars; and how corn hybridization was preceded and eased by hybrid species of the horse (Equus spp.). I discuss heterotic groups and patterns. The objective of U.S. corn breeding has been to adapt a tropical crop to a temperate climate. Adaptedness is important. Open-pollinated cultivars emphasized local adaptation, but some cultivars were more popular, widely grown, and better adapted over a broad geographic region. Hybrid seed corn companies grew larger by selling more widely adapted hybrids that favored germplasm from the more popular, widely grown, better adapted open-pollinated cultivars containing more genes for adaptedness. I examine morphological differences between inbred parents of a widely adapted hybrid. Relatively constant percentage of heterosis of well-adapted hybrids over years is due to seasonal climate affecting hybrids and their parent inbreds in a like manner because of their selection for adaptedness. Adaptedness has been more important than heterosis in the U.S. corn yield and production increases. Adaptedness in analogous heterotic species hybrids of the genus Equus, where body size is female dominant, apparently discriminates for body size between mules and hinnies that have virtually identical genotypes—adaptedness determines superiority over and above heterosis. ON 28 SEPT. 1838, Charles Darwin read Malthus’ Essay on the Principle of Population and first conceived the concept of natural selection for adaptedness (Encyclopedia Britannica, 1983). In 2001, corn became the highest tonnage crop worldwide: 557.6 million Mg of maize, 542.4 million Mg of rice (Oryza sativa L.) paddy, and 535.6 million Mg of wheat (Triticum aestivum L.) (UN/FAO, 2002). United States corn production was 53 million Mg [2 billion (2 3 10) bushels] annually in the 1930s, when corn hybrids were first commercially grown, and yield averaged 1518 kg ha (24.2 bushels acre). Corn production grew to 76 million Mg (3 billion bushels) annually in the 1950s, to 150 million Mg (6 billion bushels) annually in the 1970s, and to more than 229 million Mg (9 billion bushels) annually for the past 9 yr. The USDA-NASS (2005) estimates 2004 U.S. corn production at a record 299.7 million Mg (11.8 billion bushels) and the average yield is estimated at a record 10 059 kg ha (160.4 bushels acre) (Fig. 1). At $150 per bag for 30 million bags, the annual U.S. seed-corn cost is $4.5 billion. At $2.50 per 25.4 kg (1 bushel) and 299.7 million Mg (11.8 billion bushels), the annual U.S. farm value is $29.5 billion. The relatively high cost of seed corn (|15% of the crop’s farm value) is justified by higher yielding, newer hybrids. I relate pertinent happenings in the phenomenal increases in U.S. corn yield and production. The increases over time are associated with better adaptedness, corn breeding, hybrid corn, mechanical harvest, better soil fertility (particularly more N), single-cross hybrids that are easily identified by the farmer (better hybrid choice), field-shelling harvest (faster feed back for yield), improved cultural practices (particularly higher plant densities), and biotechnology. Heterosis in hybrid corn is well used but poorly understood. I revisit the accepted origin of Corn Belt corn and its heterotic groups and patterns. I address adaptedness due to natural and human selection that caused race and cultivar formation; evolution of heterosis, genetic diversity, and heterotic gene action; corn hybrids including combining ability, heterotic groups and patterns; and mules as the hybrid corn prototype. Major corn events are in chronological order. Mules are an afterthought. I offer the mule as a Rosetta stone providing an easily understood example of natural selection and adaptedness affecting an organism’s performance over and above its heterosis. Sections begin with a short description or history of the topic.