Do hybrids help a plant expand their range, or do they get in the way?

Over time plant ranges can expand and contract. An added complication is that species can also hybridise with their closest relatives. How do these hybrids affect the spread of species when they move into new territories? Thais Pfeilsticker and colleagues examined the rare Eucalyptus risdonii and how it interacted with E. amygdalina as it expanded into new habitats. 

E. risdonii, commonly known as the Risdon peppermint, is found in south-eastern Tasmania. You can find populations of E. risdonii among the more widespread E. amygdalina, called black peppermint locally. If there were a botanical-themed remake of the comedy Twins, then E. risdonii and E. amygdalina would be solid contenders for the lead roles. Despite being genetically close, the widespread E. amygdalina looks like Arnold Schwarzenegger growing to 30m tall compared to its sibling. E. risdonii grows to a more Danny DeVito height of just 8m.

The genetic connection means that the two trees can produce hybrids, and these are found where the trees meet and quite a way into E. amygdalina territory. There is an idea that E. risdonii is moving into E. amygdalina’s habitat and the hybrids are helping.

The hybrids are somewhere between the two trees in morphology. Of particular interest, their flowering is also somewhere between the two parent species. E. risdonii and E. amygdalina flower times overlap, but their peak flowering differs by two or three months. The hybrids fall between the two peaks and so can backcross with either parent.

Pfeilsticker and colleagues investigated the survival of the pure species and their hybrids in the natural hybrid swarm in Government Hills, Tasmania. Fires in the area seem to benefit the trees closer to the E. risdonii phenotype, so the botanists expected to see that the hybrids performed more poorly than the two species, explaining how the species maintain separate identities changing climate was favouring E. risdonii.

Sure enough, the hybrids were least likely to survive. This difference was apparent early on as the hybrids suffered more damage both from insects and pathogens. The authors suggest that the crossing of the genotypes disrupts the defences of the plants. The damage earlier may explain their increased mortality a couple of decades later.

The hybrids that do survive to reproduce have a variety of partners. Reproduction with other hybrids doesn’t seem to help survival. Reproducing with one other parent species pulls the hybrid offspring towards a more successful phenotype, and the E. risdonii phenotype was the most successful of the plants.

Yet despite being the least fit phenotype, hybrids could still have an important role in the spread of a species, say Pfeilsticker and colleagues. “While F1 hybrids may be transient in an evolutionary sense, we show that fit hybrid phenotypes resembling the putatively invading E. risdonii can be obtained from their F1 hybrids’ open-pollinated progeny after just two generations. These phenotypes are likely backcrosses, highlighting the potential for pollen-mediated species invasion through directional selection and multiple generations of backcrossing leading to ‘resurrection’ of the pollinating species…”

Effectively the hybrids mean that the whole seed of E. risdonii doesn’t have to travel to the new habitat immediately. Instead, the area can become colonised through successive waves of pollen bringing E. risdonii genes.