Rutgers geneticists discover probable causes of hybrid plant vigor

Agricultural breeders have long observed that when plants or animals from different strains are interbred, the offspring tend to be stronger, healthier or generally more fit than either of their parents, although no one knew why this occurred. Now plant geneticists investigating the maize (corn) genome at Rutgers’ Waksman Institute of Microbiology have discovered a possible explanation for this phenomenon, known as heterosis or hybrid vigor.

The Rutgers findings, presented by research associate Huihua Fu and Professor Hugo K. Dooner in the June 11 edition of the Proceedings of the National Academy of Sciences, have important implications for the understanding of plant genetics as well as applications for improving agriculture.

“A clearer understanding of the basis of heterosis could help us develop new, more productive lines of corn or other plants,” said Dooner.

The scientists began by taking a region of the maize genome and sequencing it, mapping the way the genetic material is laid out. To their surprise, when they sequenced the same region from another strain of maize, “everything was different,” Dooner reported.

Some of the genes from the first strain seemed to be missing from the second, yet the characteristics the genes controlled were still being expressed; that is, their effects were still being observed in the plants themselves. The scientists concluded that closely related genes (known as members of a gene family) that affect the plant in similar ways must be located elsewhere in the maize’s genetic material.

“This is an important finding,” said Dooner. “If you have two members of a gene family but expressing themselves in two different tissues, then a crossbred plant could contain both of the genes and may therefore be better off.”

This is particularly true under stressful environmental conditions where the offspring would be better equipped to respond to stress.

“That may well be the real basis for hybrid vigor,” said Dooner.

Conversely, Fu and Dooner found that inbreeding these hybrids — that is, breeding an individual with others from the same stock — resulted in a reduced number of gene family members.

“Ultimately, this yields inbreeding depression, which is the other side of the coin, where you have less healthy offspring,” said Dooner.

Agricultural breeders have historically developed most of their hybrids through trial and error processes, but they have gained an understanding of what it takes to create vigorous lines. With the revolution in genomics, Dooner observed, molecular biologists have begun to team up with the breeders. They are looking at the DNA, the molecular composition of their lines, trying to predict the performance of hybrids as they follow easily observed genes or molecular markers through generations.

“Now they can also take a different look at heterosis,” said Dooner. “Maybe there are gene families that should be followed together. It could be a pretty major undertaking but with new sophisticated analyses looking at tens of thousands of genes at a time, it may be possible.”