Mapping the 1970 corn blight

Here are my 2 maps1 for this discussion. I used linear regression to predict corn yield for each county in the US, using time (year) as the independent variable. I used the years 1950 to 1969 to create the model, and to predict corn yield in 1970. This should be a reasonable estimate of the ‘expected yield’ for 1970 for each county, if it had been a ‘normal year’.

I then computed the difference between the expected yield and the yield obtained by farmers, and expressed that as the percentage of the expected yield. Negative numbers mean that yields were lower than expected in a county, positive numbers mean that they were higher than expected. Counties with data for less than 9 years were excluded.

1970 corn yields were indeed much lower than expected in the southeast. Corn blight hit very hard. But also note that yield was stable or up in the north and in the west, and look were US corn was grown in 1970. The map below expresses corn area as the percentage of the total area of a county.

Most corn is grown in the corn-belt. The southern parts of it were much affected by the disease (The Illinois Secretary of Agriculture’s estimate that, by August, 25 percent of his state’s corn crop had been lost to the blight may have been spot on). But 1970 was a normal or good year for corn yield in the northern and western parts of the corn belt, and that compensated for the losses incurred elsewhere. If you sum it all up, corn production was about 15% lower than what could have been expected. That is whole lot of corn — but perhaps not that exceptional as far as bad years go.

Here is a table of estimated corn yield by state, as percentage of the expected yield for 1970, and the corn area, as percentage of the national area (only for states with more than 1% of the national corn area in the counties data set).

State Yield Area   State Yield Area
Florida -36 1   Minnesota -12 8
Georgia -33 3   Missouri -11 5
Illinois -31 18   Nebraska -9 9
Indiana -27 9   North Carolina -5 2
Iowa -26 18   Ohio -1 5
Kansas -24 2   Pennsylvania 0 2
Kentucky -22 2   South Dakota 6 4
Michigan -12 3   Wisconsin 15 3
  1. Quick & dirty, without cross checking the numbers, but I think the maps speak for themselves. []

Maize mystery solved Joost van Heerwarden and co-workers1 have solved a problem in our understanding of maize domestication. Previous work had shown that maize originated from Balsas teosinte, Zea mays subspecies parviglumis, a wild species that occurs in low and mid-elevation regions of south-west Mexico2. This made the Rio Balsas area, where parviglumis occurs, the most likely area of maize domestication. This was corroborated by Piperno et al.‘s3 discovery of 8,700 years old maize remains in that area; the oldest evidence of maize unearthed to date.

The problem was that the maize land races genetically most similar to parviglumis are not found there. They occur in the Mexican highlands. And that’s awkward, particularly because highland maize has a rather different set of ecological adaptations than lowland maize.

Van Heerwaarden et al. say this is a paradox caused by the role of another wild species: Zea mays subspecies mexicana. This species occurs in the highlands, and it is inter-fertile with cultivated maize. The tricky thing is that because the two wild species, parviglumis and mexicana, both referred to as teosinte, are closely related, more closely to each other than to their cultivated cousin, geneflow from mexicana makes the genes of highland maize look more like those of parviglumis!

This means that you cannot directly identify the most ancestral maize populations from genetic similarity with their putative ancestor. Instead, Van Heerwaarden et al. estimated ancestral gene frequencies from cultivated maize populations, without direct reference to the wild species. And, Bingo! Western lowland populations are indeed more ancestral than the highland populations. Maize did originate in the lowlands, and from there it spread to the highlands and to other parts of the Americas.

  1. van Heerwaarden J, Doebley J, Briggs WH, Glaubitz JC, Goodman MM, de Jesus Sanchez Gonzalez J, & Ross-Ibarra J (2010). Genetic signals of origin, spread, and introgression in a large sample of maize landraces. Proceedings of the National Academy of Sciences of the United States of America PMID: 21189301 []
  2. Matsuoka Y, Vigouroux Y, Goodman MM, Sanchez G J, Buckler E, & Doebley J (2002). A single domestication for maize shown by multilocus microsatellite genotyping. Proceedings of the National Academy of Sciences of the United States of America, 99 (9), 6080-4 PMID: 11983901 []
  3. Piperno DR, Ranere AJ, Holst I, Iriarte J, & Dickau R (2009). Starch grain and phytolith evidence for early ninth millennium B.P. maize from the Central Balsas River Valley, Mexico. Proceedings of the National Academy of Sciences of the United States of America, 106 (13), 5019-24 PMID: 19307570 []

Yes, maybe, no, yes: Transgenes in Mexican maize, after all

Update: The link below was behind a paywall. A new one, via, seems to be open access.
Elena Álvarez-Buylla and co-workers have found transgenes from genetically modified maize in landraces in Mexico. Their paper is to be published in Molecular Ecology, but for now we have this news article in Nature.

The evolving story has multiple layers, including the science ethics controversy. Quist and Chapela published the same finding in Nature in 2001, but their methods were questioned, and the journal made an unprecedented statement saying there had been insufficient evidence to justify the publication. Some saw the hand (and money) of Big Biotech in this1, and in the subsequent denial of tenure to Chapela at the University of California, Berkeley (that was later overturned). Now Nature reports that the Álvarez-Buylla paper was not published in the prestigious Proceedings of the National Academy of Sciences (PNAS) because the journal’s editor-in-chief Randy Schekman, also at Berkeley, considered that “the report could gain undue exposure in the press due to a political or other environmental agenda.”

We’ll see if the current paper settles the scientific controversy. Ortiz-García and colleagues did not find any transgenes in a large sample in 2003/4; a result that was found worthy of publication in PNAS. The Nature news article suggests that Álvarez-Buylla found the transgenes in only one field (out of more than 100 sampled), and that this field was also sampled by Quist and Chapela. So are we talking about a single farmer with a cousin in Iowa sending seed remittances? Or about a relatively small fraction of maize plants across the country?

It seems entirely obvious that if there are transgenes in U.S. maize, these will spread down to Mexico. Someone needs to find them first, for sure, but the more relevant question is not if transgenes spread, but rather: which, where, what mechanism(s) (long versus short distance dispersal), how fast, how much, how persistent, and what are the consequences, if any? The term “pollution” is used a lot in this debate. Me, I do not believe in pure races.

  1. Conflicts around a study of Mexican crops []

Maize and genetic engineering: why bother?

I was talking to Greg Edmeades tonight. Our conversation coalesced on the topics of the recent posts on maize water stress tolerance and on the usefulness of engineering purple tomatoes.

For many years, Greg led the maize crop physiology group at CIMMYT. He says that one of the main reasons for their success with drought tolerance is their long term institutional commitment to it: 35 years and counting. A particularly impressive feat is the widespread adoption of their maize varieties in southern Africa. For example, ZM623, selected from South African parents by Marianna Bänziger, is grown on about a million ha, says Edmeades.

His take on biotech for drought tolerance is, sure, “use whatever works, but if you are an African agricultural research institute, then, why bother?” Monsanto is reporting 10-15% yield increase under drought stress, and says it will make their technology freely available for use in Africa. Edmeades reckons that you can get a similar yield increase in about 7 years of conventional breeding and selection. And less when using molecular markers. If that is the case, it may not be worth it to deal with the complexities of genetic engineering.

Unless, perhaps, the approaches are entirely additive and you get a combined yield benefit of 30%. I think that’s unlikely. Drought tolerance is about making best use of the available water. It does not increase the amount of water.

Greg told me that I had been a little harsh in suggesting that he would not advise national programs in Africa to use a transgenic approach to drought tolerance. He would only advice against transgenes if “they had access to a steady stream of good germplasm improved for drought tolerance, and there was no regulatory framework for transgenes in place in that country. If regulatory frameworks exist, and there is no facility of improving their own varieties (or newly released commercial varieties) for drought tolerance in a systematic way, then certainly I’d take the transgenic option, especially since it is being offered on a royalty free basis.”

Single gene looking for water

Drought tolerance is the holy grail in crop improvement these days. We are running out of water; cannot easily expand irrigation; poorer farmers are affected most by it; and climate change will make things worse (etc.).

Breeding for drought tolerance has not been very successful. For lack of trying? Many years of work at CIMMYT seem to be paying off. Or is it just too damn difficult because of the multiple genes involved (from stomatal regulation to root growth), and the multiple droughts (when, how long, how much) to deal with.

Drought tolerant maize compared with local varietyCan biotech come to the rescue? This New York times article suggests that big companies and single genes may do the trick. I have to see it before I believe it, something like this picture, which shows drought-resistant corn on the right, tested next to “traditional” corn plants in Nebraska, USA. I want to see that picture in the fields of African farmers.

Or should farmers who cannot grow maize because of drought start thinking of another crop? Why not grow sorghum?