Plant scientists at the John Innes Centre have provided a new solution to an old debate on why species hybrids are often more vigorous than their parents. 
 
They found a type of genetic ‘noise’ – caused by a surprising degree of variation in gene activity even for highly similar traits in closely related species.

In this study, the scientists analysed the trait of flower asymmetry in two closely related species of snapdragon. They measured the activity of two relevant genes and its effect on the trait. 
 
In research to be published in PloS biology, the John Innes Centre scientists showed that gene activity may be free to vary during evolution within particular bounds. They theorise that when species hybridise, some of the variation in gene activity may be cancelled out, leading to greater vigour. 
 
Natural selection may be unable to eliminate the noise they identified. It only has a very minor effect on a species for any single gene, but the collective effect for many genes could be substantial, reducing overall species performance.  Hybridisation, however, might partly eliminate the noise. 
 
“This is the first study that analyses the consequences of variations in gene expression on conserved traits in closely related species,” said Professor Enrico Coen from the John Innes Centre, an institute of the Biotechnology and Biological Sciences Research Council (BBSRC), which funded the research along with a Marie Curie grant for early stage training. 
 
The results show that hybrids might be expected to exhibit increased performance in basic traits such as growth. However, they also show that in the longer term, other traits such as those involved in sexual reproduction might be expected to perform less well, accounting for reduced fertility of hybrids. 
 
“Gene expression levels are free to drift around during evolution within particular bounds,” said Professor Coen. “But the cumulative effects of variation explain the conflicting phenomena of hybrid superiority and inferiority.” 
 
This explanation of hybrid vigour covers natural species as well as domesticated varieties. The findings avoid some of the pitfalls of previous explanations. 
 
“Breeders already know there is no magic hybrid vigour gene, otherwise they would have used it by now,” said Professor Coen. “What our study shows is how and why hybridisation can have such a strong impact on performance” said Professor Coen. 
 
This was supported by a Marie Curie grant for early stage training and the BBSRC-John Innes Centre PhD Rotation Program.