Rutgers geneticists redefine the nature of hybrid corn

Scientists at Rutgers, The State University of New Jersey, have unlocked an important door to understanding one of the most important crops in the world – corn. Researchers at Rutgers’ Waksman Institute of Microbiology have redefined the nature of heterosis or hybrid vigor, the phenomenon underlying corn’s remarkable success. Heterosis is the robustness seen in hybrids when different lines are crossed and result in higher yields than either of the parental lines would produce themselves.

Maize (corn) dominates agriculture in the United States, where, according to the National Corn Growers Association, 9 billion bushels are produced annually at a value of more than $21 billion. No crop rivals its total grain yield or the diversity of its uses. Virtually all corn varieties grown today are hybrids. Understanding the genetic basis of heterosis could revolutionize our thinking about genetics and pave the way to even stronger, healthier or more productive strains.

Rentao Song and Joachim Messing of Rutgers’ Waksman Institute of Microbiology discuss their findings in a paper published in the July 22 issue of the Proceedings of the National Academy of Science. The paper is currently available online.

Waksman scientists are deeply involved in the Maize Genome Sequencing Project, an initiative to determine the order and position of the genes on the plant’s large and complex chromosomes. The heterosis investigations were a logical extension of the project.

Song and Messing took a region of a chromosome they had accurately mapped and compared it in two strains of maize – to each other, to hybrid crosses, and to corresponding regions in close relatives of maize – two kinds of rice. They also analyzed gene expression in the maize – whether genes were turned on or off – comparing the maize strains and hybrids.

They found that the same genome interval of the two maize varieties and their hybrids, all members of the same species, was substantially different in each, both in size and content. “Genes are missing or added, as are whole sequence segments that contain more than one gene,” wrote Song and Messing. The genetic differences were striking.

When they examined, for instance, the same genome interval of two rice strains, they found far less difference between them – what would typically be expected of any strains from the same species based on pure genetic data.

The significance here is that crossing two different maize strains having dramatically different genetics would be expected to produce a hybrid differing considerably from either parent. The hybrid would have accumulated genes from both parents – genes that complement each other, setting the stage for heterosis. The hybrid should exhibit characteristics with twice the vigor of the parents. To prove that this was really a source of hybrid vigor, the scientists looked at how the genes were expressed in the hybrid offspring and in the parents.

The results showed that the combination of the radically different parental genomes in the offspring produced a hybrid genome where genes absent in one parent were supplied by the other. Messing explained that this “dominance complementation,” as it is termed, might logically be viewed as the basis for hybrid vigor. However, the expression data demonstrated a vigor exceeding what would be expected from the simple addition of previously missing genes.

“The ’whole’ – the hybrid offspring – turned out to be much greater than the sum of the parts,” said Messing. “This led us to conclude that different regulatory factors from other parts of the genome were also operating in a situation we call ’overdominance.’

“What we are finding is a synergism that is much more than just combining the two parents,” he continued. “Not only do the hybrids benefit from genes added by both parents, but their inheritance also includes additional regulatory factors. These two sources of heritable information may well constitute the binary system of the genetic world.

“We got something super – much more than we anticipated. We had heterosis with a firm, verifiable genetic foundation.”