Interspecific hybridization can lead to many diverse outcomes that fall into three main categories: (i) hybrid breakdown, in which hybrids are less fit than the parents, (ii) hybrid intermediacy, where hybrid phenotypes are between the two parents, and (iii) transgressive hybrids, where hybrid phenotypes are outside the range of the parental phenotypes. Interestingly, transgressive segregation can create hybrids which are more fit than the most fit parent. Hybrids with transgressive phenotypes in traits of interest such as resistance to drought, heat, or high or low salinity, among others, are of particular interest as they can benefit agriculture, aquaculture and wild populations. Such hybrids may display important traits in the context of climate change, able to respond to novel environmental conditions not previously experienced by the parent populations. While this has been evaluated in an agricultural context, the role of transgressive hybrids under changing conditions in the wild remains largely unexplored.

In their new study titled "Transgressive gene expression and expression plasticity under thermal stress in a stable hybrid zone" and highlighted in Nature Climate Change, former graduate student Lindsey Schwartz, her mentor Dr. Jerry Hilbish and their colleagues from the USA and UK investigated gene expression plasticity — the ability to alter gene expression in response to environmental change — in blue mussel parental species (cool-temperate M. edulis and warm-temperate M. galloprovincialis) and hybrids following a two-week thermal challenge. They found that while hybrid expression plasticity was most often like one parent or the other (50%), a large fraction of genes (26%) showed transgressive expression plasticity (i.e. the change in gene expression was either greater or lesser than that of both parent species) in hybrids. By showing that gene expression plasticity was more often transgressive than intermediate in hybrids, results from Lindsey' s study suggest that transgressive hybrids could play an important role in population persistence under future warming conditions.