Researchers at the John Innes Centre in the UK have discovered that a degree of variation in gene activity gives plant hybrids the boost they need to become more vigorous. The research, funded in part by a Marie Curie grant for early stage training, sheds light on the mystery of why hybrids outpace their parents most of the time. The results of the study were recently published in the Public Library of Science (PLoS) Biology journal. One of the biggest questions puzzling botanists today is why hybrids between species exhibit two opposing features. While hybrids are usually more vigorous than their parents - what scientists call 'hybrid vigour' or 'hybrid superiority', some are less vigorous and fertile than their parents, a phenomenon known as 'hybrid inferiority'. Over time, scientists have offered their views about why two opposing features exist. In this latest study, the researchers investigated how variation affects gene expression in two species of snapdragon. They found a kind of genetic 'noise' that is triggered by a degree of variation in gene activity. Some of the variation in gene activity is wiped out when the hybridisation of species occurs, which in turn results in greater vigour. 'This is the first study that analyses the consequences of variations in gene expression on conserved traits in closely related species,' explained co-author Professor Enrico Coen of the John Innes Centre. The team said they succeeded in showing that the activity of specific genes 'may be free to vary during evolution within particular bounds by measuring this variation and its effect on phenotype', adding that, 'although such variation may have little phenotypic effect when each locus is considered individually, the collective effect of variation across multiple genes may become highly significant'. By demonstrating just how these effects could potentially trigger both hybrid superiority and inferiority, they effectively offer fresh information about hybrid performance. Led by Professor Coen, the researchers assessed the trait that causes flower asymmetry in two closely related species of snapdragon. By measuring the activity of two key genes along with its impact on flower asymmetry, the team determined that while the variation ('noise') has a minimal effect on a species for a single gene, the collective effect for several genes could be extensive, thus dwindling performance on the whole. According to them, while natural selection may fail to eliminate the identified noise, hybridisation could prove effective in blocking out the noise to some extent. Hybrids show potentially increased performance in basic traits including growth, but in the long term they also show decreased performance in other traits such as sexual reproduction. 'Gene expression levels are free to drift around during evolution within particular bounds,' Professor Coen pointed out. 'But the cumulative effects of variation explain the conflicting phenomena of hybrid superiority and inferiority.' Both natural species and domesticated varieties are covered by the researchers' hybrid vigour finding. 'Breeders already know there is no magic hybrid vigour gene, otherwise they would have used it by now,' Professor Coen said. 'What our study shows is how and why hybridisation can have such a strong impact on performance.'