Question: I asked earlier about the mixing of two species of willow and how they react together and wether one of the parents passes on a dominant gene. What is the case?

Rachael Eggleston answered on 3 Dec 2024:

Ah, yes!! Thanks so much for following up, and thanks for your patience waiting for an answer! Like I mentioned, hybridization (the mixing of two species) can lead to a lot of outcomes. I’ll do my best to explain them, but there’s a lot to cover and it’s pretty complex.

It’s important to say that the outcome you’ll get will depend a lot on which two species are mixing, sometimes which two individuals from those species, and sometimes even which species is the mother/father plant! It’s complicated stuff with a lot of factors.

1.) Upon pollination, it may just be that the pollen/egg cells don’t mix together appropriately and seeds don’t develop. In this case, hybridization could have occurred, but didn’t since the genetics don’t line up.

2.) Upon pollination, the seeds develop, but don’t germinate (sprout/grow). Something is wrong genetically- again, no hybrid.

3.) Upon pollination, the seeds develop *and* germinate! Hooray, we have a first-generation (F1) hybrid, a direct cross between parents from species A and species B. From here, we have a lot more options. (Worth noting here you *could* see ‘hybrid vigor’- where a hybrid plant grows faster/stronger/better than plants that are just species A or just species B.)

4.) The F1 hybrid could be sterile. It grows well and is healthy, but something is genetically mismatched and it can’t have any plant babies. It lives and dies without breeding.

5.) The F1 hybrid could be fertile. It grows well and is healthy, and it’s able to breed. Depending on the exact case, it could be able to breed with other F1 hybrids, with individuals of parent species A, with individuals of parent species B, or any combination of the above. Maybe two hybrids can’t breed, but they can breed with individuals from species A or B. Maybe hybrids can’t breed with species A, but they can with other hybrids or species B. (You get the point- lots of options.)

There’s a lot more options from here. You can get a hybrid swarm, where you get a ton of hybrid individuals of varying percentage ancestries (90% A/10% B, 50% A/50% B, 10% A/90%B… all the combos). These can create a hybrid zone between species A and B, where individuals of A and B can’t breed with each other since they can’t ‘reach.’ These hybrid swarms could also go extinct due to later issues with their genetics that didn’t happen at first. They can also potentially develop into new species, especially if the hybrids can only breed with one another and not with their parent species!

There’s also the chance of genetic swamping or introgression. Swamping happens when one parent species gets ‘drowned out’ by hybrids, which is usually the case if hybridization happens between a common species and a rare one. Let’s say you’ve got 100 species A and 5 species B. One A/B pair happens to hybridize in a *really* unlikely event, leading to 5 F1 hybrids. Suddenly, the 5 species B individuals only have a 50/50 chance of breeding with another B or with an A/B hybrid (we’re pretending the initial hybridization was crazy rare and can’t happen again). If this keeps going, very often you end up with no ‘pure’ species B left, only species A and A/B hybrids.

Introgression is just when a hybrid keeps breeding with one parent species (backcrossing). Like if an F1 hybrid breeds with an individual from species A, you’ll get a hybrid that’s about 75% A and 25% B. This one breeds with A- 87.5% A, 12.5% B. This one *also* breeds with A- 93.75% A, 6.25% B. (And so on and so forth!) Eventually, what you get is a plant that’s *basically* just species A… with a tiny bit of DNA from species B. This tiny bit of B DNA has ‘introgressed’ into the genome (all the DNA) of species A.

That’s all talking about genetics- how the DNA of the two species interacts (or doesn’t, depending). You were asking a little bit more about morphology, the plant’s physical appearance. This, like everything else, depends a lot on the exact situation.

Usually, when species A and B mix, you’ll get offspring that looks more or less midway between them. If A has red, round leaves and B has blue, thin leaves, the F1 hybrid will probably have purple, oval leaves. This isn’t always true; you can also see cases where the F1 will look almost exactly like one parent or the other, usually with a few tiny hints that it’s a hybrid. For example, maybe the F1 has red, round leaves, like A… but the leaves have a pointy tip, like B’s leaves do. You can also get cases where depending on the trait, one of the parent species will be dominant. For example, maybe the hybrid has red, thin leaves. In this case, A has a dominant gene for color (red), while B has the dominant gene for shape (thin).

This is true for everything, of course, not just leaves! You can pretty commonly see cases where, all on the same hybrid, some of A’s traits will be dominant, some of B’s traits will be dominant, and some traits will be an even A/B mix. You can also see cases where hybrids sharing the exact same parents will look different- some more like A and some more like B, like how human siblings may look more like their mom, dad, or an even mix of both.

All this is a lot of information and very complex stuff- so if even some of it made sense, you’re doing great! And, of course, please feel free to follow up again if you’d like more details or clarification on anything 🙂