We’ve talked about inbreeding depression a lot on this forum, but I recently came across the concept of outbreeding depression. Basically, this is the exact opposite of inbreeding depression; as more distantly related plants or animals are crossed, fitness goes down.
This could be due to lower fitness to the local environment or due to genetic factors. Here is a quote I found online:
In wild populations of the herbaceous scarlet gilia (Ipomopsis aggregata), matings between parents separated by only 100 m produced offspring with a reduced lifetime fitness that was caused by outbreeding depression, although not surprisingly the decline in fitness depended in part on environmental heterogeneity and the associated selection regimes (Waser, Price and Shaw, 2000). Effects at an even finer spatial scale were found in Nelson’s larkspur (Delphinium nelsonii) when flowers were hand-pollinated using pollen from between 1 m and 30 m away. Progeny from intermediate crossing distances (3 m and 10 m) grew approximately twice as large as the progeny that resulted from crossings between either nearby or more distant plants. This was presumably because pollen from intermediate distances did not adversely affect the fitness of offspring, whereas pollen from plants that were 1 m or 30 m away led to inbreeding depression and outbreeding depression, respectively (Waser and Price, 1994).
If it is simply a matter of fitness to the environment, this would seem suggest that once we have assembled a diverse initial grex and let it stabilize into an adapted local landrace, we should think twice about importing novel genetic material.
The other suggested mechanism was a “loss of positive epistatic interactions”; in other words, the fitness of plants doesn’t depend on individual genes, but on a whole system of genes working in concert; this can be disrupted by outbreeding. I’m not sure how fast this would be rectified in an evolving landrace.
Does anyone have thoughts on possible mechanisms and their practical implications?
Outbreeding depression is more of an issue the more genetic distance there is between the two parents.
It is much more noticable in animals than in plants, but probably begins to become a problem when trying to make hybrids of distantly related species.
That’s why it becomes a lot of work selecting for the traits you want and trying to correct any potential genetic mis-matches that appear.
Sometimes back-breeding can be helpful in those situations.
Working with a mix of heirloom genetics within a species of plants you will be dealing with inbreeding and working to create a more diverse genetic mix and adapt to your growing conditions.
Once you have a nice grex or landrace, as long as you don’t add a larger percentage of outside genetics than the current population you should be okay.
Crossing different types of corn likely won’t have serious outbreeding depression because they are closely related.
Trying to cross corn with another grass species (as some discussions here have explored) is likely to have a lot of outbreeding depression and genetic problems in the offspring.
I would have thought that would be the case, but the excerpt I posted above is talking about wild populations separated by only 300 meters having a noticeable outbreeding effect. I suppose for a wind-pollenated plant like corn the relevant distances would be greater, assuming there was a wild corn population. Any thoughts on that part of the paper?
I agree that for most of our crops, the initial inbreeding is more of a problem than any outbreeding would be.
I think they are being a little loose with what they are calling “outbreeding depression” or they just have a flawed study.
They are working with wild populations which should have more genetic diversity than our domesticated crops. They are also likely extremely well adapted to their specific growing locations.
From what I understood they are taking pollen from an outside population and pollinating a native population. Then the offspring are being grown in that native populations same location.
When the offspring fail to perform as well as the mother plants, they are calling that outbreeding depression.
But isn’t that outcome what might be expected after being crossed with genetics that have not been in that exact spot for however many generations?
Even at microclimatic levels I would think the results make sense from less than ideal genetic adaptations, not from genetic depression.
To test this they would need to grow out both the native plants and the mixed offspring in a controlled setting and then compare outcomes. I suspect the offspring would do just as well and possibly better.
If they were to underperform in a controlled setting, that would be a potential sign of outbreeding depression.
Karyotype differences and adaptive differentiations are two mechanisms which can lead to outbreeding depression. In the long term, with selection, the population having gone through the outbreeding depression can therefore perform better than the parents
See Frankham et al. https://conbio.onlinelibrary.wiley.com/doi/10.1111/j.1523-1739.2011.01662.x
My suspicion is that this is relevant to landrace gardening, in that adding more genetic diversity isn’t always going to be what you want – I have seen people state that they already have an excellent population that is well adapted to their climate with plenty of genetic diversity within it, and they’re very protective of it. I think that’s valid.
Once you reach a point where you have a local population that is everything you want, and it has sufficient genetic diversity to stay healthy long-term, it makes perfect sense to become picky about what genes you might add in. I suspect Joseph Lofthouse is extremely picky about what new tomato genetics he adds, for example – he’s selected against red flesh and inserted stigmas, and I don’t think he wants them back.
A very obvious example of outbreeding depression would be wide crosses where the parents are sufficiently distantly related – in the same family and not the same genus, for example – that the progeny can just barely survive, but only with a lot of special coddling, such as embryo rescue.
I would say such wide crosses are highly desirable for someone who wants to experiment and see if they can discover brand new traits (or combinations of traits) that aren’t available in either species alone. For someone who just wants to relax and have plants that are easy to care for, wide crosses are less likely to be a valuable tool.
Still, I personally think they’re still worth a try. A wide cross plus STUN is unlikely to work, but that doesn’t mean it never will – and if that combination does, you may have found something new that’s really cool.
I don’t worry so much with things that don’t easily cross like beans or tomatoes, but I am very careful about bringing in new things that do easily cross.
I’m very picky with those outbreeders and if I do bring in something new it is because I have reason to believe it has traits that I want to introduce or reinforce but I don’t just turn something new lose in my garden. I grow it in isolation the first year to see if really grows well and if I really want it. In some cases, I might go ahead and introduce it but only if I have plenty of my own seed in backup in case I need to write a year off. Take corn for example, I might plant a new variety to have a head start in crossing, if it turns out I like, it but be ready to discard it all and go back to my own, if I don’t.
An example of outbreeding depression for me would be if my 90 days seed to seed corn turned into 110 days or if my three or four-pound muskmelons turned back into 15-pound muskmelons, let alone if the flavor was degraded. That’s why I don’t want the unpredictability of GTS seeds.
In my situation, just growing a small garden for good things to eat and wanting to continue that such things as wide interspecies crosses are pretty worthless. I’ll leave such as that to those who work in laboratories.
There may be an exception in the case of someone willing to invest massive amounts of seed over massive amounts of land, that is many thousands of seeds over many acres, but for me this notion of STUN is synonymous with DUMB.
There is a difference between unwanted genetic traits and actual outbreeding depression.
Outbreeding depression is the result of a combination of genes from two organisms which have become separated to the point that when they are bred together there is a genetic mis-match. Meaning the genes fail to align and work together harmoniously and result in genetic disease or instability.
@Lamontana Thanks for the article, I will look into it; good point that outbreeding depression, insofar as it is a problem, could give way to greater advantages later.
@JinTX Good point that any study of outbreeding depression as a genetic phenomenon (along the lines of inbreeding depression) will be confounded by the effects of non-adapted genetics; I suppose that in the wild even a difference of 300 meters could be significant for microclimate and soil. Th Ipomopsis aggregata used in the study are linked are native to the mountain west of the USA, where differences in nearby microclimate can be very large.
@UnicornEmily Good point about wide crosses; they tend not to be vigorous, at least at first. I assume this is how many closely related species stay separate: they can cross rather easily, but the resulting hybrid seeds are initially weak and less viable, and so they are easily overwhelmed by the non-hybrid plants in a population.
@MarkReed Yes, once I have a personal landrace that I’m happy with, I will probably keep donating seeds to GTS, but might not be as interested in planting them. The mixes are a good way for people to start out, in my opinion.
One thing I’m considering in all of this; it seems that wind pollenated plants should show less of this, since they would always have crossed over much wider distances, right? On the other hand, plants that are reluctant to cross would possibly show more of it, I would assume. At least in conventional thinking, wind and insect pollenated plants show more inbreeding depression and more hybrid vigor compared to plants that tend to self-pollenated a lot.
In particular, I’m wondering if anyone has ever noted hybrid vigor in common beans; all I’ve found are people noting outbreeding depression when crossing between the Andean and Mexican common bean gene pools.
(Chuckles.) Yes, I agree. Using STUN with expensive or rare seeds is very dumb!
I like STUN as a way to sow groundcovers. Do I have thousands of seeds of that species, and I’d be happy to have some and don’t mind if I don’t get any? Do I want something in that space besides bindweed? Great! Broadcast sowing and ignoring them afterwards it is. I do this particularly often with edible native weeds.
For everything else, water and mulch are necessary, and that’s 99% of everything I care about.
Growing something new in isolation is smart! That’s a great way to determine, at no risk to your existing population, whether you want to let it cross with the rest. Put it in “quarantine,” so to speak.
Sowing ground cover or wildflowers or other things that have a reasonable chance of tolerating your climate using so called STUN is one thing. Doing it with food crops and expecting something to eat from it is another. A 1% success rate sounds about right. I love the flower Dame’s Rocket which I’m told is edible. You can throw those seeds about anywhere and they will grow. Wild daises and wild Balck Eyed Susan will too.
I’ve come across a number of crosses in common beans over the years but unfortunately, I can’t say I’ve seen much in the way of hybrid vigor. Nor have I really identified decline from inbreeding depression in varieties I’ve grown from my own seed for a long time, especially my KY Wonder beans improved over the last thirty years.
One possible exception on the hybrid vigor was with some common x runner beans that someone gave me one time. The vines were absolutely gigantic, even bigger than normal runner beans, but despite massive blooming they made even less beans. Three or four pods with two or three seeds each. The next year those seeds did the same thing. I didn’t waste any more time or trellis space on them after that.
Emily: You sure got that right. People from thousands of miles away want to send me the amazingly astonishingly magnificent seeds from their great-grandmother’s garden on the coast, and my heart sinks. I don’t want them, they’ll do poorly here. Those seeds, or some just like them, already failed for me, decades ago.
That actually sounds like an example of potential outbreeding depression. Yes the plants grew well, but because they were a cross of two distantly related beans they probably experienced some problems when it came to pollination, possibly being semi-sterile.
Yes, because in the case of easily cross-pollinating plants some percentage of your seed is now going to be 50% the new one and if it was bad for some reason that’s a problem. It’s ok if you have plenty of your own seeds in back up and can just go back to them the next year. If it turns out the new one is great, and you want your whole population to become 50% it, then you can save mostly seeds from those mother plants.
I have an example not exactly like that but along those lines. My broccol-ish is doing well but I’m pretty sure the best genetics as far as cold tolerance, production and flavor came from January King cabbage and Long Island Brussels sprouts. I have those growing now and if they survive the winter and bloom next year, I’ll let them cross with each other and with the rest of the broccol-ish which is now feral. Then I’ll save seed about 40% from JK mothers, 40% from LIBS mothers and 20% existing broccol-ish mothers and start over on selection. I think it might be possible to have something that makes Brussels spouts-ish things in winter and broccol-ish things in early spring. All while it’s too cold for bugs and too damp to hassel with watering.
That is interesting, but mostly just interesting rather than actionable because the result of continuing with it to find out what might eventually come from it just involves too much time and effort all of which detracts from my most important goal of filling the pantry each season.
I do have another possible example of it as well. When I attempted to cross Ipomoea batatas with Ipomoea pandurata I managed to obtain a few seeds. The seeds from batatas mothers did not sprout, I still have the seeds from the pandurata mothers. I say possible because although I covered the flowers with empty tea bags, I can’t be 100% sure a bee didn’t sneak in some batatas pollen on the batatas flowers and vice versa. I could have also self-pollinated the flowers by accident while transferring pollen. It’s commonly believed that both of these species are self-incompatible, but I know that isn’t true, at least not universally so. Sweet potato seeds are a bit finicky too, when it comes to sprouting, so I also can’t say the germination failure had anything to do with foreign pollen even if it was truly involved.
I may take up this experiment again with more attempts and better methods because both are perennial but batatas only so in warm climates. Pandurata is cold hardy but tastes like tofu dipped in cat pee, or at least I imagine so. Still all the pieces of something great are there. They also easily take to grafting in both directions, in case that would help.
I don’t know that this could ever work because I’ve done a lot of research and people with far more knowledge than me and with much greater resources have tried it before, with no real results.
It’s cool to hear the Long Island brussels sprouts have been so good for you, because I’ve been finding their genetics great, too!
Last spring, I saved seeds from several Long Island brussels sprouts plants that were flowering next to several kohlrabi plants (probably Early White Vienna), and I sowed a bunch of those seeds in the fall. I got around 200 seedlings, some of which I accidentally stepped on later, and others which got selected out by the winter.
I now have six large, healthy plants with huge swollen stems that look like kohlrabi with brussels sprouts growing on them. They survived last winter with no care, and actively grew through the winter. They survived the summer with no bug damage, even though an unknown kale on the other side of the garden was covered in aphids. They didn’t even flower in spring, despite having gone through an entire winter; it seems they’re determined to be biennial. So that means they’ve gotten nice and big now. I’m finding that the results of this cross is really pleasing me.
I’m expecting them to grow through the winter again, like they did last year. My loose plan is to cut off the leaves to eat during the winter, starting from bottom to top, to encourage the underarm buds to get huge. Then I’ll let some of those go to seed and eat others as brussels sprouts. I’ll probably also cut some off and plant them to clone them, since I’ve heard that’s something you can get away with.
So yeah, my experience with Long Island brussels sprouts has so far been great, too. That variety likely has excellent overwintering genes.
Just wanted to say that issues with fertility and genetic regulatory networks in wide crosses can often be solved with further crossing, either back cross or going down the F2, etc. Sometimes they can’t, the plant is weak for whatever reason, or won’t flower, or something. But in my experience doing wide crosses, hybrid vigor occurs more often than not, despite some seedlings that don’t combine well. It such a powerful effect that it keeps inspiring me to do more and more wide crosses simply because of the strength of the resultant seedlings, which despite some fertility issues is really impressive.
The problem seems to be caused by incompatible genes in the Andean and Mesoamerican genetic pools, and the results are very severe.
“The F1 plants were characterized by retarded growth, chlorosis of the trifoliolate leaves, the formation of adventitious roots on the hypocotyl, and reduced root growth.”
Does anyone know which gene pool the various bean varieties available in the USA belong to? Is there a paper on this somewhere? I imagine this sort of mismatch could hamper the development of common bean landraces, because it might not be sorted out over time; instead, the two pools would exist side-by-side without mixing, since any hybrids would be “crippled” and probably rouged out.
@MalcolmS, I didn’t know that there were differing origins of common beans, but I’ve never seen anything like that when they cross so mine must be all the same. When they cross, I see different color seeds show up, longer or flatter or curved or straight pods, differences in growth, bush or climbing but I don’t see much as far a hybrid vigor or improved production.
I have to turn this back to sweet potatoes because I eat, sleep and breath sweet potatoes and I have seen some weird things with them. The university guys say there are four different compatibility groups with sweet potatoes. I have no clue what the origins of these groups are or how they are defined nor which one or one(s) mine are.
From what I’ve learned every seed spouted sweet potato is genetically unique and over the years I have seen hundreds of them, maybe thousands. Out of those I have seen some weird things like spindly little sprouts all curled up like a spring and that don’t live very long after sprouting, I’ve seen maybe three or four of them.
My favorites for the novelty of it are the “vampires”. They don’t make chlorophyll; the leaves are white or pale yellow. I’ve probably seen ten of them over the years and they will grow and even bloom although very slowly, unless they are touched even briefly by direct sunlight in which case they are dead in a few minutes. That’s why I call them vampires.
Do you think I might have genetics from more than one of these compatibility groups and rarely but once in a while the wrong two manage to make a seed?
