Heterosis: A Case Study

When two species are relatively closely related, and not separated by a direct reproductive barrier, not only are hybrids sometimes formed, but those hybrids can be far more vigorous than either parent originally was. These hybrids are sometimes still sterile (they can't make functional gametes or seeds), but in physical characteristics they are often larger, more colorful, more hardy, and so on. In agriculture, such hybrids are often sought after to increase crop production (modern tomatoes, corn, cucumbers and squash, and watermelons are all examples of hybrid species or strains selectively bred for vigor far and above their progenitors), and heterosis is currently thought to be more common in plants than in animals (in no small part due to the high flexibility plants have when it comes to surviving with odd gene combinations).

I've made a bit of a name for myself in creating a large number of Drosera hybrids, finding crosses that work, crosses that don't, and in the process determining which ones work exceptionally well. This hybrid is one of those that has been slow so far to vegetatively propagate (it can be picky about striking from leaf or flower cuttings, but we're getting there), but once started it is a truly impressive plant, and the vigor it displays beyond its parents can't be missed.

A mature D. aliciae plant.
This is the most commonly kept form of D. natalensis, with small flat rosettes of paddle-shaped leaves and distinct, tapered petioles.

Drosera aliciae is a classic South African representative, broad flat rosettes of wedge-shaped leaves up to 5 cm across and typically a species that grows and propagates rapidly, is tolerant of a wide range of conditions, and produces fairly large flower stalks with broad, bright pink blooms. D. natalensis is a close relative and a highly variable, though smaller, species with more paddle shaped leaves and thinner flower stalks with more delicate, thin-petaled light pink blooms. Phylogenetically, these two species are both located along the same evolutionary branch and relatively closely to each other, though they rarely if ever coexist in the wild. D. aliciae is a South African cape endemic, while D. natalensis ranges further east and north, even up into Madagascar.

I have two forms of the latter species, and when D. aliciae and the more common natalensis form in the photo above bloomed simultaneously I took the opportunity and crossed the two together. The result:

D. natalensis x aliciae, mature and in bloom; you can see the base of the inflorescence snaking out at the top left, so heavy it cannot stay upright.

D. natalensis x aliciae shows clear traits of both parents, with the broad wedge-leaved rosette of aliciae but sporting a longer petiole and broader lamina tip inherited from natalensis.

Side view of the cross, where the multiple layers of active leaves are visible.

Heterosis shows in this cross in two distinct ways though: large as aliciae can get, the hybrid beats it out in having rosettes that can exceed 8 cm in diameter, and the number of active leaves on the plant can be nearly twice that of the Alice sundew. The other trait is in the flower stalk. Blooms are slightly smaller themselves than aliciae, but the stalk itself is an impressive structure:

Obscenely lanky flower stalk, in the growing space coiled around several pots.

Not the greatest picture, I know, but it's impossible to capture such a long and lanky structure in any decent image. While both parental species may produce stalks reaching 20-40 cm in length and up to 15 blooms, the hybrid's inflorescence can be more than 100 cm long, and sport over 30 flowers.

Interestingly enough though, while the large inflorescences are similar (and this is a trait I have witnessed in many of the hybrids I've created) the reverse cross with D. aliciae as the maternal parent has not yet reached the same size. It is highly vigorous and more readily divides, a trait of the maternal species in this case, but rosettes are more intermediate between the two species in diameter and possess fewer active leaves at a time.

D. aliciae x natalensis; more rounded leaf tips like natalensis, but more wedge-shaped leaves like aliciae, and smaller than the reverse cross (but more prone to dividing).

Creating two-way crosses this way then allows one to see the effects of one species being the pod parent vs. the other, as the directionality is often crucial to the result of the offspring. Whether it is traits within the mitochondrial or plastid genes inherited from the pod parent that affects the difference in size here, is unknown, but may be a fun project for anyone interested in studying plant genetics and their effects.