Compounding corn genetics

Good evening folks,

I have a theory question for you corn breeders out there. I know that in a landrace, the more genetic material you have in a given crop means there's a reasonable chance some of the plants contain the right combination of genes to survive whatever natural threat might harm the crop in a given season.

My question is this:  What would happen if, instead of having a wide variety of plants with a wide variance of resistance to many things, you bred a single strain that had built in resistance to most known threats?  

For example, say you started with one variety of corn, detassled it, and pollinated it with "disease resistant variety 1".  The next generation,  you do the same,  but pollinate with "disease resistant variety 2". The next generation,  you do the same, but pollinate with "disease resistant variety 3", & so on until you have developed a single variety that contains compounded genetic resistance to all known threats.  Is this possible?  Has it been done?

I'm sure this is much more complicated than my simple way of thinking, otherwise all modern corn varieties would be resistant to everything known by now.  What are your thoughts?

I think your idea will work, but you'll still be mixing up the genetics enough to have differences between plants. A lot of us like this, but that's a move toward landracing. If you want uniformity, you'll have to stabilize your variety by selection and inbreeding. In that part of the process, you'll lose some of the genetics you bred in. Being sure to retain resistance to diseases that are present will probably be easy; save seeds from plants that don't get sick. Retaining resistance to diseases that aren't present would be tougher. I don't see how you'd select for that, so it's a crap shoot whether you lose those genes or not.

You bring up a lot of good points.  I especially liked the point about how can you select for resistance to something that isn't there.  I guess visually, you can't.  

Perhaps that's a perfect opportunity in which a group of growers from different areas could start with the original multi-threat resistant variety that genetically has as much resistance to known threats as possible, then each grows out that variety in their own areas with eventual swapping back to the original stain.

Ha!  Reading my own words above, I think I've arrived at the same theory for landrace corn development as so many others have already started before me.  I just had to think it through for myself. Thanks for sharing your wisdom.

Local acclimation and intensive selection against the negatives inherently breeds resistance to knowns and unknowns by increasing health, vigor, and overall quality of the landrace.  I do select for positive traits on occasion and I experiment a lot in that regard, but ultimately I prefer the landrace do its own thing.  However, I am constantly selecting against the negatives.  This takes my continual commitment and observation which I enjoy anyway so no big deal to me.  To me human-created landraces are not "plant and forget", they require guidance, vigilance, and maintenance in perpetuity in order to achieve the desired outcomes that the human wants.  There is no such thing as "finished".

While continually introducing new acquisitions from outside sources into the landrace works against local acclimation it helps to increase diversity and new resistance opportunities.  This open landrace scheme is what I do but I am aware that I might be introducing weak/susceptible strains as well.  I am willing to take the risk with all my landraces, for now, but I always reserve the option of temporarily or permanently close-looping any or all of my landraces for any reason.

With every landrace I have created I have seen an early diversity, health, and vigor crash followed by a gradual and noticeable increase in all three categories over the course of years, which I consider to be proof of increasing local acclimation.  Incorporating seeds from outside sources does not seem to adversely affect this trajectory (other than perhaps slowing down the rate of that gradual increase) but I always limit what I introduce as a small percentage of any annual landrace planting.  Some landraces like tomatoes and beans I choose to trial new acquisitions in isolation before I incorporate those seeds into my landrace, but I do not quite know how to follow this strategy with corns due to land constraints and corns' promiscuous open pollination characteristics.

All these discussions and strategies aside, corn landraces are really fun to grow and tinker with.  Just make every effort to avoid different corn types crossing or you will have a mess on your hands.  Be aware of what other folks might be planting in the vicinity of your corn plot, while being knowledgeable of best corn isolation distances.  Due to my circumstances I have to plant my corn landraces on a rotational schedule, some years I do not plant corn at all because neighboring farmers are planting field corn well within the minimum isolation distance.  Just a fact of life for me.

I've had many of those same thoughts/plans for long term sweet corn landraces, but I'm just restarting in corn breeding, so that's good to hear. Perhaps you can explain the below quote from Joseph Lofthouse on another old post I read?

"Note: If SH2 crosses with regular sweet corn, a non-sweet corn can be produced, but that's a special case."

I know that SH2 corn instructions state to isolate from other types,  but I always thought that was because the pollen from those other types of sweet corn would "water down" the sweetness & "toughen" the texture of the SH2 variety. I'm just curious what the story is on this?

Cy Cobb wrote:
I know that SH2 corn instructions state to isolate from other types,  but I always thought that was because the pollen from those other types of sweet corn would "water down" the sweetness & "toughen" the texture of the SH2 variety. I'm just curious what the story is on this?

It's not quite as simple as that since, minimally, there are two known genes or genetic loci that contribute to kernal sweetness in maize.  It appears that sh2, a naturally occuring 'mutant' gene, contributes maximum sweetness when two copies of the sh2 "allele" (one copy from each parent) are present in the progeny.....in the seed that gets planted.  By outcrossing with other pollen, ...... for example a sweet corn that does NOT use sh2 to achieve its sweetness.....it's possible to bring in a different "allele" of sh2  that somewhat negates the effect of the single-copy sh2 that may be in any one progeny plant.  In fact, the 'non-mutant' version of sh2 in wild maize stocks quite rightly would probably add toughness to the kernals based on its normal function.  But additionally as noted in the link below, there can be interactions between sh and su genes which complicates it a bit further.  So perhaps just best to keep selecting for desired traits within your landrace and get the best that you can for your needs and interests.

https://horticulture.oregonstate.edu/oregon-vegetables/corn-sweet-processing

With respect to combining all possible disease resistances into a landrace or variety, that has been the "holy grail" of plant breeding for over a century.  The main observation comes in the form of "yield penalty".....for incrementally more resistance you breed in, then generally speaking you unfortunately also observe incrementally small reductions in yield of your crop....*under disease-free conditions*.   If you are accepting of the yield penalty, however, then you can take solace during years of heavy disease pressure that you were able to harvest *something* while those growing less resistant material would have lost everything.  An extreme example to be sure, but not without historical precedent.

Excellent read! Thanks for that link. I imagine this is why the taste test is so important. I am reconsidering my planned mix now, & may keep the SH2's in a group of their own.  I was thinking the combination of flavors/ sweetness in fresh eating sweet corn wouldn't be a bad thing.

The su and sh alleles are on different chromosomes. When combined, they produce flour corn, instead of sweet corn. It is possible (with several years of effort) to select for strains that have compatible alleles to produce sweet corn. These varieties are called "synergistic".

In my own plant breeding, I will not use the sh2 allele, because it produces seeds that lack vigor, and fail to thrive. They are only viable with heavy application of poisons to the seed.

I minimize the use of the se allele for the same reason.

The problem with breeding for "all known resistances", is that we can't possibly know what that would entail. There are resistances to soil, farmer's habits, sunlight, clouds, humidity, aridity, viruses, microbes, fungi, bugs, wind, mammals, birds, reptiles, etc, etc, etc. If we inbreed a crop enough to say that it is resistant to every known thing, then it becomes an industrialized variety that is doomed to failure when something unknown comes along.

I don't even know what all the corn diseases are, I just select for things I like and cull out things I don't. That said, there is one corn disease that has my interest the one we always called smut, properly known as huitlacoche. Growing up if my granddad or dad found smut on corn, they ripped that plant out. My granddad might have been thinking of breeding, but my dad didn't even save seeds. Mostly I think they just didn't want the smut to spread to other plants. I remember back then that it mostly was found on the ears.  In more recent decades I had almost forgotten about it because I grew mostly modern SE hybrids which I think have been selected for resistance it, that's one of the reasons I did use SE corn in my sweet corn landrace.

In the last ten or so years I have rediscovered huitlacoche, probably because of all the old sweet, flint and flour corns I've brought in and maybe especially because of the worm resistant variety Zapalote Chico from Mexico. I was alarmed about it at first and was selecting against it. It isn't a terrible problem and even if left alone generally does not spread to other plants in the patch. Like I said, in my memory it always grew in and ruined the ears but last two or three years I have found plants that grew lots of it, mostly low down on the stalks and didn't bother the ears at all. I detassel those plants to keep their genes out of the rest but now have seeds from three different perfectly matured ears whose stalks made lots of huitlacoche. I'm pretty sure that those three plants have a maternal line back to the ZC.

I still want to keep it separated from the primary patches but am mulling the idea of a corn breeding project for disease. Perfectly matured corn and a fungal delicacy on the same plant? Could it be?

Good points all, lots to think about.  Growing up,  anytime we had corn smut appear on an ear, we removed it for fear of spreading the fungus to other ears.

All of the seed I have is untreated seed, so no worries about the pink stuff here.

I will say, that I'd like to extend seed viability if I can. However, I think that has more to do with the amount of starch in the seed.  Sweet corn is already lacking in that department, & SH2 even more so.  In your experience, does flour or flint corn have a longer shelf life as far as germinating?

Thanks for the clarification in my way of thinking.

Well, I did some research into each of my seed varieties that will go into future plantings, & only one out of the 16 was SH2.  The rest are SU & SE.  I think I will keep the SH2 out of my mix for the time being, & experiment with it separately since it's pretty unique.