How do plant hybrids aid invasions?

There’s a common belief that hybridization of plants can aid invasions through adaptive introgression. This is where a hybrid breeds with one of its parent species, so that the parent species eventually adopts a new gene. An example might be herbicide resistance transferring from crops to wild plants. In a new paper in Annals of Botany, Chengjun Li and colleagues consider how hybrids may help in a different way.

A limiting factor on invasions is finding plants to mate with. “Following long-distance dispersal, a self-incompatible (SI) colonizer may experience an Allee effect because of limited mate availability,” write the authors. “However, if hybridization can occur with a resident species (or another invader), preferential backcrossing in further generations to the phenotypes most similar to the incoming species in future generations could reconstitute the species’ genome. Thus, the incoming SI species can establish, persist and proliferate without any conspecific mates initially being available.“

To test to see what effects matter in a plant invasion Li and colleagues examined the breeding systems of Cakile edentula, American searocket, which is self-compatible (SC) and C. maritima, European searocket, a self-incompatible (SI) species . Both species can be invasive outside their home ranges.

The team produced a series of crosses over generations to see how reproductive success was affected. “In our study, the early generations of backcrosses between hybrids and the SI parent had high similarity in many traits to C. maritima and better performance compared with other hybrids, with showier floral display, greater flower production and larger plants. Therefore, after repeated backcrossing, the breeding system in subsequent generations may shift over to completely SI with its corresponding phenotypic characteristics. Such hybrids would be barely distinguishable from the original species since they have the same phenotypes and breeding systems,” say Li and colleagues. However, they warn that their results are specifically tied to Cakile.

“[W]hile the non-adaptive mechanism of invasion following hybridization is supported in our study and it is tempting to generalize, caution is needed when making predictions in the absence of detailed information on plant breeding systems, their inheritance and reproductive isolation.”