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PERSPECTIVES

Adaptedness and Heterosis in Corn and Mule Hybrids

Department of Crop Sciences, University of Illinois, Urbana, IL 61801

^* Corresponding author (atroyer{at}uiuc.edu)

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.

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