Invasion of the hybrids.

Human activity and climate change are increasingly driving species, which were once separate together, leading to the potential for gene flow. Hybridization between diverged species brings together two genomes which have evolved to meet different adaptive requirements. The unique combination of these traits in a hybrid may be beneficial or maladaptive, but either way it results in increased phenotypic variation. A percentage of hybrid individuals may, therefore, find themselves able to exploit environmental niches which their progenitors cannot, leading to invasive hybrid swarms becoming established in new habitats. Previous research into hybrids, most famously that of Loren Rieseberg and co-workers (Rieseberg et al. 1999, 2003) in sunflowers, demonstrated that hybridization can give rise to transgressive segregation of adaptive traits, wherein the combination of favourable alleles from both parents in hybrids can enable them to outperform either. However, the question still remains as to how much of the competitive ability of hybrids is a direct result of admixture and how much is the result of selection after the fact. In this issue of Molecular Ecology, (Czypionka et al. 2012) describe their study of transcriptional changes resulting from hybridization in a fish hybrid termed invasive sculpins (Cottus). Using gene expression microarray assays, they compare gene expression in both wild and lab-reared invasive hybrids to the progenitor species and experimentally produced F(2) hybrids. They demonstrate that whilst hybridization alone does result in higher variance in gene expression (some of which is transgressive), many of the transgressive changes distinguishing the invasives appear to have come about subsequent to the initial natural hybridization event. They speculate that initial success of the hybrids in their new habitat is facilitated by hybridization, but that optimization of the invasive phenotype and removal of maladaptive traits rapidly reduces the variation in gene expression seen in early hybrids.