understanding corn genetics

[DarJones]

Keen,

You can find a lot of references to mitosis and the pachytene phase of cell division and the problems that occur when a cell has mismatched chromosomes. Long story short, for a cell to enter division, the chromosomes must first pair up, then they duplicate, then they split into two cells with identical chromosome complements. If there is a chromosome that can't pair up because it does not have a complement, then cell division is halted in the worst possible case or else chromosome duplication does not occur properly resulting in one cell with an extra chromosome and the other missing that chromosome. That is the basic cause of loss of chromosomes.

Meiosis is the special form of cell division that results in a reproductive cell forming. These cells typically have 1/2 the chromosome complement of a normal cell. Meiosis is a special case because the chromosomes go through a process of multiplication followed by radical division down to the 1N chromosome complement. If a cell starts out as a triploid, then it can't go through the normal pairing up and multiplication and therefore is sterile.

For corn, there are 10 pairs of chromosomes. Teosinte comes in two flavors, some varieties have 10 pairs and others such as Zea Perennis have 20 pairs. In other words, the chromosomes in Perennis have doubled. A cross between maize and Perennis results in a triploid which is effectively sterile as per the above description of meiosis.

en.wikipedia.org/wiki/Zea_%28genus%29

Some of the more interesting traits in teosinte include flood tolerance, perennial plants, pest tolerance, disease tolerance, heat and drought tolerance, plant structure, etc.

DarJones

[12540dumont]

Dar,
When fooling around with corn, how do I know which plant to choose as the mother?

For example,
I have Carol's corn, which fills nicely, is worm free, fast and of good size, I have Florianni another flint/polenta corn. It does not have the robustness of Carol's corn, but it's beautiful and tastes terrific. I think it could be improved. So, does it matter which corn gets to be the daddy? Or is there a better way to go about improving a variety without breeding it to something else?

Thanks, Holly

[Joseph Lofthouse]

Oct 26, 2011 12:20:32 GMT -5 12540dumont said:

Dar,
When fooling around with corn, how do I know which plant to choose as the mother?

For example,
I have Carol's corn, which fills nicely, is worm free, fast and of good size, I have Florianni another flint/polenta corn. It does not have the robustness of Carol's corn, but it's beautiful and tastes terrific. I think it could be improved. So, does it matter which corn gets to be the daddy? Or is there a better way to go about improving a variety without breeding it to something else?

Thanks, Holly

In my garden, I generally choose the wildest corn to be the mother, believing that there is more diversity in the organelles of older corn than there is in modern inbreeds. I like genetic diversity, and I like diversity of organelles. I wonder for example if the lush green color of Hopi Blue flour corn is due to it having more efficient chloroplasts, or more of them. Chloroplasts are inherited only from the mother.

I do lots of crop improvement projects that don't involve purposeful crossing: by selecting those individuals that grow best in my garden each year. These are narrowing the diversity of my crops, but improving their productivity in my garden.

[DarJones]

The variety picked to be the mother with all else being equal should be the most vigorous of the two varieties. If one of the two varieties carries a strong recessive trait such as sweet corn being crossed to a flint/dent variety, then it is best to use the recessive carrying variety as the female parent because it makes picking out the crossed kernels easier. In general, crosses will give the same effect regardless of which is the male and which the female. As Joseph has alluded above, there are some maternal traits that should be considered. For example, if you have a variety that happens to carry cytoplasmic sterility, you may only be able to use it as a female parent.

DarJones

[Deleted]

Glad I found a thread about corn genetics. I mentioned on the corn colors thread about breeding a new variety of corn. I have the pedigree more or less figured out, but I still have a few genetic questions. First, what is the most dominant kernel type? I know all sweets are recessive, but what about flint? Some have said it is dominant, while others say it's recessive (or at least to dent types). Purely floury starch is recessive, right? I've read that kernel color usually comes from the pericarp or aleurone. I'm under the impression that a red pericarp (think Bloody Butcher) is a recessive mutation (at least it is in Abenaki / Roy's Calais). Is the dark purple color in the Peruvian Morado's pericarp recessive also? How about the deep reddish purple in Double Red Sweet? In my experience with Bloody Butcher, the kernels were all uniformly red, but a few red seeds with mixed parents wound up in the planting stock, and a dozen plants produced entirely white & blue ears, giving me the impression that the red pericarp is recessive. However, Painted Mountain has reds mixed in with blues, purples, and oranges on the same ear. Does its red source from the aleurone instead and is therefore codominant? I've been under the impression that the Double Red Sweet's and Peruvian Morado's anthocyanin-rich colors come from the pericarp and are recessive, thus I would have to spend a lot of extra time to breed back out and maintain the kernel pigments when crossed with a genetically domineering white or yellow kernelled variety (like Hickory King for the composite's size). Any help would be much appreciated.

[raymondo]

This may help bjargakarlinn: www.grochbiology.org/CornGeneticsArticle.pdf

[blueadzuki]

while we are on the subject, anyone have a clue at to how the sharp tight kernel specking I prize so highly is transmitted? The murky "mossy" is see fairly often is presumably incomplete dominance (since one color in a speckle is always the "base color" be it white or yellow) but the tight, well delinated spots found on corns like Robin's egg seem to indicate a gene, probably a fairly rare one since the trait shows up so infrequently (at least in North American corns, it seems commoner in Andean). At one point, Allen said that speckling had someting to do with chinmarking, but I have difficuty making sense of that, as specling occurs in the aluerone and chinmarking occurs in the pericarp which is, if I recall 100% maternal tissue. (so it should not have much effect on the activation or lack therof of genes in the aleurone.)

[keen101 (Biolumo / Andrew B.)]

All i know about the speckled colorings is that just like the chinmarked (striped) kernels, both are the result of the transposons or the jumping genes. I would assume they would be mostly if not entirely inherited from the maternal side, but I'm not sure.

[Joseph Lofthouse]

Nov 1, 2011 14:56:22 GMT -5 @bjargakarlinn said:

However, Painted Mountain has reds mixed in with blues, purples, and oranges on the same ear.

Look again...

We can observe how a crop has segregated to determine where the various colors are coming from. First thing we know is that we look at a corn kernel from the outside. I'm not going to address the dominance/recessiveness of each gene because I think something else is going on here.... Perhaps it could be called visible/obscured.

Pericarp is the outermost layer and it's color is due only to the mother. If color is present it mostly masks the colors underneath it. In my version of Painted Mountain something like 60% of the cobs are a single color (perhaps with slight variations, so I'll attribute those colors [dark purple and red; eggplant and pomegranate to be more accurate] to the pericarp.

The aleurone is the next layer deep. These colors are due to triploid tissue, so they don't undergo simple segregation, but they are segregating. So we get multi-colored cobs. These multi-colored cobs do not have dark purple or red kernels on them, again confirming that dark purple and red are coming only from the pericarp.

The whites and yellows in Painted Mountain have colorless pericarps and aleurones. Any other colors that show up on cobs with white or yellow kernels [pinks, blues, lavenders, grays, and browns] are due to colors in the aleurone. There are also no dark purple or red kernels on cobs with white or yellow kernels. Therefore, the blues, lavenders, grays, greens, pinks, and browns in Painted Mountain are due to aleurone coloring.

I left orange out of my analysis. I don't have enough data points....

So in conclusion, if pericarp color is present [dark purple or red] it masks any other colors.

If aleurone color is present [lavender, gray, brown, pink, blue] it masks endosperm color.

If pericarp color and aleurone color are both absent, only then can endosperm color be seen.

For the sake of simplicity I'm ignoring: speckling, spotting, blotching, transposon striping, and partial color blushes on kernels.

[Deleted]

Okay, maybe an example would help me more. Say, I want to cross the antioxidant laden Double Red Sweet as the mother with the girthy Hickory King as the father. Would the F1 offspring have their mother's purplish red pericarp, or would that trait be masked by the father's lack of color in the first generation?

[Joseph Lofthouse]

I crossed some purple/red corns with sugary enhanced sweet corn. Here is what the children looked like.





If red/purple was a recessive trait then we'd expect only 1/4 of these to be red/purple. However 1/2 to 3/4 of them are red/purple implying that red/purple pericarp is dominant over clear pericarp.

The trouble with red/purple pericarp, is that you can't conclusively select for red/purple in this year's seed, because even though it is a dominant trait, it doesn't manifest itself until the following year. You end up selecting based on pedigree: what it's mother was like. Half of those yellow kernels are carrying a dominant red/purple pericarp gene that we can't observe. So we select based on what the mother was like, slowly, slowly getting closer to our desired phenotype.

I know Dar: Just self it and grow out two generations of the children....

[Joseph Lofthouse]

Nov 1, 2011 19:46:52 GMT -5 @bjargakarlinn said:

Okay, maybe an example would help me more. Say, I want to cross the antioxidant laden Double Red Sweet as the mother with the girthy Hickory King as the father. Would the F1 offspring have their mother's purplish red pericarp, or would that trait be masked by the father's lack of color in the first generation?

Too complicated to answer....
Especially considering the pedigree of Double Red.

White is the most recessive of colors so it's unlikely to mask anything.

Color depends on many genes that interact with each other....

Since Double Red is the mother, all F1 seeds would be red the first year.

Then, if Double Red is homozygous for Purple pericarp (doubtful) and if all the other genes necessary are present, then in the F2, all of the cobs would contain only purple kernels.... If Double Red isn't homozygous, then some other colors could start showing up.

In the F3 generation I would expect to start seeing normal segregation: 3 cobs purple to 1 cob of some other color.

[Deleted]

Wow, those are very pretty ears, Joseph. What was the parentage, if I may ask? And one more genetics question: I asked earlier about what type of kernel was dominant. I'm pretty sure floury is recessive, but I'm not sure about dent vs. flint. Would you happen to know? I'd like my Nanapu composite to have flint kernels for better storage and insect & mold resistance.

[Joseph Lofthouse]

Nov 1, 2011 21:26:06 GMT -5 @bjargakarlinn said:

Wow, those are very pretty ears, Joseph. What was the parentage, if I may ask? And one more genetics question: I asked earlier about what type of kernel was dominant. I'm pretty sure floury is recessive, but I'm not sure about dent vs. flint. Would you happen to know? I'd like my Nanapu composite to have flint kernels for better storage and insect & mold resistance.

The origins are unknown. The photo labeled cherry flour is the F2 of a cross between purple kernels from my Indian corn landrace (collected from all over) and sugary enhanced sweet corn (Precocious, Bodacious, or Incredible.)

The variety labeled "su purple" originated at Walmart. I noticed a few sweet corn kernels in an ear of dark purple Indian corn, so I brought it home and crossed the sweet kernels with the same sugary enhanced sweet corns as listed above. It is also an F2.

I don't know about the dominance of kernel types... However, in the barn, I have enough segregating lines cross pollinated with each other, that I should be able to figure it out. Check back in a couple days. Recessive genes are the easiest to work with in a segregating population. Dominant genes are a pain! Dominant genes that are only expressed in maternal tissue are even more painful!!

[Joseph Lofthouse]

So more photos... Only white seeds were planted. However one cob showed up containing kernels that were all dark purple/red. That cob has a dominant purple pericarp gene, but that gene was not expressed in the pericarp of the mother plant, which means that the gene arrived in the kernel via the pollen and lay dormant.



Something similar happened among the gray kernels: Also, in this case the pericarp color is light enough that the aleurone color can show through somewhat.



Since these were pollinated as patches, it also means that in this year's harvest there is a dormant-dominant gene for red pericarp color hiding inside the white and gray kernels. For the whites we can expect about 1 red cob next year per 34 plants grown. For the grays about 2 red cobs per 32 plants.